Merge //aos/linux_code/logging into //aos/common/logging

Shuffling files+targets to clean it up further is coming next.

Change-Id: Iae716b07b340a66502f8e0964281c10fae0fcb7a
diff --git a/aos/common/logging/BUILD b/aos/common/logging/BUILD
index 78d1cbd..b3e358d 100644
--- a/aos/common/logging/BUILD
+++ b/aos/common/logging/BUILD
@@ -1,5 +1,108 @@
 package(default_visibility = ['//visibility:public'])
 
+cc_library(
+  name = 'log_replay',
+  srcs = [
+    'log_replay.cc',
+  ],
+  hdrs = [
+    'log_replay.h',
+  ],
+  deps = [
+    ':binary_log_file',
+    '//aos/common:queues',
+    '//aos/common/logging',
+    '//aos/linux_code/ipc_lib:queue',
+  ],
+)
+
+cc_binary(
+  name = 'binary_log_writer',
+  srcs = [
+    'binary_log_writer.cc',
+  ],
+  deps = [
+    '//aos/common/logging',
+    '//aos/linux_code:init',
+    '//aos/linux_code:configuration',
+    '//aos/common:die',
+    ':binary_log_file',
+    '//aos/common:queue_types',
+  ],
+)
+
+cc_binary(
+  name = 'log_streamer',
+  srcs = [
+    'log_streamer.cc',
+  ],
+  deps = [
+    '//aos/common/logging',
+    '//aos/linux_code:init',
+    '//aos/common:time',
+    '//aos/linux_code/ipc_lib:queue',
+  ],
+)
+
+cc_binary(
+  name = 'log_displayer',
+  srcs = [
+    'log_displayer.cc',
+  ],
+  deps = [
+    '//aos/common/logging',
+    '//aos/linux_code:init',
+    ':binary_log_file',
+    '//aos/common:queue_types',
+    '//aos/linux_code:configuration',
+    '//aos/common/util:string_to_num',
+  ],
+)
+
+cc_library(
+  name = 'binary_log_file',
+  srcs = [
+    'binary_log_file.cc',
+  ],
+  hdrs = [
+    'binary_log_file.h',
+  ],
+  deps = [
+    '//aos/common/logging',
+  ],
+)
+
+cc_library(
+  name = 'linux_interface',
+  visibility = ['//aos/common/logging:__pkg__'],
+  srcs = [
+    'linux_interface.cc',
+  ],
+  deps = [
+    '//aos/linux_code:complex_thread_local',
+    '//aos/common:die',
+    '//aos/common/logging:context',
+  ],
+)
+
+cc_library(
+  name = 'linux_logging',
+  visibility = [
+    '//aos/linux_code:__subpackages__'
+  ],
+  hdrs = [
+    'linux_logging.h',
+  ],
+  srcs = [
+    'linux_logging.cc',
+  ],
+  deps = [
+    '//aos/linux_code/ipc_lib:queue',
+    '//aos/common:time',
+    '//aos/common/logging:logging',
+  ],
+)
+
 cc_test(
   name = 'logging_impl_test',
   srcs = [
@@ -74,7 +177,7 @@
   deps = [
     '//aos/common:die',
     '//aos/common/libc:aos_strerror',
-    '//aos/linux_code/logging:linux_interface',
+    '//aos/common/logging:linux_interface',
     '//aos/common:macros',
   ],
 )
diff --git a/aos/common/logging/binary_log_file.cc b/aos/common/logging/binary_log_file.cc
new file mode 100644
index 0000000..be58e11
--- /dev/null
+++ b/aos/common/logging/binary_log_file.cc
@@ -0,0 +1,263 @@
+#include "aos/common/logging/binary_log_file.h"
+
+#include <stdio.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <signal.h>
+#include <setjmp.h>
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+namespace {
+
+unsigned long SystemPageSize() {
+  static unsigned long r = sysconf(_SC_PAGESIZE);
+  return r;
+}
+
+}  // namespace
+
+LogFileAccessor::LogFileAccessor(int fd, bool writable)
+    : fd_(fd), writable_(writable), offset_(0), current_(0), position_(0) {
+  // Check to make sure that mmap will allow mmaping in chunks of kPageSize.
+  if (SystemPageSize() > kPageSize || (kPageSize % SystemPageSize()) != 0) {
+    LOG(FATAL, "system page size (%lu) not factor of kPageSize (%zd).\n",
+        SystemPageSize(), kPageSize);
+  }
+
+  MapNextPage();
+}
+
+void LogFileAccessor::Sync() const {
+  msync(current_, kPageSize, MS_ASYNC | MS_INVALIDATE);
+}
+
+void LogFileAccessor::SkipToLastSeekablePage() {
+  CHECK(definitely_use_mmap());
+
+  struct stat info;
+  if (fstat(fd_, &info) == -1) {
+    PLOG(FATAL, "fstat(%d, %p) failed", fd_, &info);
+  }
+
+  CHECK((info.st_size % kPageSize) == 0);
+  const auto last_readable_page_number = (info.st_size / kPageSize) - 1;
+  const auto last_seekable_page_number =
+      last_readable_page_number / kSeekPages * kSeekPages;
+  const off_t new_offset = last_seekable_page_number * kPageSize;
+  // We don't want to go backwards...
+  if (new_offset > offset_) {
+    Unmap(current_);
+    offset_ = new_offset;
+    MapNextPage();
+  }
+}
+
+// The only way to tell is using fstat, but we don't really want to be making a
+// syscall every single time somebody wants to know the answer, so it gets
+// cached in is_last_page_.
+bool LogFileAccessor::IsLastPage() {
+  if (is_last_page_ != Maybe::kUnknown) {
+    return is_last_page_ == Maybe::kYes;
+  }
+
+  struct stat info;
+  if (fstat(fd_, &info) == -1) {
+    PLOG(FATAL, "fstat(%d, %p) failed", fd_, &info);
+  }
+  bool r = offset_ == static_cast<off_t>(info.st_size - kPageSize);
+  is_last_page_ = r ? Maybe::kYes : Maybe::kNo;
+  return r;
+}
+
+void LogFileAccessor::MapNextPage() {
+  if (writable_) {
+    if (ftruncate(fd_, offset_ + kPageSize) == -1) {
+      PLOG(FATAL, "ftruncate(%d, %zd) failed", fd_, kPageSize);
+    }
+  }
+
+  if (use_read_ == Maybe::kYes) {
+    ssize_t todo = kPageSize;
+    while (todo > 0) {
+      ssize_t result = read(fd_, current_ + (kPageSize - todo), todo);
+      if (result == -1) {
+        PLOG(FATAL, "read(%d, %p, %zu) failed", fd_,
+             current_ + (kPageSize - todo), todo);
+      } else if (result == 0) {
+        memset(current_, 0, todo);
+        result = todo;
+      }
+      todo -= result;
+    }
+    CHECK_EQ(0, todo);
+  } else {
+    current_ = static_cast<char *>(
+        mmap(NULL, kPageSize, PROT_READ | (writable_ ? PROT_WRITE : 0),
+             MAP_SHARED, fd_, offset_));
+    if (current_ == MAP_FAILED) {
+      if (!writable_ && use_read_ == Maybe::kUnknown && errno == ENODEV) {
+        LOG(INFO, "Falling back to reading the file using read(2).\n");
+        use_read_ = Maybe::kYes;
+        current_ = new char[kPageSize];
+        MapNextPage();
+        return;
+      } else {
+        PLOG(FATAL,
+             "mmap(NULL, %zd, PROT_READ [ | PROT_WRITE], MAP_SHARED, %d, %jd)"
+             " failed",
+             kPageSize, fd_, static_cast<intmax_t>(offset_));
+      }
+    } else {
+      use_read_ = Maybe::kNo;
+    }
+    if (madvise(current_, kPageSize, MADV_SEQUENTIAL | MADV_WILLNEED) == -1) {
+      PLOG(WARNING, "madvise(%p, %zd, MADV_SEQUENTIAL | MADV_WILLNEED) failed",
+           current_, kPageSize);
+    }
+  }
+  offset_ += kPageSize;
+}
+
+void LogFileAccessor::Unmap(void *location) {
+  CHECK_NE(Maybe::kUnknown, use_read_);
+
+  if (use_read_ == Maybe::kNo) {
+    if (munmap(location, kPageSize) == -1) {
+      PLOG(FATAL, "munmap(%p, %zd) failed", location, kPageSize);
+    }
+  }
+  is_last_page_ = Maybe::kUnknown;
+  position_ = 0;
+}
+
+const LogFileMessageHeader *LogFileReader::ReadNextMessage(bool wait) {
+  LogFileMessageHeader *r;
+  do {
+    r = static_cast<LogFileMessageHeader *>(
+        static_cast<void *>(&current()[position()]));
+    if (wait) {
+      CHECK(definitely_use_mmap());
+      if (futex_wait(&r->marker) != 0) continue;
+    }
+    if (r->marker == 2) {
+      Unmap(current());
+      MapNextPage();
+      CheckCurrentPageReadable();
+      r = static_cast<LogFileMessageHeader *>(static_cast<void *>(current()));
+    }
+  } while (wait && r->marker == 0);
+  if (r->marker == 0) {
+    return NULL;
+  }
+  IncrementPosition(sizeof(LogFileMessageHeader) + r->name_size +
+                    r->message_size);
+  if (position() >= kPageSize) {
+    // It's a lot better to blow up here rather than getting SIGBUS errors the
+    // next time we try to read...
