diagnostics_log_encoding/

encode.rs

// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! Encoding diagnostic records using the Fuchsia Tracing format.

use crate::{constants, ArgType, Argument, Header, Metatag, RawSeverity, Record, Value};
use fidl_fuchsia_diagnostics::Severity;
use std::array::TryFromSliceError;
use std::borrow::{Borrow, Cow};
use std::fmt::Debug;
use std::io::Cursor;
use std::ops::Deref;
use thiserror::Error;

/// An `Encoder` wraps any value implementing `MutableBuffer` and writes diagnostic stream records
/// into it.
pub struct Encoder<B> {
    pub(crate) buf: B,
    /// Encoder options
    options: EncoderOpts,
}

/// Options for the encoder
#[derive(Default)]
pub struct EncoderOpts {
    /// Whether or not to always log the line/file information
    /// Defaults to false. If false, the line/file information
    /// will only be logged for ERROR and above.
    pub always_log_file_line: bool,
}

/// Parameters for `Encoder/write_event`.
pub struct WriteEventParams<'a, E, T, MS> {
    /// The event to write as a record.
    pub event: E,
    /// Tags associated with the log event.
    pub tags: &'a [T],
    /// Metatags associated with the log event.
    pub metatags: MS,
    /// The process that emitted the log.
    pub pid: zx::Koid,
    /// The thread that emitted the log.
    pub tid: zx::Koid,
    /// Number of events that were dropped before this one.
    pub dropped: u64,
}

impl<B> Encoder<B>
where
    B: MutableBuffer,
{
    /// Create a new `Encoder` from the provided buffer.
    pub fn new(buf: B, options: EncoderOpts) -> Self {
        Self { buf, options }
    }

    /// Returns a reference to the underlying buffer being used for encoding.
    pub fn inner(&self) -> &B {
        &self.buf
    }

    /// Returns a reference to the underlying buffer being used for encoding.
    pub fn take(self) -> B {
        self.buf
    }

    /// Writes an event to to the buffer as a record.
    ///
    /// Fails if there is insufficient space in the buffer for encoding.
    pub fn write_event<'a, E, MS, T>(
        &mut self,
        params: WriteEventParams<'a, E, T, MS>,
    ) -> Result<(), EncodingError>
    where
        E: RecordEvent,
        MS: Iterator<Item = &'a Metatag>,
        T: AsRef<str>,
    {
        let WriteEventParams { event, tags, metatags, pid, tid, dropped } = params;
        let severity = event.raw_severity();
        self.write_inner(event.timestamp(), severity, |this| {
            this.write_raw_argument(constants::PID, pid.raw_koid())?;
            this.write_raw_argument(constants::TID, tid.raw_koid())?;
            if dropped > 0 {
                this.write_raw_argument(constants::NUM_DROPPED, dropped)?;
            }
            if this.options.always_log_file_line || severity >= Severity::Error.into_primitive() {
                // If the severity is ERROR or higher, we add the file and line information.
                if let Some(mut file) = event.file() {
                    let split = file.split("../");
                    file = split.last().unwrap();
                    this.write_raw_argument(constants::FILE, Value::Text(Cow::Borrowed(file)))?;
                }

                if let Some(line) = event.line() {
                    this.write_raw_argument(constants::LINE, line as u64)?;
                }
            }

            // Write the metatags as tags (if any were given)
            for metatag in metatags {
                match metatag {
                    Metatag::Target => this.write_raw_argument(constants::TAG, event.target())?,
                }
            }

            event.write_arguments(this)?;

            for tag in tags {
                this.write_raw_argument(constants::TAG, tag.as_ref())?;
            }
            Ok(())
        })?;
        Ok(())
    }

