rust: add event file reading module

Summary:
We implement a module for reading a TensorFlow event file. This wraps
the TFRecord reading module added in #4307 and parses records as protos.
The reader also stores the last-read event wall time, so that we can
stop reading from event files after a while (cf. `--reload_multifile`).

Along the way, we add a couple utility methods to `TfRecord` that make
it easier to use records in test code.

Test Plan:
Unit tests included.

wchargin-branch: rust-event-file-reading
wchargin-source: 18183bce70f95cdef820853b67f5a0930076ba15

Author: wchargin

tensorboard/data/server/BUILD

@@ -23,6 +23,7 @@ rust_library(
     name = "rustboard_core",
     srcs = [
         "lib.rs",
+        "event_file.rs",
         "masked_crc.rs",
         "scripted_reader.rs",
         "tf_record.rs",

tensorboard/data/server/event_file.rs

@@ -0,0 +1,223 @@
+/* Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================*/
+
+//! Parsing for event files containing a stream of `Event` protos.
+
+use prost::{DecodeError, Message};
+use std::io::Read;
+
+use crate::proto::tensorboard::Event;
+use crate::tf_record::{ChecksumError, ReadRecordError, TfRecordReader};
+
+/// A reader for a stream of `Event` protos framed as TFRecords.
+///
+/// As with [`TfRecordReader`], an event may be read over one or more underlying reads, to support
+/// growing, partially flushed files.
+pub struct EventFileReader<R> {
+    /// Wall time of the record most recently read from this event file, or `None` if no records
+    /// have been read. Used for determining when to consider this file dead and abandon it.
+    last_wall_time: Option<f64>,
+    /// Underlying record reader owned by this event file.
+    reader: TfRecordReader<R>,
+}
+
+/// Error returned by [`EventFileReader::read_event`].
+#[derive(Debug)]
+pub enum ReadEventError {
+    /// The record failed its checksum. This may only be detected if the protocol buffer fails to
+    /// decode: records with bad checksum that still happen to parse as valid protos may be
+    /// returned silently.
+    InvalidRecord(ChecksumError),
+    /// The record passed its checksum, but the contained protocol buffer is invalid.
+    InvalidProto(DecodeError),
+    /// The record is a valid `Event` proto, but its `wall_time` is `NaN`.
+    NanWallTime(Event),
+    /// An error occurred reading the record. May or may not be fatal.
+    ReadRecordError(ReadRecordError),
+}
+
+impl From<DecodeError> for ReadEventError {
+    fn from(e: DecodeError) -> Self {
+        ReadEventError::InvalidProto(e)
+    }
+}
+
+impl From<ChecksumError> for ReadEventError {
+    fn from(e: ChecksumError) -> Self {
+        ReadEventError::InvalidRecord(e)
+    }
+}
+
+impl From<ReadRecordError> for ReadEventError {
+    fn from(e: ReadRecordError) -> Self {
+        ReadEventError::ReadRecordError(e)
+    }
+}
+
+impl ReadEventError {
+    /// Checks whether this error indicates a truncated record. This is a convenience method, since
+    /// the end of a file always implies a truncation event.
+    pub fn truncated(&self) -> bool {
+        matches!(
+            self,
+            ReadEventError::ReadRecordError(ReadRecordError::Truncated)
+        )
+    }
+}
+
+impl<R: Read> EventFileReader<R> {
+    /// Creates a new `EventFileReader` wrapping the given reader.
+    pub fn new(reader: R) -> Self {
+        Self {
+            last_wall_time: None,
+            reader: TfRecordReader::new(reader),
+        }
+    }
+
+    /// Reads the next event from the file.
+    pub fn read_event(&mut self) -> Result<Event, ReadEventError> {
+        let record = self.reader.read_record()?;
+        let event = match Event::decode(&record.data[..]) {
+            Ok(ev) => ev,
+            Err(err) => {
+                // On proto decoding failure, check the record checksum first.
+                record.checksum()?;
+                return Err(err.into());
+            }
+        };
+        let wall_time = event.wall_time;
+        if wall_time.is_nan() {
+            return Err(ReadEventError::NanWallTime(event));
+        }
+        self.last_wall_time = Some(wall_time);
+        Ok(event)
+    }
+
+    /// Gets the wall time of the event most recently read from the event file, or `None` if no
+    /// events have yet been read.
+    pub fn last_wall_time(&self) -> &Option<f64> {