+    LOG(FATAL, "corrupt log file running over page size\n");
+  }
+  return r;
+}
+
+// This mess is because the only not completely hackish way to do this is to set
+// up a handler for SIGBUS/SIGSEGV that siglongjmps out to avoid either the
+// instruction being repeated infinitely (and more signals being delivered) or
+// (with SA_RESETHAND) the signal killing the process.
+namespace {
+
+void *volatile fault_address;
+sigjmp_buf jump_context;
+
+void CheckCurrentPageReadableHandler(int /*signal*/, siginfo_t *info, void *) {
+  fault_address = info->si_addr;
+
+  siglongjmp(jump_context, 1);
+}
+
+}  // namespace
+void LogFileReader::CheckCurrentPageReadable() {
+  if (definitely_use_read()) return;
+
+  if (sigsetjmp(jump_context, 1) == 0) {
+    struct sigaction action;
+    action.sa_sigaction = CheckCurrentPageReadableHandler;
+    sigemptyset(&action.sa_mask);
+    action.sa_flags = SA_RESETHAND | SA_SIGINFO;
+    struct sigaction previous_bus, previous_segv;
+    if (sigaction(SIGBUS, &action, &previous_bus) == -1) {
+      PLOG(FATAL, "sigaction(SIGBUS(=%d), %p, %p) failed",
+           SIGBUS, &action, &previous_bus);
+    }
+    if (sigaction(SIGSEGV, &action, &previous_segv) == -1) {
+      PLOG(FATAL, "sigaction(SIGSEGV(=%d), %p, %p) failed",
+           SIGSEGV, &action, &previous_segv);
+    }
+
+    char __attribute__((unused)) c = current()[0];
+
+    if (sigaction(SIGBUS, &previous_bus, NULL) == -1) {
+      PLOG(FATAL, "sigaction(SIGBUS(=%d), %p, NULL) failed",
+           SIGBUS, &previous_bus);
+    }
+    if (sigaction(SIGSEGV, &previous_segv, NULL) == -1) {
+      PLOG(FATAL, "sigaction(SIGSEGV(=%d), %p, NULL) failed",
+           SIGSEGV, &previous_segv);
+    }
+  } else {
+    if (fault_address == current()) {
+      LOG(FATAL, "could not read 1 byte at offset 0x%jx into log file\n",
+          static_cast<uintmax_t>(offset()));
+    } else {
+      LOG(FATAL, "faulted at %p, not %p like we were (maybe) supposed to\n",
+          fault_address, current());
+    }
+  }
+}
+
+LogFileMessageHeader *LogFileWriter::GetWritePosition(size_t message_size) {
+  if (NeedNewPageFor(message_size)) ForceNewPage();
+  LogFileMessageHeader *const r = static_cast<LogFileMessageHeader *>(
+      static_cast<void *>(&current()[position()]));
+  IncrementPosition(message_size);
+  return r;
+}
+
+// A number of seekable pages, not the actual file offset, is stored in *cookie.
+bool LogFileWriter::ShouldClearSeekableData(off_t *cookie,
+                                            size_t next_message_size) const {
+  off_t next_message_page = (offset() / kPageSize) - 1;
+  if (NeedNewPageFor(next_message_size)) {
+    ++next_message_page;
+  }
+  const off_t current_seekable_page = next_message_page / kSeekPages;
+  CHECK_LE(*cookie, current_seekable_page);
+  const bool r = *cookie != current_seekable_page;
+  *cookie = current_seekable_page;
+  return r;
+}
+
+bool LogFileWriter::NeedNewPageFor(size_t bytes) const {
+  return position() + bytes + (kAlignment - (bytes % kAlignment)) +
+             sizeof(aos_futex) >
+         kPageSize;
+}
+
+void LogFileWriter::ForceNewPage() {
+  char *const temp = current();
+  MapNextPage();
+  if (futex_set_value(
+          static_cast<aos_futex *>(static_cast<void *>(&temp[position()])),
+          2) == -1) {
+    PLOG(WARNING, "readers will hang because futex_set_value(%p, 2) failed",
+         &temp[position()]);
+  }
+  Unmap(temp);
+}
+
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
diff --git a/aos/common/logging/binary_log_file.h b/aos/common/logging/binary_log_file.h
new file mode 100644
index 0000000..b753699
--- /dev/null
+++ b/aos/common/logging/binary_log_file.h
@@ -0,0 +1,207 @@
+#ifndef AOS_COMMON_LOGGING_BINARY_LOG_FILE_H_
+#define AOS_COMMON_LOGGING_BINARY_LOG_FILE_H_
+
+#include <sys/types.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include <algorithm>
+
+#include "aos/common/logging/logging_impl.h"
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+
+// What to align messages to. A macro because it gets used in attributes.
+// This definition gets #undefed later. Use LogFileAccessor::kAlignment instead.
+#define MESSAGE_ALIGNMENT 8
+
+// File format: {
+//   LogFileMessageHeader header;
+//   char *name;  // of the process that wrote the message
+//   void *message;
+// } not crossing kPageSize boundaries into the file and aligned to
+// MESSAGE_ALIGNMENT.
+//
+// Field sizes designed to fit the various values from LogMessage even on
+// other machines (hopefully) because they're baked into the files. They are
+// layed out so that all of the fields are aligned even though the whole thing
+// is packed.
+//
+// A lot of the fields don't have comments because they're the same as the
+// identically named fields in LogMessage.
+struct __attribute__((aligned(MESSAGE_ALIGNMENT))) __attribute__((packed))
+    LogFileMessageHeader {
+  // Represents the type of an individual message.
+  enum class MessageType : uint16_t {
+    // char[] (no '\0' on the end).
+    kString,
+    kStructType,
+    kStruct,
+    kMatrix,
+  };
+
+  // Gets futex_set once this one has been written
+  // for readers keeping up with a live writer.
+  //
+  // Gets initialized to 0 by ftruncate.
+  //
+  // There will be something here after the last message on a "page" set to 2
+  // (by the futex_set) to indicate that the next message is on the next page.
+  aos_futex marker;
+  static_assert(sizeof(marker) == 4, "mutex changed size!");
+  static_assert(MESSAGE_ALIGNMENT >= alignof(aos_futex),
+                "MESSAGE_ALIGNMENT is too small");
+
+  uint32_t time_sec;
+  static_assert(sizeof(time_sec) >= sizeof(LogMessage::seconds),
+                "tv_sec won't fit");
+  uint32_t time_nsec;
+  static_assert(sizeof(time_nsec) >= sizeof(LogMessage::nseconds),
+                "tv_nsec won't fit");
+
+  int32_t source;
+  static_assert(sizeof(source) >= sizeof(LogMessage::source), "PIDs won't fit");
+
+  // Both including all of the bytes in that part of the message.
+  uint32_t name_size, message_size;
+
+  uint16_t sequence;
+  static_assert(sizeof(sequence) == sizeof(LogMessage::sequence),
+                "something changed");
+
+  MessageType type;
+
+  log_level level;
+  static_assert(sizeof(level) == 1, "log_level changed size!");
+};
+static_assert(std::is_pod<LogFileMessageHeader>::value,
+              "LogFileMessageHeader will to get dumped to a file");
+static_assert(offsetof(LogFileMessageHeader, marker) == 0,
+              "marker has to be at the start so readers can find it");
+
+// Handles the mmapping and munmapping for reading and writing log files.