    /// Writes a Record to the buffer.
    pub fn write_record<R>(&mut self, record: R) -> Result<(), EncodingError>
    where
        R: RecordFields,
    {
        self.write_inner(record.timestamp(), record.raw_severity(), |this| {
            record.write_arguments(this)
        })
    }

    fn write_inner<F>(
        &mut self,
        timestamp: zx::BootInstant,
        severity: RawSeverity,
        write_args: F,
    ) -> Result<(), EncodingError>
    where
        F: FnOnce(&mut Self) -> Result<(), EncodingError>,
    {
        // TODO(https://fxbug.dev/42138121): on failure, zero out the region we were using
        let starting_idx = self.buf.cursor();
        // Prepare the header, we'll finish writing once we know the full size of the record.
        let header_slot = self.buf.put_slot(std::mem::size_of::<u64>())?;
        self.write_i64(timestamp.into_nanos())?;

        write_args(self)?;

        let mut header = Header(0);
        header.set_type(crate::TRACING_FORMAT_LOG_RECORD_TYPE);
        header.set_severity(severity);

        let length = self.buf.cursor() - starting_idx;
        header.set_len(length);

        assert_eq!(length % 8, 0, "all records must be written 8-byte aligned");
        self.buf.fill_slot(header_slot, &header.0.to_le_bytes());
        Ok(())
    }

    /// Writes an argument with this encoder with the given name and value.
    pub fn write_raw_argument(
        &mut self,
        name: &str,
        value: impl WriteArgumentValue<B>,
    ) -> Result<(), EncodingError> {
        self.inner_write_argument(move |header, encoder| {
            encoder.write_argument_name(header, name)?;
            value.write_value(header, encoder)?;
            Ok(())
        })
    }

    /// Writes an argument with this encoder.
    pub fn write_argument<'a>(
        &mut self,
        argument: impl Borrow<Argument<'a>>,
    ) -> Result<(), EncodingError> {
        let argument = argument.borrow();
        self.inner_write_argument(move |header, encoder| {
            encoder.write_argument_name(header, argument.name())?;
            argument.write_value(header, encoder)?;
            Ok(())
        })
    }

    fn write_argument_name(
        &mut self,
        header: &mut Header,
        name: &str,
    ) -> Result<(), EncodingError> {
        self.write_string(name)?;
        header.set_name_ref(string_mask(name));
        Ok(())
    }

    fn inner_write_argument(
        &mut self,
        cb: impl FnOnce(&mut Header, &mut Self) -> Result<(), EncodingError>,
    ) -> Result<(), EncodingError> {
        let starting_idx = self.buf.cursor();
        let header_slot = self.buf.put_slot(std::mem::size_of::<Header>())?;

        let mut header = Header(0);
        cb(&mut header, self)?;

        let record_len = self.buf.cursor() - starting_idx;
        assert_eq!(record_len % 8, 0, "arguments must be 8-byte aligned");

        header.set_size_words((record_len / 8) as u16);
        self.buf.fill_slot(header_slot, &header.0.to_le_bytes());

        Ok(())
    }

    /// Write an unsigned integer.
    fn write_u64(&mut self, n: u64) -> Result<(), EncodingError> {
        self.buf.put_u64_le(n).map_err(|_| EncodingError::BufferTooSmall)
    }

    /// Write a signed integer.
    fn write_i64(&mut self, n: i64) -> Result<(), EncodingError> {
        self.buf.put_i64_le(n).map_err(|_| EncodingError::BufferTooSmall)
    }

    /// Write a floating-point number.
    fn write_f64(&mut self, n: f64) -> Result<(), EncodingError> {
        self.buf.put_f64(n).map_err(|_| EncodingError::BufferTooSmall)
    }

    /// Write a string padded to 8-byte alignment.
    fn write_string(&mut self, src: &str) -> Result<(), EncodingError> {
        self.write_bytes(src.as_bytes())
    }

    /// Write bytes padded to 8-byte alignment.
    #[doc(hidden)]
    pub fn write_bytes(&mut self, src: &[u8]) -> Result<(), EncodingError> {
        self.buf.put_slice(src).map_err(|_| EncodingError::BufferTooSmall)?;
        unsafe {
            let align = std::mem::size_of::<u64>();
            let num_padding_bytes = (align - src.len() % align) % align;
            // TODO(https://fxbug.dev/42138122) need to enforce that the buffer is zeroed
            self.buf.advance_cursor(num_padding_bytes);
        }
        Ok(())
    }
}

mod private {
    use super::*;

    pub trait Sealed {}
    impl Sealed for Value<'_> {}
    impl Sealed for Argument<'_> {}
    impl Sealed for u64 {}
    impl Sealed for f64 {}
    impl Sealed for i64 {}
    impl Sealed for bool {}
    impl Sealed for String {}
    impl Sealed for &str {}
    impl Sealed for Cow<'_, str> {}
}