+        &self.last_wall_time
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::masked_crc::MaskedCrc;
+    use crate::proto::tensorboard as pb;
+    use crate::scripted_reader::ScriptedReader;
+    use crate::tf_record::TfRecord;
+    use std::io::Cursor;
+
+    /// Encodes an `Event` proto to bytes.
+    fn encode_event(e: &Event) -> Vec<u8> {
+        let mut encoded = Vec::new();
+        Event::encode(&e, &mut encoded).expect("failed to encode event");
+        encoded
+    }
+
+    #[test]
+    fn test() {
+        let good_event = Event {
+            what: Some(pb::event::What::FileVersion("good event".to_string())),
+            wall_time: 1234.5,
+            ..Event::default()
+        };
+        let mut nan_event = Event {
+            what: Some(pb::event::What::FileVersion("bad wall time".to_string())),
+            wall_time: f64::NAN,
+            ..Event::default()
+        };
+        let records = vec![
+            TfRecord::from_data(encode_event(&good_event)),
+            TfRecord::from_data(encode_event(&nan_event)),
+            TfRecord::from_data(b"failed proto, OK record".to_vec()),
+            TfRecord {
+                data: b"failed proto, failed checksum, OK record structure".to_vec(),
+                data_crc: MaskedCrc(0x12345678),
+            },
+        ];
+        let mut file = Vec::new();
+        for record in records {
+            record.write(&mut file).expect("writing record");
+        }
+        let mut reader = EventFileReader::new(Cursor::new(file));
+
+        assert_eq!(reader.last_wall_time(), &None);
+        assert_eq!(reader.read_event().unwrap(), good_event);
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+        match reader.read_event() {
+            Err(ReadEventError::NanWallTime(mut e)) => {
+                // can't just check `e == nan_event` because `NaN != NaN`
+                assert!(e.wall_time.is_nan());
+                e.wall_time = 0.0;
+                nan_event.wall_time = 0.0;
+                assert_eq!(e, nan_event);
+            }
+            other => panic!("{:?}", other),
+        };
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+        match reader.read_event() {
+            Err(ReadEventError::InvalidProto(_)) => (),
+            other => panic!("{:?}", other),
+        };
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+        match reader.read_event() {
+            Err(ReadEventError::InvalidRecord(ChecksumError {
+                got: _,
+                want: MaskedCrc(0x12345678),
+            })) => (),
+            other => panic!("{:?}", other),
+        };
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+        // After end of file, should get a truncation error.
+        let last = reader.read_event();
+        assert!(last.as_ref().unwrap_err().truncated(), "{:?}", last);
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+    }
+
+    #[test]
+    fn test_resume() {
+        let event = Event {
+            what: Some(pb::event::What::FileVersion("good event".to_string())),
+            wall_time: 1234.5,
+            ..Event::default()
+        };
+        let mut file = Cursor::new(Vec::<u8>::new());
+        TfRecord::from_data(encode_event(&event))
+            .write(&mut file)
+            .unwrap();
+        let record_bytes = file.into_inner();
+        let (beginning, end) = record_bytes.split_at(6);
+
+        let sr = ScriptedReader::new(vec![beginning.to_vec(), end.to_vec()]);
+        let mut reader = EventFileReader::new(sr);
+
+        // first read should be truncated
+        let result = reader.read_event();
+        assert!(result.as_ref().unwrap_err().truncated(), "{:?}", result);
+        assert_eq!(reader.last_wall_time(), &None);
+
+        // second read should be the full record
+        let result = reader.read_event();
+        assert_eq!(result.unwrap(), event);
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+
+        // further reads should be truncated again
+        let result = reader.read_event();
+        assert!(result.as_ref().unwrap_err().truncated(), "{:?}", result);
+        assert_eq!(reader.last_wall_time(), &Some(1234.5));
+    }
+}

tensorboard/data/server/lib.rs

@@ -15,6 +15,7 @@ limitations under the License.
 
 //! Core functionality for TensorBoard data loading.
 