+class LogFileAccessor {
+ public:
+  LogFileAccessor(int fd, bool writable);
+  ~LogFileAccessor() {
+    if (use_read_ == Maybe::kYes) {
+      delete[] current_;
+    }
+  }
+
+  // Asynchronously syncs all open mappings.
+  void Sync() const;
+
+  // Returns true iff we currently have the last page in the file mapped.
+  // This is fundamentally a racy question, so the return value may not be
+  // accurate by the time this method returns.
+  bool IsLastPage();
+
+  // Skips to the last page which is an even multiple of kSeekPages.
+  // This is fundamentally racy, so it may not actually be on the very last
+  // possible multiple of kSeekPages when it returns, but it should be close.
+  // This will never move backwards.
+  void SkipToLastSeekablePage();
+
+  size_t file_offset(const void *msg) {
+    return offset() + (static_cast<const char *>(msg) - current());
+  }
+
+ protected:
+  // The size of the chunks that get mmaped/munmapped together. Large enough so
+  // that not too much space is wasted and it's hopefully bigger than and a
+  // multiple of the system page size but small enough so that really large
+  // chunks of memory don't have to get mapped at the same time.
+  static const size_t kPageSize = 16384;
+  // What to align messages to, copied into an actual constant.
+  static const size_t kAlignment = MESSAGE_ALIGNMENT;
+#undef MESSAGE_ALIGNMENT
+  // Pages which are multiples of this from the beginning of a file start with
+  // no saved state (ie struct types). This allows seeking immediately to the
+  // largest currently written interval of this number when following.
+  static const size_t kSeekPages = 256;
+
+  char *current() const { return current_; }
+  size_t position() const { return position_; }
+  off_t offset() const { return offset_; }
+
+  void IncrementPosition(size_t size) {
+    position_ += size;
+    AlignPosition();
+  }
+
+  void MapNextPage();
+  void Unmap(void *location);
+
+  // Advances position to the next (aligned) location.
+  void AlignPosition() {
+    position_ += kAlignment - (position_ % kAlignment);
+  }
+
+ protected:
+  bool definitely_use_read() const { return use_read_ == Maybe::kYes; }
+  bool definitely_use_mmap() const { return use_read_ == Maybe::kNo; }
+
+ private:
+  // Used for representing things that we might know to be true/false or we
+  // might not know (yet).
+  enum class Maybe { kUnknown, kYes, kNo };
+
+  const int fd_;
+  const bool writable_;
+
+  // Into the file. Always a multiple of kPageSize.
+  off_t offset_;
+  char *current_;
+  size_t position_;
+
+  Maybe is_last_page_ = Maybe::kUnknown;
+
+  // Use read instead of mmap (necessary for fds that don't support mmap).
+  Maybe use_read_ = Maybe::kUnknown;
+};
+
+class LogFileReader : public LogFileAccessor {
+ public:
+  LogFileReader(int fd) : LogFileAccessor(fd, false) {}
+
+  // May return NULL iff wait is false.
+  const LogFileMessageHeader *ReadNextMessage(bool wait);
+
+ private:
+  // Tries reading from the current page to see if it fails because the file
+  // isn't big enough.
+  void CheckCurrentPageReadable();
+};
+
+class LogFileWriter : public LogFileAccessor {
+ public:
+  LogFileWriter(int fd) : LogFileAccessor(fd, true) {}
+
+  // message_size should be the total number of bytes needed for the message.
+  LogFileMessageHeader *GetWritePosition(size_t message_size);
+
+  // Returns true exactly once for each unique cookie on each page where cached
+  // data should be cleared.
+  // Call with a non-zero next_message_size to determine if cached data should
+  // be forgotten before writing a next_message_size-sized message.
+  // cookie should be initialized to 0.
+  bool ShouldClearSeekableData(off_t *cookie, size_t next_message_size) const;
+
+  // Forces a move to a new page for the next message.
+  // This is important when there is cacheable data that needs to be re-written
+  // before a message which will spill over onto the next page but the cacheable
+  // message being refreshed is smaller and won't get to a new page by itself.
+  void ForceNewPage();
+
+ private:
+  bool NeedNewPageFor(size_t bytes) const;
+};
+
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
+
+#endif  // AOS_COMMON_LOGGING_BINARY_LOG_FILE_H_
diff --git a/aos/common/logging/binary_log_writer.cc b/aos/common/logging/binary_log_writer.cc
new file mode 100644
index 0000000..d4e6685
--- /dev/null
+++ b/aos/common/logging/binary_log_writer.cc
@@ -0,0 +1,311 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <time.h>
+#include <string.h>
+#include <string>
+#include <unistd.h>
+#include <sys/types.h>
+#include <pwd.h>
+#include <fcntl.h>
+#include <dirent.h>
+#include <mntent.h>
+
+#include <map>
+#include <unordered_set>
+
+#include "aos/common/logging/linux_logging.h"
+#include "aos/common/logging/binary_log_file.h"
+#include "aos/linux_code/init.h"
+#include "aos/linux_code/configuration.h"
+#include "aos/common/queue_types.h"
+#include "aos/common/die.h"
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+namespace {
+
+void CheckTypeWritten(uint32_t type_id, LogFileWriter *writer,
+                      ::std::unordered_set<uint32_t> *written_type_ids) {
+  if (written_type_ids->count(type_id) > 0) return;
+  if (MessageType::IsPrimitive(type_id)) return;
+
+  const MessageType &type = type_cache::Get(type_id);
+  for (int i = 0; i < type.number_fields; ++i) {
+    CheckTypeWritten(type.fields[i]->type, writer, written_type_ids);
+  }
+
+  char buffer[1024];
+  ssize_t size = type.Serialize(buffer, sizeof(buffer));
+  if (size == -1) {
+    LOG(WARNING, "%zu-byte buffer is too small to serialize type %s\n",
+        sizeof(buffer), type.name.c_str());
+    return;
+  }
+  LogFileMessageHeader *const output =
+      writer->GetWritePosition(sizeof(LogFileMessageHeader) + size);
+
+  output->time_sec = output->time_nsec = 0;
+  output->source = getpid();
+  output->name_size = 0;
+  output->sequence = 0;
+  output->level = FATAL;
+
+  memcpy(output + 1, buffer, size);
+  output->message_size = size;
+
+  output->type = LogFileMessageHeader::MessageType::kStructType;
+  futex_set(&output->marker);
+
+  written_type_ids->insert(type_id);
+}
+
+void AllocateLogName(char **filename, const char *directory) {
+  int fileindex = 0;
+  DIR *const d = opendir(directory);
+  if (d == nullptr) {
+    PDie("could not open directory %s", directory);
+  }
+  int index = 0;
+  while (true) {
+    errno = 0;
+    struct dirent *const dir = readdir(d);
+    if (dir == nullptr) {
+      if (errno == 0) {
+        break;
+      } else {
+        PLOG(FATAL, "readdir(%p) failed", d);
+      }
+    } else {
+      if (sscanf(dir->d_name, "aos_log-%d", &index) == 1) {
+        if (index >= fileindex) {
+          fileindex = index + 1;
+        }
+      }
+    }
+  }
+  closedir(d);
+
+  char previous[512];
+  ::std::string path = ::std::string(directory) + "/aos_log-current";
+  ssize_t len = ::readlink(path.c_str(), previous, sizeof(previous));
+  if (len != -1) {
+    previous[len] = '\0';
+  } else {
+    previous[0] = '\0';
+    LOG(INFO, "Could not find aos_log-current\n");
+    printf("Could not find aos_log-current\n");
+  }
+  if (asprintf(filename, "%s/aos_log-%03d", directory, fileindex) == -1) {
+    PDie("couldn't create final name");
+  }
+  LOG(INFO, "Created log file (aos_log-%d) in directory (%s). Previous file "
+            "was (%s).\n",
+      fileindex, directory, previous);
+  printf("Created log file (aos_log-%d) in directory (%s). Previous file was "
+         "(%s).\n",
+         fileindex, directory, previous);