/// Trait implemented by types which can be written to the encoder.
pub trait WriteArgumentValue<B>: private::Sealed {
    /// Writes the value of the argument.
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError>;
}

impl<B: MutableBuffer> WriteArgumentValue<B> for Argument<'_> {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        match self {
            Self::Pid(value) | Self::Tid(value) => value.raw_koid().write_value(header, encoder),
            Self::Line(value) | Self::Dropped(value) => value.write_value(header, encoder),
            Self::Tag(value) | Self::File(value) | Self::Message(value) => {
                value.write_value(header, encoder)
            }
            Self::Other { value, .. } => value.write_value(header, encoder),
        }
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for i64 {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        header.set_type(ArgType::I64 as u8);
        encoder.write_i64(*self)
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for u64 {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        header.set_type(ArgType::U64 as u8);
        encoder.write_u64(*self)
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for f64 {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        header.set_type(ArgType::F64 as u8);
        encoder.write_f64(*self)
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for bool {
    fn write_value(
        &self,
        header: &mut Header,
        _encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        header.set_type(ArgType::Bool as u8);
        header.set_bool_val(*self);
        Ok(())
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for &str {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        header.set_type(ArgType::String as u8);
        header.set_value_ref(string_mask(self));
        encoder.write_string(self)
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for String {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        self.as_str().write_value(header, encoder)
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for Cow<'_, str> {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        self.as_ref().write_value(header, encoder)
    }
}

impl<B: MutableBuffer> WriteArgumentValue<B> for Value<'_> {
    fn write_value(
        &self,
        header: &mut Header,
        encoder: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        match self {
            Value::SignedInt(s) => s.write_value(header, encoder),
            Value::UnsignedInt(u) => u.write_value(header, encoder),
            Value::Floating(f) => f.write_value(header, encoder),
            Value::Text(t) => t.write_value(header, encoder),
            Value::Boolean(b) => b.write_value(header, encoder),
        }
    }
}

fn string_mask(s: &str) -> u16 {
    let len = s.len();
    if len == 0 {
        return 0;
    }
    (len as u16) | (1 << 15)
}

/// Trait implemented by types which can be written by the Encoder.
pub trait RecordEvent {
    /// Returns the record severity.
    fn raw_severity(&self) -> RawSeverity;
    /// Returns the name of the file where the record was emitted.
    fn file(&self) -> Option<&str>;
    /// Returns the number of the line in the file where the record was emitted.
    fn line(&self) -> Option<u32>;
    /// Returns the target of the record.
    fn target(&self) -> &str;
    /// Consumes this type and writes all the arguments.
    fn write_arguments<B: MutableBuffer>(
        self,
        writer: &mut Encoder<B>,
    ) -> Result<(), EncodingError>;
    /// Returns the timestamp associated to this record.
    fn timestamp(&self) -> zx::BootInstant;
}

/// Trait implemented by complete Records.
pub trait RecordFields {
    /// Returns the record severity.
    fn raw_severity(&self) -> RawSeverity;

    /// Returns the timestamp associated to this record.
    fn timestamp(&self) -> zx::BootInstant;

    /// Consumes this type and writes all the arguments.
    fn write_arguments<B: MutableBuffer>(
        self,
        writer: &mut Encoder<B>,
    ) -> Result<(), EncodingError>;
}

/// Arguments to create a record for testing purposes.
pub struct TestRecord<'a> {
    /// Severity of the log
    pub severity: RawSeverity,
    /// Timestamp of the test record.
    pub timestamp: zx::BootInstant,
    /// File that emitted the log.
    pub file: Option<&'a str>,
    /// Line in the file that emitted the log.
    pub line: Option<u32>,
    /// Additional record arguments.
    pub record_arguments: Vec<Argument<'a>>,
}

impl TestRecord<'_> {
    /// Creates a test record from a record.
    pub fn from<'a>(file: &'a str, line: u32, record: &'a Record<'a>) -> TestRecord<'a> {
        TestRecord {
            severity: record.severity,
            timestamp: record.timestamp,
            file: Some(file),
            line: Some(line),
            record_arguments: record.arguments.clone(),
        }
    }
}

