Don't return an error after a partial write in `LineWriter`

[Stebalien]

LineWriter can return an error after a partial write if flushing the inner BufWriter fails (

rust/src/libstd/io/buffered.rs

Line 766 in cb343c3

try!(self.inner.flush());

). It should instead return Ok(bytes_written), ignoring the error.

OLD REPORT:

I originally thought this problem was systematic so I audited stdlib. Apparently it isn't... See below.

The write trait guarantees:

If an error is returned then no bytes in the buffer were written to this writer.

However, this is violated all over stdlib. For example, if "abc\nde" is written to a LineWriter, the LineWriter could write "abc", try to flush, and then return an error due to a failed flush. However, "abc" has been written violating the spec.

IMO, the solution is to return Ok(amount_written) for partial writes and drop the error (if any). This should be safe because of write's write-xor-error guarantee. If the error was transient, the caller never needs to know. Otherwise, they will will learn of it on the next write.

Here's everything in the stdlib that implements write (and should be audited):

• Sink in librustdoc/test.rs
• Sink in libstd/io/util.rs
• &'a mut W in libstd/io/impls.rs
• Box<W> in libstd/io/impls.rs
• &'a mut [u8] in libstd/io/impls.rs
• Vec<u8> in libstd/io/impls.rs
• Cursor<&'a mut [u8]> in libstd/io/cursor.rs
• Cursor<Vec<u8>> in libstd/io/cursor.rs
• Cursor<Box<[u8]>> in libstd/io/cursor.rs
• StdoutRaw in libstd/io/stdio.rs
• StderrRaw in libstd/io/stdio.rs
• Maybe<W> in libstd/io/stdio.rs
• Stdout in libstd/io/stdio.rs
• StdoutLock<'a> in libstd/io/stdio.rs
• Stderr in libstd/io/stdio.rs
• StderrLock<'a> in libstd/io/stdio.rs
• BufWriter<W> in libstd/io/buffered.rs
• LineWriter<W> in libstd/io/buffered.rs (buggy)
• Broadcast<T, U> in libstd/io/mod.rs (buggy but unfixable and deprecated).
• TcpStream in libstd/net/tcp.rs
• &'a TcpStream in libstd/net/tcp.rs
• Stderr in libstd/sys/unix/stdio.rs
• Stderr in libstd/sys/windows/stdio.rs (panics on partial write)
• ChildStdin in libstd/process.rs
• File in libstd/fs.rs
• &'a File in libstd/fs.rs

[Stebalien]

Note: this could, e.g., theoretically cause line_writer.write_all("abc\nde"); line_writer.flush() to write "abcabc\nde" on interrupt.

[Stebalien]

Ok. Actually, it turns out that only BufWriter and LineWriter are really problematic. Stderr on windows is bad but at least it's acknowledged in the code with a FIXME...

[Stebalien]

So, BufWriter is actually correct (I misread some of the code). This is just a bug in LineWriter, nothing more.

[bluss]

Nice effort to audit it all!