+}
+
+#ifdef AOS_ARCHITECTURE_arm_frc
+bool FoundThumbDrive(const char *path) {
+  FILE *mnt_fp = setmntent("/etc/mtab", "r");
+  if (mnt_fp == nullptr) {
+    Die("Could not open /etc/mtab");
+  }
+
+  bool found = false;
+  struct mntent mntbuf;
+  char buf[256];
+  while (!found) {
+    struct mntent *mount_list = getmntent_r(mnt_fp, &mntbuf, buf, sizeof(buf));
+    if (mount_list == nullptr) {
+      break;
+    }
+    if (strcmp(mount_list->mnt_dir, path) == 0) {
+      found = true;
+    }
+  }
+  endmntent(mnt_fp);
+  return found;
+}
+
+bool FindDevice(char *device, size_t device_size) {
+  char test_device[10];
+  for (char i = 'a'; i < 'z'; ++i) {
+    snprintf(test_device, sizeof(test_device), "/dev/sd%c", i);
+    LOG(INFO, "Trying to access %s\n", test_device);
+    if (access(test_device, F_OK) != -1) {
+      snprintf(device, device_size, "sd%c", i);
+      return true;
+    }
+  }
+  return false;
+}
+#endif
+
+int BinaryLogReaderMain() {
+  InitNRT();
+
+#ifdef AOS_ARCHITECTURE_arm_frc
+  char folder[128];
+
+  {
+    char dev_name[8];
+    while (!FindDevice(dev_name, sizeof(dev_name))) {
+      LOG(INFO, "Waiting for a device\n");
+      printf("Waiting for a device\n");
+      sleep(5);
+    }
+    snprintf(folder, sizeof(folder), "/media/%s1", dev_name);
+    while (!FoundThumbDrive(folder)) {
+      LOG(INFO, "Waiting for %s\n", folder);
+      printf("Waiting for %s\n", folder);
+      sleep(1);
+    }
+    snprintf(folder, sizeof(folder), "/media/%s1/", dev_name);
+  }
+
+  if (access(folder, F_OK) == -1) {
+#else
+  const char *folder = configuration::GetLoggingDirectory();
+  if (access(folder, R_OK | W_OK) == -1) {
+#endif
+    LOG(FATAL, "folder '%s' does not exist. please create it\n", folder);
+  }
+  LOG(INFO, "logging to folder '%s'\n", folder);
+
+  char *tmp;
+  AllocateLogName(&tmp, folder);
+  char *tmp2;
+  if (asprintf(&tmp2, "%s/aos_log-current", folder) == -1) {
+    PLOG(WARNING, "couldn't create current symlink name");
+  } else {
+    if (unlink(tmp2) == -1 && (errno != EROFS && errno != ENOENT)) {
+      LOG(WARNING, "unlink('%s') failed", tmp2);
+    }
+    if (symlink(tmp, tmp2) == -1) {
+      PLOG(WARNING, "symlink('%s', '%s') failed", tmp, tmp2);
+    }
+    free(tmp2);
+  }
+  int fd = open(tmp, O_SYNC | O_APPEND | O_RDWR | O_CREAT,
+                S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);
+  free(tmp);
+  if (fd == -1) {
+    PLOG(FATAL, "opening file '%s' failed", tmp);
+  }
+  LogFileWriter writer(fd);
+
+  ::std::unordered_set<uint32_t> written_type_ids;
+  off_t clear_type_ids_cookie = 0;
+
+  while (true) {
+    const LogMessage *const msg = ReadNext();
+    if (msg == NULL) continue;
+
+    const size_t raw_output_length =
+        sizeof(LogFileMessageHeader) + msg->name_length + msg->message_length;
+    size_t output_length = raw_output_length;
+    if (msg->type == LogMessage::Type::kStruct) {
+      output_length += sizeof(msg->structure.type_id) + sizeof(uint32_t) +
+                       msg->structure.string_length;
+      if (writer.ShouldClearSeekableData(&clear_type_ids_cookie,
+                                         output_length)) {
+        writer.ForceNewPage();
+        written_type_ids.clear();
+      }
+      CheckTypeWritten(msg->structure.type_id, &writer, &written_type_ids);
+    } else if (msg->type == LogMessage::Type::kMatrix) {
+      output_length +=
+          sizeof(msg->matrix.type) + sizeof(uint32_t) + sizeof(uint16_t) +
+          sizeof(uint16_t) + msg->matrix.string_length;
+      CHECK(MessageType::IsPrimitive(msg->matrix.type));
+    }
+    LogFileMessageHeader *const output = writer.GetWritePosition(output_length);
+    char *output_strings = reinterpret_cast<char *>(output) + sizeof(*output);
+    output->name_size = msg->name_length;
+    output->message_size = msg->message_length;
+    output->source = msg->source;
+    output->level = msg->level;
+    output->time_sec = msg->seconds;
+    output->time_nsec = msg->nseconds;
+    output->sequence = msg->sequence;
+    memcpy(output_strings, msg->name, msg->name_length);
+
+    switch (msg->type) {
+      case LogMessage::Type::kString:
+        memcpy(output_strings + msg->name_length, msg->message,
+               msg->message_length);
+        output->type = LogFileMessageHeader::MessageType::kString;
+        break;
+      case LogMessage::Type::kStruct: {
+        char *position = output_strings + msg->name_length;
+
+        memcpy(position, &msg->structure.type_id,
+               sizeof(msg->structure.type_id));
+        position += sizeof(msg->structure.type_id);
+        output->message_size += sizeof(msg->structure.type_id);
+
+        const uint32_t length = msg->structure.string_length;
+        memcpy(position, &length, sizeof(length));
+        position += sizeof(length);
+        memcpy(position, msg->structure.serialized,
+               length + msg->message_length);
+        position += length + msg->message_length;
+        output->message_size += sizeof(length) + length;
+
+        output->type = LogFileMessageHeader::MessageType::kStruct;
+      } break;
+      case LogMessage::Type::kMatrix: {
+        char *position = output_strings + msg->name_length;
+
+        memcpy(position, &msg->matrix.type, sizeof(msg->matrix.type));
+        position += sizeof(msg->matrix.type);
+        output->message_size += sizeof(msg->matrix.type);
+
+        uint32_t length = msg->matrix.string_length;
+        memcpy(position, &length, sizeof(length));
+        position += sizeof(length);
+        output->message_size += sizeof(length);
+
+        uint16_t rows = msg->matrix.rows, cols = msg->matrix.cols;
+        memcpy(position, &rows, sizeof(rows));
+        position += sizeof(rows);
+        memcpy(position, &cols, sizeof(cols));
+        position += sizeof(cols);
+        output->message_size += sizeof(rows) + sizeof(cols);
+        CHECK_EQ(msg->message_length,
+                 MessageType::Sizeof(msg->matrix.type) * rows * cols);
+
+        memcpy(position, msg->matrix.data, msg->message_length + length);
+        output->message_size += length;
+
+        output->type = LogFileMessageHeader::MessageType::kMatrix;
+      } break;
+    }
+
+    if (output->message_size - msg->message_length !=
+        output_length - raw_output_length) {
+      LOG(FATAL, "%zu != %zu\n", output->message_size - msg->message_length,
+          output_length - raw_output_length);
+    }
+
+    futex_set(&output->marker);
+
+    logging::linux_code::Free(msg);
+  }
+
+  Cleanup();
+  return 0;
+}
+
+}  // namespace
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
+
+int main() {
+  return ::aos::logging::linux_code::BinaryLogReaderMain();
+}
diff --git a/aos/common/logging/linux_interface.cc b/aos/common/logging/linux_interface.cc
new file mode 100644
index 0000000..dff48e1
--- /dev/null
+++ b/aos/common/logging/linux_interface.cc
@@ -0,0 +1,89 @@
+#include "aos/common/logging/context.h"
+
+#include <string>
+#include <string.h>
+#include <sys/prctl.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "aos/linux_code/complex_thread_local.h"
+#include "aos/common/die.h"
+
+namespace aos {
+namespace logging {
+namespace internal {
+namespace {
+
+// TODO(brians): Differentiate between threads with the same name in the same
+// process.
+
+::std::string GetMyName() {
+  // The maximum number of characters that can make up a thread name.
+  // The docs are unclear if it can be 16 characters with no '\0', so we'll be
+  // safe by adding our own where necessary.