impl RecordEvent for TestRecord<'_> {
    fn raw_severity(&self) -> RawSeverity {
        self.severity
    }

    fn file(&self) -> Option<&str> {
        self.file
    }

    fn line(&self) -> Option<u32> {
        self.line
    }

    fn target(&self) -> &str {
        unimplemented!("Unused at the moment");
    }

    fn timestamp(&self) -> zx::BootInstant {
        self.timestamp
    }

    fn write_arguments<B: MutableBuffer>(
        self,
        writer: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        for argument in self.record_arguments {
            writer.write_argument(argument)?;
        }
        Ok(())
    }
}

impl RecordFields for Record<'_> {
    fn raw_severity(&self) -> RawSeverity {
        self.severity
    }

    fn write_arguments<B: MutableBuffer>(
        self,
        writer: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        for arg in self.arguments {
            writer.write_argument(arg)?;
        }
        Ok(())
    }

    fn timestamp(&self) -> zx::BootInstant {
        self.timestamp
    }
}

#[cfg(test)]
impl RecordFields for &Record<'_> {
    fn raw_severity(&self) -> RawSeverity {
        self.severity
    }

    fn write_arguments<B: MutableBuffer>(
        self,
        writer: &mut Encoder<B>,
    ) -> Result<(), EncodingError> {
        for arg in &self.arguments {
            writer.write_argument(arg)?;
        }
        Ok(())
    }

    fn timestamp(&self) -> zx::BootInstant {
        self.timestamp
    }
}

/// Analogous to `bytes::BufMut` with some additions to be able to write at specific offsets.
pub trait MutableBuffer {
    /// Returns the number of total bytes this container can store. Shared memory buffers are not
    /// expected to resize and this should return the same value during the entire lifetime of the
    /// buffer.
    fn capacity(&self) -> usize;

    /// Returns the current position into which the next write is expected.
    fn cursor(&self) -> usize;

    /// Advance the write cursor by `n` bytes.
    ///
    /// # Safety
    ///
    /// This is marked unsafe because a malformed caller may
    /// cause a subsequent out-of-bounds write.
    unsafe fn advance_cursor(&mut self, n: usize);

    /// Write a copy of the `src` slice into the buffer, starting at the provided offset.
    ///
    /// # Safety
    ///
    /// Implementations are not expected to bounds check the requested copy, although they may do
    /// so and still satisfy this trait's contract.
    unsafe fn put_slice_at(&mut self, src: &[u8], offset: usize);

    /// Returns whether the buffer has sufficient remaining capacity to write an incoming value.
    fn has_remaining(&self, num_bytes: usize) -> bool;

    /// Advances the write cursor without immediately writing any bytes to the buffer. The returned
    /// struct offers the ability to later write to the provided portion of the buffer.
    fn put_slot(&mut self, width: usize) -> Result<WriteSlot, EncodingError> {
        if self.has_remaining(width) {
            let slot = WriteSlot { range: self.cursor()..(self.cursor() + width) };
            unsafe {
                self.advance_cursor(width);
            }
            Ok(slot)
        } else {
            Err(EncodingError::BufferTooSmall)
        }
    }

    /// Write `src` into the provided slot that was created at a previous point in the stream.
    fn fill_slot(&mut self, slot: WriteSlot, src: &[u8]) {
        assert_eq!(
            src.len(),
            slot.range.end - slot.range.start,
            "WriteSlots can only insert exactly-sized content into the buffer"
        );
        unsafe {
            self.put_slice_at(src, slot.range.start);
        }
    }

    /// Writes the contents of the `src` buffer to `self`, starting at `self.cursor()` and
    /// advancing the cursor by `src.len()`.
    ///
    /// # Panics
    ///
    /// This function panics if there is not enough remaining capacity in `self`.
    fn put_slice(&mut self, src: &[u8]) -> Result<(), EncodingError> {
        if self.has_remaining(src.len()) {
            unsafe {
                self.put_slice_at(src, self.cursor());
                self.advance_cursor(src.len());
            }
            Ok(())
        } else {
            Err(EncodingError::NoCapacity)
        }
    }