+pub mod event_file;
 pub mod masked_crc;
 pub mod tf_record;
 

tensorboard/data/server/tf_record.rs

@@ -16,7 +16,7 @@ limitations under the License.
 //! Resumable reading for TFRecord streams.
 
 use byteorder::{ByteOrder, LittleEndian};
-use std::io::{self, Read};
+use std::io::{self, Read, Write};
 
 use crate::masked_crc::MaskedCrc;
 
@@ -54,11 +54,12 @@ pub struct TfRecordReader<R> {
 
 /// A TFRecord with a data buffer and expected checksum. The checksum may or may not match the
 /// actual contents.
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Eq)]
 pub struct TfRecord {
     /// The payload of the TFRecord.
     pub data: Vec<u8>,
-    data_crc: MaskedCrc,
+    /// The data CRC listed in the record, which may or not actually match the payload.
+    pub data_crc: MaskedCrc,
 }
 
 /// A buffer's checksum was computed, but it did not match the expected value.
@@ -82,6 +83,31 @@ impl TfRecord {
             Err(ChecksumError { got, want })
         }
     }
+
+    /// Creates a TFRecord from a data vector, computing the correct data CRC. Calling `checksum()`
+    /// on this record will always succeed.
+    pub fn from_data(data: Vec<u8>) -> Self {
+        let data_crc = MaskedCrc::compute(&data);
+        TfRecord { data, data_crc }
+    }
+
+    /// Encodes the record to an output stream. The data CRC will be taken from the `TfRecord`
+    /// value, not recomputed from the payload. This means that reading a valid record and writing
+    /// it back out will always produce identical input. It also means that the written data CRC
+    /// may not be valid.
+    ///
+    /// This may call [`Write::write`] multiple times; consider providing a buffered output stream
+    /// if this is an issue.
+    ///
+    /// A record can always be serialized. This method fails only due to underlying I/O errors.
+    pub fn write<W: Write>(&self, mut writer: W) -> io::Result<()> {
+        let len_buf: [u8; 8] = (self.data.len() as u64).to_le_bytes();
+        writer.write_all(&len_buf)?;
+        writer.write_all(&MaskedCrc::compute(&len_buf).0.to_le_bytes())?;
+        writer.write_all(&self.data)?;
+        writer.write_all(&self.data_crc.0.to_le_bytes())?;
+        Ok(())
+    }
 }
 
 /// Error returned by [`TfRecordReader::read_record`].
@@ -128,6 +154,11 @@ impl<R: Read> TfRecordReader<R> {
         }
     }
 
+    /// Consumes this `TfRecordReader<R>`, returning the underlying reader `R`.
+    pub fn into_inner(self) -> R {
+        self.reader
+    }
+
     /// Attempts to read a TFRecord, pausing gracefully in the face of truncations. If the record
     /// is truncated, the result is a `Truncated` error; call `read_record` again once more data
     /// may have been written to resume reading where it left off. If the record is read
@@ -352,4 +383,46 @@ mod tests {
             other => panic!("{:?}", other),
         }
     }
+
+    #[test]
+    fn test_from_data() {
+        let test_cases = vec![
+            b"".to_vec(),
+            b"\x00".to_vec(),
+            b"the quick brown fox jumped over the lazy dog".to_vec(),
+        ];
+        for data in test_cases {
+            TfRecord::from_data(data).checksum().unwrap();
+        }
+    }
+
+    fn test_write_read_roundtrip(record: &TfRecord) {
+        let mut cursor = Cursor::new(Vec::<u8>::new());
+        record.write(&mut cursor).expect("failed to write record");
+        let written_len = cursor.position();
+        cursor.set_position(0);
+        let mut reader = TfRecordReader::new(cursor);
+        let output_record = reader.read_record().expect("read_record");
+        assert_eq!(&output_record, record);
+        assert_eq!(reader.into_inner().position(), written_len); // should have read all the bytes and not more
+    }
+
+    #[test]
+    fn test_write_read_roundtrip_valid_data_crc() {
+        let data = b"hello world".to_vec();
+        let record = TfRecord {
+            data_crc: MaskedCrc::compute(&data),
+            data,
+        };
+        test_write_read_roundtrip(&record);
+    }
+
+    #[test]
+    fn test_write_read_roundtrip_invalid_data_crc() {
+        let record = TfRecord {
+            data: b"hello world".to_vec(),
+            data_crc: MaskedCrc(0x12345678),
+        };
+        test_write_read_roundtrip(&record);
+    }
 }