+  static const size_t kThreadNameLength = 16;
+
+  ::std::string process_name(program_invocation_short_name);
+
+  char thread_name_array[kThreadNameLength + 1];
+  if (prctl(PR_GET_NAME, thread_name_array) != 0) {
+    PDie("prctl(PR_GET_NAME, %p) failed", thread_name_array);
+  }
+  thread_name_array[sizeof(thread_name_array) - 1] = '\0';
+  ::std::string thread_name(thread_name_array);
+
+  // If the first bunch of characters are the same.
+  // We cut off comparing at the shorter of the 2 strings because one or the
+  // other often ends up cut off.
+  if (strncmp(thread_name.c_str(), process_name.c_str(),
+              ::std::min(thread_name.length(), process_name.length())) == 0) {
+    // This thread doesn't have an actual name.
+    return process_name;
+  }
+
+  return process_name + '.' + thread_name;
+}
+
+::aos::ComplexThreadLocal<Context> my_context;
+
+// True if we're going to delete the current Context object ASAP. The
+// reason for doing this instead of just deleting them is that tsan (at least)
+// doesn't like it when pthread_atfork handlers do complicated stuff and it's
+// not a great idea anyways.
+thread_local bool delete_current_context(false);
+
+}  // namespace
+
+// Used in aos/linux_code/init.cc when a thread's name is changed.
+void ReloadThreadName() {
+  if (my_context.created()) {
+    ::std::string my_name = GetMyName();
+    if (my_name.size() + 1 > sizeof(Context::name)) {
+      Die("logging: process/thread name '%s' is too long\n",
+          my_name.c_str());
+    }
+    strcpy(my_context->name, my_name.c_str());
+    my_context->name_size = my_name.size();
+  }
+}
+
+Context *Context::Get() {
+  if (__builtin_expect(delete_current_context, false)) {
+    my_context.Clear();
+    delete_current_context = false;
+  }
+  if (__builtin_expect(!my_context.created(), false)) {
+    my_context.Create();
+    ReloadThreadName();
+    my_context->source = getpid();
+  }
+  return my_context.get();
+}
+
+void Context::Delete() {
+  delete_current_context = true;
+}
+
+}  // namespace internal
+}  // namespace logging
+}  // namespace aos
diff --git a/aos/common/logging/linux_logging.cc b/aos/common/logging/linux_logging.cc
new file mode 100644
index 0000000..14ce72b
--- /dev/null
+++ b/aos/common/logging/linux_logging.cc
@@ -0,0 +1,145 @@
+#include "aos/common/logging/linux_logging.h"
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+#include <time.h>
+#include <sys/types.h>
+#include <errno.h>
+#include <unistd.h>
+#include <limits.h>
+#include <inttypes.h>
+
+#include <algorithm>
+
+#include "aos/common/die.h"
+#include "aos/common/logging/logging_interface.h"
+#include "aos/linux_code/ipc_lib/queue.h"
+#include "aos/common/time.h"
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+namespace {
+
+RawQueue *queue = NULL;
+
+int dropped_messages = 0;
+::aos::time::Time dropped_start, backoff_start;
+// Wait this long after dropping a message before even trying to write any more.
+constexpr ::aos::time::Time kDropBackoff = ::aos::time::Time::InSeconds(0.1);
+
+LogMessage *GetMessageOrDie() {
+  LogMessage *message = static_cast<LogMessage *>(queue->GetMessage());
+  if (message == NULL) {
+    LOG(FATAL, "%p->GetMessage() failed\n", queue);
+  } else {
+    return message;
+  }
+}
+
+class LinuxQueueLogImplementation : public LogImplementation {
+  __attribute__((format(GOOD_PRINTF_FORMAT_TYPE, 3, 0)))
+  virtual void DoLog(log_level level, const char *format, va_list ap) override {
+    LogMessage *message = GetMessageOrDie();
+    internal::FillInMessage(level, format, ap, message);
+    Write(message);
+  }
+
+  virtual void LogStruct(log_level level, const ::std::string &message_string,
+                         size_t size, const MessageType *type,
+                         const ::std::function<size_t(char *)> &serialize)
+      override {
+    LogMessage *message = GetMessageOrDie();
+    internal::FillInMessageStructure(level, message_string, size, type,
+                                     serialize, message);
+    Write(message);
+  }
+
+  virtual void LogMatrix(log_level level, const ::std::string &message_string,
+                         uint32_t type_id, int rows, int cols, const void *data)
+      override {
+    LogMessage *message = GetMessageOrDie();
+    internal::FillInMessageMatrix(level, message_string, type_id, rows, cols,
+                                  data, message);
+    Write(message);
+  }
+};
+
+}  // namespace
+
+void Register() {
+  Init();
+
+  queue = RawQueue::Fetch("LoggingQueue", sizeof(LogMessage), 1323, 40000);
+  if (queue == NULL) {
+    Die("logging: couldn't fetch queue\n");
+  }
+
+  AddImplementation(new LinuxQueueLogImplementation());
+}
+
+const LogMessage *ReadNext(Options<RawQueue> flags, int *index) {
+  return static_cast<const LogMessage *>(queue->ReadMessageIndex(flags, index));
+}
+
+const LogMessage *ReadNext() {
+  return ReadNext(RawQueue::kBlock);
+}
+
+const LogMessage *ReadNext(Options<RawQueue> flags) {
+  const LogMessage *r = NULL;
+  do {
+    r = static_cast<const LogMessage *>(queue->ReadMessage(flags));
+    // not blocking means return a NULL if that's what it gets
+  } while ((flags & RawQueue::kBlock) && r == NULL);
+  return r;
+}
+
+LogMessage *Get() {
+  return static_cast<LogMessage *>(queue->GetMessage());
+}
+
+void Free(const LogMessage *msg) {
+  queue->FreeMessage(msg);
+}
+
+void Write(LogMessage *msg) {
+  if (__builtin_expect(dropped_messages > 0, false)) {
+    ::aos::time::Time message_time =
+        ::aos::time::Time(msg->seconds, msg->nseconds);
+    if (message_time - backoff_start < kDropBackoff) {
+      ++dropped_messages;
+      queue->FreeMessage(msg);
+      return;
+    }
+
+    LogMessage *dropped_message = GetMessageOrDie();
+    internal::FillInMessageVarargs(
+        ERROR, dropped_message,
+        "%d logs starting at %" PRId32 ".%" PRId32 " dropped\n",
+        dropped_messages, dropped_start.sec(), dropped_start.nsec());
+    if (queue->WriteMessage(dropped_message, RawQueue::kNonBlock)) {
+      dropped_messages = 0;
+    } else {
+      // Don't even bother trying to write this message because it's not likely
+      // to work and it would be confusing to have one log in the middle of a
+      // string of failures get through.
+      ++dropped_messages;
+      backoff_start = message_time;
+      queue->FreeMessage(msg);
+      return;
+    }
+  }
+  if (!queue->WriteMessage(msg, RawQueue::kNonBlock)) {
+    if (dropped_messages == 0) {
+      dropped_start = backoff_start =
+          ::aos::time::Time(msg->seconds, msg->nseconds);
+    }
+    ++dropped_messages;
+  }
+}
+
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
diff --git a/aos/common/logging/linux_logging.h b/aos/common/logging/linux_logging.h
new file mode 100644
index 0000000..3f74fa8
--- /dev/null
+++ b/aos/common/logging/linux_logging.h
@@ -0,0 +1,34 @@
+#ifndef AOS_COMMON_LOGGING_LINUX_LOGGING_H_
+#define AOS_COMMON_LOGGING_LINUX_LOGGING_H_
+
+#include "aos/common/logging/logging_impl.h"
+#include "aos/common/util/options.h"
+
+namespace aos {
+
+class RawQueue;
+
+namespace logging {
+namespace linux_code {
+
+// Calls AddImplementation to register the usual linux logging implementation
+// which sends the messages through a queue. This implementation relies on
+// another process(es) to read the log messages that it puts into the queue.
+// This function is usually called by aos::Init*.
+void Register();
+
+// Fairly simple wrappers around the raw queue calls.
+
+// This one never returns NULL if flags contains BLOCK.