    /// Writes an unsigned 64 bit integer to `self` in little-endian byte order.
    ///
    /// Advances the cursor by 8 bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::BufMut;
    ///
    /// let mut buf = vec![0; 8];
    /// buf.put_u64_le_at(0x0102030405060708, 0);
    /// assert_eq!(buf, b"\x08\x07\x06\x05\x04\x03\x02\x01");
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if there is not enough remaining capacity in `self`.
    fn put_u64_le(&mut self, n: u64) -> Result<(), EncodingError> {
        self.put_slice(&n.to_le_bytes())
    }

    /// Writes a signed 64 bit integer to `self` in little-endian byte order.
    ///
    /// The cursor position is advanced by 8.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::BufMut;
    ///
    /// let mut buf = vec![0; 8];
    /// buf.put_i64_le_at(0x0102030405060708, 0);
    /// assert_eq!(buf, b"\x08\x07\x06\x05\x04\x03\x02\x01");
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if there is not enough remaining capacity in `self`.
    fn put_i64_le(&mut self, n: i64) -> Result<(), EncodingError> {
        self.put_slice(&n.to_le_bytes())
    }

    /// Writes a double-precision IEEE 754 floating point number to `self`.
    ///
    /// The cursor position is advanced by 8.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::BufMut;
    ///
    /// let mut buf = vec![];
    /// buf.put_i64_le(0x0102030405060708);
    /// assert_eq!(buf, b"\x08\x07\x06\x05\x04\x03\x02\x01");
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if there is not enough remaining capacity in `self`.
    fn put_f64(&mut self, n: f64) -> Result<(), EncodingError> {
        self.put_slice(&n.to_bits().to_ne_bytes())
    }
}

/// A region of the buffer which was advanced past and can later be filled in.
#[must_use]
pub struct WriteSlot {
    range: std::ops::Range<usize>,
}

/// Wrapper for a vector that allows us to implement necessary traits.
#[derive(Debug, Default)]
pub struct ResizableBuffer(Vec<u8>);

impl From<Vec<u8>> for ResizableBuffer {
    fn from(buf: Vec<u8>) -> Self {
        Self(buf)
    }
}

impl Deref for ResizableBuffer {
    type Target = Vec<u8>;

    // Required method
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl ResizableBuffer {
    /// Return the inner vector.
    pub fn into_inner(self) -> Vec<u8> {
        self.0
    }
}

impl MutableBuffer for Cursor<ResizableBuffer> {
    fn capacity(&self) -> usize {
        self.get_ref().0.len()
    }

    fn cursor(&self) -> usize {
        self.position() as usize
    }

    fn has_remaining(&self, _num_bytes: usize) -> bool {
        true
    }

    unsafe fn advance_cursor(&mut self, n: usize) {
        self.set_position(self.position() + n as u64);
    }

    unsafe fn put_slice_at(&mut self, to_put: &[u8], offset: usize) {
        let this = &mut self.get_mut().0;
        if offset < this.len() {
            let available = this.len() - offset;

            // Copy the elements that fit into the buffer.
            let min = available.min(to_put.len());
            let dest = &mut this[offset..(offset + min)];
            dest.copy_from_slice(&to_put[..min]);

            // If we couldn't fit all elements, then extend the buffer with the remaining elements.
            if available < to_put.len() {
                this.extend_from_slice(&to_put[available..]);
            }
        } else {
            // If the offset is bigger than the length, fill with zeros up to the offset and then
            // write the slice.
            this.resize(offset, 0);
            this.extend_from_slice(to_put);
        }
    }
}

impl<T: MutableBuffer + ?Sized> MutableBuffer for &mut T {
    fn has_remaining(&self, num_bytes: usize) -> bool {
        (**self).has_remaining(num_bytes)
    }
    fn capacity(&self) -> usize {
        (**self).capacity()
    }

    fn cursor(&self) -> usize {
        (**self).cursor()
    }

    unsafe fn advance_cursor(&mut self, n: usize) {
        (**self).advance_cursor(n);
    }

    unsafe fn put_slice_at(&mut self, to_put: &[u8], offset: usize) {
        (**self).put_slice_at(to_put, offset);
    }
}

impl<T: MutableBuffer + ?Sized> MutableBuffer for Box<T> {
    fn has_remaining(&self, num_bytes: usize) -> bool {
        (**self).has_remaining(num_bytes)
    }
    fn capacity(&self) -> usize {
        (**self).capacity()
    }

