Add FileReader flag to decide if errors in opening file fatal/non-fatal

Flag default is fatal and will crash if errors in opening. If non-fatal,
log error instead of crashing. Added tests for the same.

Change-Id: I788416b86628055f137d0fc4a18fb62c0ca9177a
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/util/file.h b/aos/util/file.h
index 2fc4b2b..d0037fd 100644
--- a/aos/util/file.h
+++ b/aos/util/file.h
@@ -52,6 +52,8 @@
 
 enum class FileOptions { kReadable, kWriteable };
 
+enum class FileReaderErrorType { kFatal, kNonFatal };
+
 // Maps file from disk into memory
 std::shared_ptr<absl::Span<uint8_t>> MMapFile(
     const std::string &path, FileOptions options = FileOptions::kReadable);
@@ -61,7 +63,10 @@
 // but where you still want to read a file.
 class FileReader {
  public:
-  FileReader(std::string_view filename);
+  // Opens the file for reading. Crashes if file does not open when flag is set
+  // to fatal (default). Logs error if file does not open and flag is non-fatal.
+  FileReader(std::string_view filename,
+             FileReaderErrorType error_type = FileReaderErrorType::kFatal);
   // Reads the entire contents of the file into the internal buffer and returns
   // a string_view of it.  Returns nullopt if we failed to read the contents
   // (currently only on EREMOTEIO). Note: The result may not be null-terminated.
@@ -89,6 +94,9 @@
   // must be base 10.  Returns nullopt if we failed to read the file.
   std::optional<int32_t> ReadInt32();
 
+  // Checks if file could be opened.
+  bool is_open() const { return file_.get() != -1; }
+
  private:
   aos::ScopedFD file_;
 };