+const LogMessage *ReadNext(Options<RawQueue> flags);
+const LogMessage *ReadNext(Options<RawQueue> flags, int *index);
+const LogMessage *ReadNext();
+LogMessage *Get();
+void Free(const LogMessage *msg);
+void Write(LogMessage *msg);
+
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
+
+#endif  // AOS_COMMON_LOGGING_LINUX_LOGGING_H_
diff --git a/aos/common/logging/log_displayer.cc b/aos/common/logging/log_displayer.cc
new file mode 100644
index 0000000..70d2ad5
--- /dev/null
+++ b/aos/common/logging/log_displayer.cc
@@ -0,0 +1,410 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <getopt.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <inttypes.h>
+
+#include <algorithm>
+#include <memory>
+#include <string>
+
+#include "aos/linux_code/configuration.h"
+#include "aos/common/logging/binary_log_file.h"
+#include "aos/common/queue_types.h"
+#include "aos/common/logging/logging_impl.h"
+#include "aos/common/logging/logging_printf_formats.h"
+#include "aos/common/util/string_to_num.h"
+
+using ::aos::logging::linux_code::LogFileMessageHeader;
+
+namespace {
+
+const char *kArgsHelp = "[OPTION]... [FILE]\n"
+    "Display log file FILE (created by BinaryLogReader) to stdout.\n"
+    "FILE is \"aos_log-current\" by default.\n"
+    "FILE can also be \"-\" to read from standard input.\n"
+    "\n"
+    "  -n, --name-prefix NAME  "
+    "only display entries from processes which share NAME as a prefix\n"
+    "  -N, --name NAME         only display entries from processes named NAME\n"
+    "  -l, --level LEVEL       "
+      "only display log entries at least as important as LEVEL\n"
+    "  -p, --pid PID           only display log entries from process PID\n"
+    "  -f, --follow            "
+      "wait when the end of the file is reached (implies --end)\n"
+    "  -t, --terminate         stop when the end of file is reached (default)\n"
+    "  -b, --beginning         start at the beginning of the file (default)\n"
+    "  -e, --end               start at the end of the file\n"
+    "  -s, --skip NUMBER       skip NUMBER matching logs\n"
+    "  -m, --max NUMBER        only display up to NUMBER logs\n"
+    "  // -o, --format FORMAT  use FORMAT to display log entries\n"
+    "  -h, --help              display this help and exit\n"
+    "\n"
+    "LEVEL must be DEBUG, INFO, WARNING, ERROR, or FATAL.\n"
+    "  It defaults to INFO.\n"
+    "\n"
+    "TODO(brians) implement the commented out ones.\n";
+
+const char *kExampleUsages = "To view logs from the shooter:\n"
+    "\t`log_displayer -n shooter`\n"
+    "To view debug logs from the shooter:\n"
+    "\t`log_displayer -n shooter -l DEBUG`\n"
+    "To view what the shooter is logging in realtime:\n"
+    "\t`log_displayer -f -n shooter`\n"
+    "To view shooter logs from an old log file:\n"
+    "\t`log_displayer aos_log-<number> -n shooter`\n"
+    "To view the statuses of the shooter hall effects in realtime:\n"
+    "\t`log_displayer -f -n shooter -l DEBUG | grep .Position`\n";
+
+void PrintHelpAndExit() {
+  fprintf(stderr, "Usage: %s %s", program_invocation_name, kArgsHelp);
+  fprintf(stderr, "\nExample usages:\n\n%s", kExampleUsages);
+
+  // Get the possible executables from start_list.txt.
+  FILE *start_list = fopen("start_list.txt", "r");
+  if (!start_list) {
+    ::std::string path(::aos::configuration::GetRootDirectory());
+    path += "/start_list.txt";
+    start_list = fopen(path.c_str(), "r");
+    if (!start_list) {
+      printf("\nCannot open start_list.txt. This means that the\n"
+      "possible arguments for the -n option cannot be shown. log_displayer\n"
+      "looks for start_list.txt in the current working directory and in\n"
+      "%s.\n\n", ::aos::configuration::GetRootDirectory());
+      PLOG(FATAL, "Unable to open start_list.txt");
+    }
+  }
+
+  // Get file size.
+  if (fseek(start_list, 0, SEEK_END)) {
+    PLOG(FATAL, "fseek() failed while reading start_list.txt");
+  }
+  int size = ftell(start_list);
+  if (size < 0) {
+    PLOG(FATAL, "ftell() failed while reading start_list.txt");
+  }
+  rewind(start_list);
+
+  ::std::unique_ptr<char[]> contents(new char[size + 1]);
+  if (contents == NULL) {
+    LOG(FATAL, "malloc() failed while reading start_list.txt.\n");
+  }
+  size_t bytes_read = fread(contents.get(), 1, size, start_list);
+  if (bytes_read < static_cast<size_t>(size)) {
+    LOG(FATAL, "Read %zu bytes from start_list.txt, expected %d.\n",
+        bytes_read, size);
+  }
+
+  // printf doesn't like strings without the \0.
+  contents[size] = '\0';
+  fprintf(stderr, "\nPossible arguments for the -n option:\n%s", contents.get());
+
+  if (fclose(start_list)) {
+    LOG(FATAL, "fclose() failed.\n");
+  }
+
+  exit(EXIT_SUCCESS);
+}
+
+}  // namespace
+
+int main(int argc, char **argv) {
+  const char *filter_name = nullptr, *filter_exact_name = nullptr;
+  size_t filter_length = 0;
+  log_level filter_level = INFO;
+  bool follow = false;
+  // Whether we need to skip everything until we get to the end of the file.
+  bool skip_to_end = false;
+  const char *filename = "aos_log-current";
+  int display_max = 0;
+  int32_t source_pid = -1;
+
+  ::aos::logging::Init();
+  ::aos::logging::AddImplementation(
+      new ::aos::logging::StreamLogImplementation(stdout));
+
+  while (true) {
+    static struct option long_options[] = {
+      {"name-prefix", required_argument, NULL, 'n'},
+      {"name", required_argument, NULL, 'N'},
+      {"level", required_argument, NULL, 'l'},
+      {"pid", required_argument, NULL, 'p'},
+
+      {"follow", no_argument, NULL, 'f'},
+      {"terminate", no_argument, NULL, 't'},
+      {"beginning", no_argument, NULL, 'b'},
+      {"end", no_argument, NULL, 'e'},
+      {"skip", required_argument, NULL, 's'},
+      {"max", required_argument, NULL, 'm'},
+
+      {"format", required_argument, NULL, 'o'},
+
+      {"help", no_argument, NULL, 'h'},
+      {0, 0, 0, 0}
+    };
+    int option_index = 0;
+
+    const int c = getopt_long(argc, argv, "N:n:l:p:fts:m:o:h",
+                    long_options, &option_index);
+    if (c == -1) { // if we're at the end
+      break;
+    }
+    switch (c) {
+      case 0:
+        fputs("LogDisplayer: got a 0 option but didn't set up any\n", stderr);
+        abort();
+      case 'n':
+        filter_name = optarg;
+        filter_exact_name = nullptr;
+        filter_length = strlen(filter_name);
+        break;
+      case 'N':
+        filter_exact_name = optarg;
+        filter_name = nullptr;
+        filter_length = strlen(filter_name);
+        break;
+      case 'l':
+        filter_level = ::aos::logging::str_log(optarg);
+        if (filter_level == LOG_UNKNOWN) {
+          fprintf(stderr, "LogDisplayer: unknown log level '%s'\n", optarg);
+          exit(EXIT_FAILURE);
+        }
+        break;
+      case 'p':
+        if (!::aos::util::StringToNumber(::std::string(optarg), &source_pid)) {
+          fprintf(stderr, "ERROR: -p expects a number, not '%s'.\n", optarg);
+          exit(EXIT_FAILURE);
+        }
+        if (source_pid < 0) {
+          fprintf(stderr, "LogDisplayer: invalid pid '%s'\n", optarg);
+          exit(EXIT_FAILURE);
+        }
+        break;
+      case 'f':
+        follow = true;
+        skip_to_end = true;
+        break;
+      case 't':
+        follow = false;
+        break;
+      case 'b':
+        skip_to_end = false;
+        break;
+      case 'e':
+        skip_to_end = true;
+        break;
+      case 'm':
+        if (!::aos::util::StringToNumber(::std::string(optarg), &display_max)) {
+          fprintf(stderr, "ERROR: -m expects a number, not '%s'.\n", optarg);
+          exit(EXIT_FAILURE);
+        }
+        if (display_max <= 0) {
+          fprintf(stderr, "LogDisplayer: invalid max log number '%s'\n",
+              optarg);
+          exit(EXIT_FAILURE);
+        }
+        break;
+      case 'o':
+        abort();
+        break;
+      case 'h':
+        PrintHelpAndExit();
+        break;
+      case '?':
+        break;
+      default:
+        fprintf(stderr, "LogDisplayer: in a bad spot (%s: %d)\n",
+                __FILE__, __LINE__);
+        abort();
+    }
+  }
+
+  if (optind < argc) {
+    // We got a filename.