    fn cursor(&self) -> usize {
        (**self).cursor()
    }

    unsafe fn advance_cursor(&mut self, n: usize) {
        (**self).advance_cursor(n);
    }

    unsafe fn put_slice_at(&mut self, to_put: &[u8], offset: usize) {
        (**self).put_slice_at(to_put, offset);
    }
}

impl MutableBuffer for Cursor<Vec<u8>> {
    fn has_remaining(&self, num_bytes: usize) -> bool {
        (self.cursor() + num_bytes) <= self.capacity()
    }

    fn capacity(&self) -> usize {
        self.get_ref().len()
    }

    fn cursor(&self) -> usize {
        self.position() as usize
    }

    unsafe fn advance_cursor(&mut self, n: usize) {
        self.set_position(self.position() + n as u64);
    }

    unsafe fn put_slice_at(&mut self, to_put: &[u8], offset: usize) {
        let dest = &mut self.get_mut()[offset..(offset + to_put.len())];
        dest.copy_from_slice(to_put);
    }
}

impl MutableBuffer for Cursor<&mut [u8]> {
    fn has_remaining(&self, num_bytes: usize) -> bool {
        (self.cursor() + num_bytes) <= self.capacity()
    }

    fn capacity(&self) -> usize {
        self.get_ref().len()
    }

    fn cursor(&self) -> usize {
        self.position() as usize
    }

    unsafe fn advance_cursor(&mut self, n: usize) {
        self.set_position(self.position() + n as u64);
    }

    unsafe fn put_slice_at(&mut self, to_put: &[u8], offset: usize) {
        let dest = &mut self.get_mut()[offset..(offset + to_put.len())];
        dest.copy_from_slice(to_put);
    }
}

impl<const N: usize> MutableBuffer for Cursor<[u8; N]> {
    fn has_remaining(&self, num_bytes: usize) -> bool {
        (self.cursor() + num_bytes) <= self.capacity()
    }
    fn capacity(&self) -> usize {
        self.get_ref().len()
    }

    fn cursor(&self) -> usize {
        self.position() as usize
    }

    unsafe fn advance_cursor(&mut self, n: usize) {
        self.set_position(self.position() + n as u64);
    }

    unsafe fn put_slice_at(&mut self, to_put: &[u8], offset: usize) {
        let dest = &mut self.get_mut()[offset..(offset + to_put.len())];
        dest.copy_from_slice(to_put);
    }
}

/// An error that occurred while encoding data to the stream format.
#[derive(Debug, Error)]
pub enum EncodingError {
    /// The provided buffer is too small.
    #[error("buffer is too small")]
    BufferTooSmall,

    /// We attempted to encode values which are not yet supported by this implementation of
    /// the Fuchsia Tracing format.
    #[error("unsupported value type")]
    Unsupported,

    /// We attempted to write to a buffer with no remaining capacity.
    #[error("the buffer has no remaining capacity")]
    NoCapacity,

    /// Some other error happened. Useful for integrating with this crate, but providing custom
    /// errors.
    #[error(transparent)]
    Other(Box<dyn std::error::Error + Send + Sync>),
}

impl EncodingError {
    /// Treat a custom error as an encoding error.
    pub fn other<E>(err: E) -> Self
    where
        E: std::error::Error + Send + Sync + 'static,
    {
        Self::Other(err.into())
    }
}

impl From<TryFromSliceError> for EncodingError {
    fn from(_: TryFromSliceError) -> Self {
        EncodingError::BufferTooSmall
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::parse::parse_record;

    #[fuchsia::test]
    fn build_basic_record() {
        let mut encoder = Encoder::new(Cursor::new([0u8; 1024]), EncoderOpts::default());
        encoder
            .write_event(WriteEventParams::<_, &str, _> {
                event: TestRecord {
                    severity: Severity::Info as u8,
                    timestamp: zx::BootInstant::from_nanos(12345),
                    file: None,
                    line: None,
                    record_arguments: vec![],
                },
                tags: &[],
                metatags: std::iter::empty(),
                pid: zx::Koid::from_raw(0),
                tid: zx::Koid::from_raw(0),
                dropped: 0,
            })
            .expect("wrote event");
        let (record, _) = parse_record(encoder.inner().get_ref()).expect("wrote valid record");
        assert_eq!(
            record,
            Record {
                timestamp: zx::BootInstant::from_nanos(12345),
                severity: Severity::Info as u8,
                arguments: vec![
                    Argument::pid(zx::Koid::from_raw(0)),
                    Argument::tid(zx::Koid::from_raw(0)),
                ]
            }
        );
    }

