Add support for editing the logfile headers

This lets us fix bugs in logfiles if the need arises.

Change-Id: I374c4f6b3745835af2279d4b29f6f8098a5d7770
diff --git a/aos/events/logging/log_edit.cc b/aos/events/logging/log_edit.cc
new file mode 100644
index 0000000..5db503a
--- /dev/null
+++ b/aos/events/logging/log_edit.cc
@@ -0,0 +1,62 @@
+#include <iostream>
+
+#include "aos/configuration.h"
+#include "aos/events/logging/logger.h"
+#include "aos/events/simulated_event_loop.h"
+#include "aos/init.h"
+#include "aos/json_to_flatbuffer.h"
+#include "aos/util/file.h"
+#include "gflags/gflags.h"
+
+DEFINE_string(logfile, "/tmp/logfile.bfbs",
+              "Name of the logfile to read from.");
+DEFINE_bool(
+    replace, false,
+    "If true, replace the header on the log file with the JSON header.");
+DEFINE_string(
+    header, "",
+    "If provided, this is the path to the JSON with the log file header.");
+
+int main(int argc, char **argv) {
+  gflags::SetUsageMessage(
+      R"(This tool lets us manipulate log files.)");
+  aos::InitGoogle(&argc, &argv);
+
+  if (!FLAGS_header.empty()) {
+    if (FLAGS_replace) {
+      const ::std::string header_json =
+          aos::util::ReadFileToStringOrDie(FLAGS_header);
+      flatbuffers::FlatBufferBuilder fbb;
+      fbb.ForceDefaults(1);
+      flatbuffers::Offset<aos::logger::LogFileHeader> header =
+          aos::JsonToFlatbuffer<aos::logger::LogFileHeader>(header_json, &fbb);
+
+      fbb.FinishSizePrefixed(header);
+
+      const std::string orig_path = FLAGS_logfile + ".orig";
+      PCHECK(rename(FLAGS_logfile.c_str(), orig_path.c_str()) == 0);
+
+      aos::logger::SpanReader span_reader(orig_path);
+      CHECK(span_reader.ReadMessage().empty());
+
+      aos::logger::DetachedBufferWriter buffer_writer(FLAGS_logfile);
+      buffer_writer.QueueSizedFlatbuffer(&fbb);
+
+      while (true) {
+        absl::Span<const uint8_t> msg_data = span_reader.ReadMessage();
+        if (msg_data == absl::Span<const uint8_t>()) {
+          break;
+        }
+
+        buffer_writer.WriteSizedFlatbuffer(msg_data);
+      }
+    } else {
+      aos::logger::MessageReader reader(FLAGS_logfile);
+      aos::util::WriteStringToFileOrDie(
+          FLAGS_header, aos::FlatbufferToJson(reader.log_file_header(), true));
+    }
+  }
+
+  aos::Cleanup();
+  return 0;
+}