+    filename = argv[optind++];
+  }
+  if (optind < argc) {
+    fputs("non-option ARGV-elements: ", stderr);
+    while (optind < argc) {
+      fprintf(stderr, "%s\n", argv[optind++]);
+    }
+  }
+
+  int fd;
+  if (strcmp(filename, "-") == 0) {
+    if (skip_to_end) {
+      fputs("Can't skip to end of stdin!\n", stderr);
+      return EXIT_FAILURE;
+    }
+    fd = STDIN_FILENO;
+  } else {
+    fd = open(filename, O_RDONLY);
+  }
+
+  fprintf(stderr, "displaying down to level %s from file '%s'\n",
+      ::aos::logging::log_str(filter_level), filename);
+
+  if (fd == -1) {
+    PLOG(FATAL, "couldn't open file '%s' for reading", filename);
+  }
+  ::aos::logging::linux_code::LogFileReader reader(fd);
+
+  if (skip_to_end) {
+    fputs("skipping old logs...\n", stderr);
+    reader.SkipToLastSeekablePage();
+  }
+
+  const LogFileMessageHeader *msg;
+  int displayed = 0;
+  do {
+    msg = reader.ReadNextMessage(follow);
+    if (msg == NULL) {
+      fputs("reached end of file\n", stderr);
+      return 0;
+    }
+
+    if (msg->type == LogFileMessageHeader::MessageType::kStructType) {
+      size_t bytes = msg->message_size;
+      ::aos::MessageType *type = ::aos::MessageType::Deserialize(
+          reinterpret_cast<const char *>(msg + 1), &bytes);
+      if (type == nullptr) {
+        LOG(INFO, "Trying old version of type decoding.\n");
+        bytes = msg->message_size;
+        type = ::aos::MessageType::Deserialize(
+            reinterpret_cast<const char *>(msg + 1), &bytes, false);
+      }
+
+      if (type == nullptr) {
+        LOG(WARNING, "Error deserializing MessageType of size %" PRIx32
+                     " starting at %zx.\n",
+            msg->message_size, reader.file_offset(msg + 1));
+      } else {
+        ::aos::type_cache::Add(*type);
+      }
+      continue;
+    }
+
+    if (source_pid >= 0 && msg->source != source_pid) {
+      // Message is from the wrong process.
+      continue;
+    }
+
+    if (skip_to_end) {
+      if (reader.IsLastPage()) {
+        fputs("done skipping old logs\n", stderr);
+        skip_to_end = false;
+      } else {
+        continue;
+      }
+    }
+
+    if (::aos::logging::log_gt_important(filter_level, msg->level)) continue;
+
+    const char *position =
+        reinterpret_cast<const char *>(msg + 1);
+
+    if (filter_name != nullptr) {
+      const size_t compare_length =
+          ::std::min<size_t>(filter_length, msg->name_size);
+      if (memcmp(filter_name, position, compare_length) != 0) {
+        continue;
+      }
+      if (compare_length < msg->name_size) {
+        if (position[compare_length] != '.') continue;
+      }
+    }
+
+    if (filter_exact_name != nullptr) {
+      if (filter_length != msg->name_size) continue;
+      if (memcmp(filter_exact_name, position, filter_length) != 0) {
+        continue;
+      }
+    }
+
+    if (display_max && displayed++ >= display_max) {
+      fputs("Not displaying the rest of the messages.\n", stderr);
+      return 0;
+    }
+
+    position += msg->name_size;
+
+#define BASE_ARGS                                                           \
+  AOS_LOGGING_BASE_ARGS(                                                    \
+      msg->name_size, reinterpret_cast<const char *>(msg + 1), msg->source, \
+      msg->sequence, msg->level, msg->time_sec, msg->time_nsec)
+    switch (msg->type) {
+      case LogFileMessageHeader::MessageType::kString:
+        fprintf(stdout, AOS_LOGGING_BASE_FORMAT "%.*s", BASE_ARGS,
+                static_cast<int>(msg->message_size), position);
+        break;
+      case LogFileMessageHeader::MessageType::kStruct: {
+        uint32_t type_id;
+        memcpy(&type_id, position, sizeof(type_id));
+        position += sizeof(type_id);
+
+        uint32_t string_length;
+        memcpy(&string_length, position, sizeof(string_length));
+        position += sizeof(string_length);
+
+        char buffer[2048];
+        size_t output_length = sizeof(buffer);
+        size_t input_length =
+            msg->message_size -
+            (sizeof(type_id) + sizeof(uint32_t) + string_length);
+        if (!PrintMessage(buffer, &output_length, position + string_length,
+                          &input_length, ::aos::type_cache::Get(type_id))) {
+          LOG(FATAL, "printing message (%.*s) of type %s into %zu-byte buffer "
+                     "failed\n",
+              static_cast<int>(string_length), position,
+              ::aos::type_cache::Get(type_id).name.c_str(), sizeof(buffer));
+        }
+        if (input_length > 0) {
+          LOG(WARNING, "%zu extra bytes on message of type %s\n",
+              input_length, ::aos::type_cache::Get(type_id).name.c_str());
+        }
+        fprintf(stdout, AOS_LOGGING_BASE_FORMAT "%.*s: %.*s\n", BASE_ARGS,
+                static_cast<int>(string_length), position,
+                static_cast<int>(sizeof(buffer) - output_length), buffer);
+      } break;
+      case LogFileMessageHeader::MessageType::kMatrix: {
+        uint32_t type;
+        memcpy(&type, position, sizeof(type));
+        position += sizeof(type);
+
+        uint32_t string_length;
+        memcpy(&string_length, position, sizeof(string_length));
+        position += sizeof(string_length);
+
+        uint16_t rows;
+        memcpy(&rows, position, sizeof(rows));
+        position += sizeof(rows);
+        uint16_t cols;
+        memcpy(&cols, position, sizeof(cols));
+        position += sizeof(cols);
+
+        const size_t matrix_bytes =
+            msg->message_size -
+            (sizeof(type) + sizeof(uint32_t) + sizeof(uint16_t) +
+             sizeof(uint16_t) + string_length);
+        CHECK_EQ(matrix_bytes, ::aos::MessageType::Sizeof(type) * rows * cols);
+        char buffer[2048];
+        size_t output_length = sizeof(buffer);
+        if (!::aos::PrintMatrix(buffer, &output_length,
+                                position + string_length, type, rows, cols)) {
+          LOG(FATAL, "printing %dx%d matrix of type %" PRIu32 " failed\n", rows,
+              cols, type);
+        }
+        fprintf(stdout, AOS_LOGGING_BASE_FORMAT "%.*s: %.*s\n", BASE_ARGS,
+                static_cast<int>(string_length), position,
+                static_cast<int>(sizeof(buffer) - output_length), buffer);
+      } break;
+      case LogFileMessageHeader::MessageType::kStructType:
+        LOG(FATAL, "shouldn't get here\n");
+        break;
+    }
+#undef BASE_ARGS
+  } while (msg != NULL);
+}
diff --git a/aos/common/logging/log_replay.cc b/aos/common/logging/log_replay.cc
new file mode 100644
index 0000000..a1efcca
--- /dev/null
+++ b/aos/common/logging/log_replay.cc
@@ -0,0 +1,44 @@
+#include "aos/common/logging/log_replay.h"
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+
+bool LogReplayer::ProcessMessage() {
+  const LogFileMessageHeader *message = reader_->ReadNextMessage(false);
+  if (message == nullptr) return true;
+  if (message->type != LogFileMessageHeader::MessageType::kStruct) return false;
+
+  const char *position = reinterpret_cast<const char *>(message + 1);
+
+  ::std::string process(position, message->name_size);
+  position += message->name_size;
+
+  uint32_t type_id;
+  memcpy(&type_id, position, sizeof(type_id));
+  position += sizeof(type_id);
+
+  uint32_t message_length;
+  memcpy(&message_length, position, sizeof(message_length));
+  position += sizeof(message_length);
+  ::std::string message_text(position, message_length);
+  position += message_length;
+
+  size_t split_index = message_text.find_first_of(':') + 2;
+  split_index = message_text.find_first_of(':', split_index) + 2;
+  message_text = message_text.substr(split_index);
+
+  auto handler = handlers_.find(Key(process, message_text));
+  if (handler == handlers_.end()) return false;
+
+  handler->second->HandleStruct(
+      ::aos::time::Time(message->time_sec, message->time_nsec), type_id,
+      position,
+      message->message_size -
+          (sizeof(type_id) + sizeof(message_length) + message_length));
+  return false;
+}
+
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
diff --git a/aos/common/logging/log_replay.h b/aos/common/logging/log_replay.h
new file mode 100644
index 0000000..6fb230c
--- /dev/null
+++ b/aos/common/logging/log_replay.h
@@ -0,0 +1,164 @@
+#ifndef AOS_COMMON_LOGGING_LOG_REPLAY_H_
+#define AOS_COMMON_LOGGING_LOG_REPLAY_H_
+
+#include <unordered_map>
+#include <string>
+#include <functional>
+#include <memory>
+
+#include "aos/common/logging/binary_log_file.h"
+#include "aos/common/queue.h"
+#include "aos/common/logging/logging.h"
+#include "aos/common/macros.h"
+#include "aos/linux_code/ipc_lib/queue.h"
+#include "aos/common/queue_types.h"
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+
+// Manages pulling logged queue messages out of log files.