    #[fuchsia::test]
    fn build_records_with_location() {
        let mut encoder = Encoder::new(Cursor::new([0u8; 1024]), EncoderOpts::default());
        encoder
            .write_event(WriteEventParams::<_, &str, _> {
                event: TestRecord {
                    severity: Severity::Error as u8,
                    timestamp: zx::BootInstant::from_nanos(12345),
                    file: Some("foo.rs"),
                    line: Some(10),
                    record_arguments: vec![],
                },
                tags: &[],
                metatags: std::iter::empty(),
                pid: zx::Koid::from_raw(0),
                tid: zx::Koid::from_raw(0),
                dropped: 0,
            })
            .expect("wrote event");
        let (record, _) = parse_record(encoder.inner().get_ref()).expect("wrote valid record");
        assert_eq!(
            record,
            Record {
                timestamp: zx::BootInstant::from_nanos(12345),
                severity: Severity::Error as u8,
                arguments: vec![
                    Argument::pid(zx::Koid::from_raw(0)),
                    Argument::tid(zx::Koid::from_raw(0)),
                    Argument::file("foo.rs"),
                    Argument::line(10),
                ]
            }
        );
    }

    #[fuchsia::test]
    fn build_record_with_dropped_count() {
        let mut encoder = Encoder::new(Cursor::new([0u8; 1024]), EncoderOpts::default());
        encoder
            .write_event(WriteEventParams::<_, &str, _> {
                event: TestRecord {
                    severity: Severity::Warn as u8,
                    timestamp: zx::BootInstant::from_nanos(12345),
                    file: None,
                    line: None,
                    record_arguments: vec![],
                },
                tags: &[],
                metatags: std::iter::empty(),
                pid: zx::Koid::from_raw(0),
                tid: zx::Koid::from_raw(0),
                dropped: 7,
            })
            .expect("wrote event");
        let (record, _) = parse_record(encoder.inner().get_ref()).expect("wrote valid record");
        assert_eq!(
            record,
            Record {
                timestamp: zx::BootInstant::from_nanos(12345),
                severity: Severity::Warn as u8,
                arguments: vec![
                    Argument::pid(zx::Koid::from_raw(0)),
                    Argument::tid(zx::Koid::from_raw(0)),
                    Argument::dropped(7),
                ]
            }
        );
    }

    #[test]
    fn resizable_vec_mutable_buffer() {
        // Putting a slice at offset=len is equivalent to concatenating.
        let mut vec = Cursor::new(ResizableBuffer(vec![1u8, 2, 3]));
        unsafe {
            vec.put_slice_at(&[4, 5, 6], 3);
        }
        assert_eq!(vec.get_ref().0, vec![1, 2, 3, 4, 5, 6]);

        // Putting a slice at an offset inside the buffer, is equivalent to replacing the items
        // there.
        let mut vec = Cursor::new(ResizableBuffer(vec![1, 3, 7, 9, 11, 13, 15]));
        unsafe {
            vec.put_slice_at(&[2, 4, 6], 2);
        }
        assert_eq!(vec.get_ref().0, vec![1, 3, 2, 4, 6, 13, 15]);

        // Putting a slice at an index in range replaces all the items and extends if needed.
        let mut vec = Cursor::new(ResizableBuffer(vec![1, 2, 3]));
        unsafe {
            vec.put_slice_at(&[4, 5, 6, 7], 0);
        }
        assert_eq!(vec.get_ref().0, vec![4, 5, 6, 7]);

        // Putting a slice at an offset beyond the buffer, fills with zeros the items in between.
        let mut vec = Cursor::new(ResizableBuffer(vec![1, 2, 3]));
        unsafe {
            vec.put_slice_at(&[4, 5, 6], 5);
        }
        assert_eq!(vec.get_ref().0, vec![1, 2, 3, 0, 0, 4, 5, 6]);
    }
}