+//
+// Basic usage:
+//   - Use the Add* methods to register handlers for various message sources.
+//   - Call OpenFile to open a log file.
+//   - Call ProcessMessage repeatedly until it returns true.
+//
+// This code could do something to adapt similar-but-not-identical
+// messages to the current versions, but currently it will LOG(FATAL) if any of
+// the messages don't match up exactly.
+class LogReplayer {
+ public:
+  LogReplayer() {}
+
+  // Gets ready to read messages from fd.
+  // Does not take ownership of fd.
+  void OpenFile(int fd) {
+    reader_.reset(new LogFileReader(fd));
+  }
+  // Closes the currently open file.
+  void CloseCurrentFile() { reader_.reset(); }
+  // Returns true if we have a file which is currently open.
+  bool HasCurrentFile() const { return reader_.get() != nullptr; }
+
+  // Processes a single message from the currently open file.
+  // Returns true if there are no more messages in the file.
+  // This will not call any of the handlers if the next message either has no
+  // registered handlers or is not a queue message.
+  bool ProcessMessage();
+
+  // Adds a handler for messages with a certain string from a certain process.
+  // T must be a Message with the same format as the messages generated by
+  // the .q files.
+  // LOG(FATAL)s for duplicate handlers.
+  template <class T>
+  void AddHandler(const ::std::string &process_name,
+                  const ::std::string &log_message,
+                  ::std::function<void(const T &message)> handler) {
+    CHECK(handlers_.emplace(
+        ::std::piecewise_construct,
+        ::std::forward_as_tuple(process_name, log_message),
+        ::std::forward_as_tuple(::std::unique_ptr<StructHandlerInterface>(
+            new TypedStructHandler<T>(handler)))).second);
+  }
+
+  // Adds a handler which takes messages and places them directly on a queue.
+  // T must be a Message with the same format as the messages generated by
+  // the .q files.
+  template <class T>
+  void AddDirectQueueSender(const ::std::string &process_name,
+                            const ::std::string &log_message,
+                            const ::aos::Queue<T> &queue) {
+    AddHandler(process_name, log_message,
+               ::std::function<void(const T &)>(
+                   QueueDumpStructHandler<T>(queue.name())));
+  }
+
+ private:
+  // A generic handler of struct log messages.
+  class StructHandlerInterface {
+   public:
+    virtual ~StructHandlerInterface() {}
+
+    virtual void HandleStruct(::aos::time::Time log_time, uint32_t type_id,
+                              const void *data, size_t data_size) = 0;
+  };
+
+  // Converts struct log messages to a message type and passes it to an
+  // ::std::function.
+  template <class T>
+  class TypedStructHandler : public StructHandlerInterface {
+   public:
+    TypedStructHandler(::std::function<void(const T &message)> handler)
+        : handler_(handler) {}
+
+    void HandleStruct(::aos::time::Time log_time, uint32_t type_id,
+                      const void *data, size_t data_size) override {
+      CHECK_EQ(type_id, T::GetType()->id);
+      T message;
+      CHECK_EQ(data_size, T::Size());
+      CHECK_EQ(data_size, message.Deserialize(static_cast<const char *>(data)));
+      message.sent_time = log_time;
+      handler_(message);
+    }
+
+   private:
+    const ::std::function<void(T message)> handler_;
+  };
+
+  // A callable class which dumps messages straight to a queue.
+  template <class T>
+  class QueueDumpStructHandler {
+   public:
+    QueueDumpStructHandler(const ::std::string &queue_name)
+        : queue_(RawQueue::Fetch(queue_name.c_str(), sizeof(T), T::kHash,
+                                 T::kQueueLength)) {}
+
+    void operator()(const T &message) {
+      LOG_STRUCT(DEBUG, "re-sending", message);
+      void *raw_message = queue_->GetMessage();
+      CHECK_NOTNULL(raw_message);
+      memcpy(raw_message, &message, sizeof(message));
+      CHECK(queue_->WriteMessage(raw_message, RawQueue::kOverride));
+    }
+
+   private:
+    ::aos::RawQueue *const queue_;
+  };
+
+  // A key for specifying log messages to give to a certain handler.
+  struct Key {
+    Key(const ::std::string &process_name, const ::std::string &log_message)
+        : process_name(process_name), log_message(log_message) {}
+
+    ::std::string process_name;
+    ::std::string log_message;
+  };
+
+  struct KeyHash {
+    size_t operator()(const Key &key) const {
+      return string_hash(key.process_name) ^
+             (string_hash(key.log_message) << 1);
+    }
+
+   private:
+    const ::std::hash<::std::string> string_hash = ::std::hash<::std::string>();
+  };
+  struct KeyEqual {
+    bool operator()(const Key &a, const Key &b) const {
+      return a.process_name == b.process_name && a.log_message == b.log_message;
+    }
+  };
+
+  ::std::unordered_map<const Key, ::std::unique_ptr<StructHandlerInterface>,
+                       KeyHash, KeyEqual> handlers_;
+  ::std::unique_ptr<LogFileReader> reader_;
+
+  DISALLOW_COPY_AND_ASSIGN(LogReplayer);
+};
+
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
+
+#endif  // AOS_COMMON_LOGGING_LOG_REPLAY_H_
diff --git a/aos/common/logging/log_streamer.cc b/aos/common/logging/log_streamer.cc
new file mode 100644
index 0000000..506a92b
--- /dev/null
+++ b/aos/common/logging/log_streamer.cc
@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <time.h>
+#include <string.h>
+#include <string>
+#include <unistd.h>
+#include <sys/types.h>
+#include <pwd.h>
+#include <fcntl.h>
+#include <inttypes.h>
+
+#include "aos/common/logging/linux_logging.h"
+#include "aos/linux_code/init.h"
+#include "aos/linux_code/ipc_lib/queue.h"
+#include "aos/common/logging/logging_impl.h"
+#include "aos/common/time.h"
+
+namespace aos {
+namespace logging {
+namespace linux_code {
+namespace {
+
+int LogStreamerMain() {
+  InitNRT();
+
+  const time::Time now = time::Time::Now();
+  printf("starting at %" PRId32 "s%" PRId32 "ns-----------------------------\n",
+         now.sec(), now.nsec());
+
+  while (true) {
+    const LogMessage *const msg = ReadNext();
+    if (msg == NULL) continue;
+
+    internal::PrintMessage(stdout, *msg);
+
+    logging::linux_code::Free(msg);
+  }
+
+  Cleanup();
+  return 0;
+}
+
+}  // namespace
+}  // namespace linux_code
+}  // namespace logging
+}  // namespace aos
+
+int main() {
+  return ::aos::logging::linux_code::LogStreamerMain();
+}