view tests/test-arbitraryfilectx.t @ 47965:f9e6f2bb721d

rhg: Don’t compare ambiguous files one byte at a time

Even though the use of `BufReader` reduces the number of syscalls to read
the file from disk, `.bytes()` yields a separate `Result` for every byte.
Creating those results and dispatching on them is most likely costly.

Instead, this commit opts for simplicity by reading the entire file into memory
and comparing a single pair of byte strings. Note that memory already needs to
contain the entire previous contents of the file, as read from the filelog.
So with an extremely large file this doubles memory use but does not make it
grow by orders of magnitude.

At first I wrote code that still avoids reading the entire file into memory
and compares one buffer at a time with `BufReader`. Find this code below for
posterity. However its correctness is subtle. I ended up preferring the
simplicity of the obviously-correct single comparison.

```rust
let mut reader = BufReader::new(fobj);
let mut expected = &contents_in_p1[..];
loop {
    let buf = reader.fill_buf().when_reading_file(&fs_path)?;
    if buf.is_empty() {
        // Found EOF
        return Ok(expected.is_empty());
    } else if let Some(rest) = expected.drop_prefix(buf) {
        // What we read so far matches the expected content, continue reading
        let buf_len = buf.len();
        reader.consume(buf_len);
        expected = rest
    } else {
        // Found different content
        return Ok(false);
    }
}
```

Differential Revision: https://phab.mercurial-scm.org/D11412

Setup:
  $ cat > eval.py <<EOF
  > from __future__ import absolute_import
  > import filecmp
  > from mercurial import commands, context, pycompat, registrar
  > cmdtable = {}
  > command = registrar.command(cmdtable)
  > @command(b'eval', [], b'hg eval CMD')
  > def eval_(ui, repo, *cmds, **opts):
  >     cmd = b" ".join(cmds)
  >     res = pycompat.bytestr(eval(cmd, globals(), locals()))
  >     ui.warn(b"%s" % res)
  > EOF

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "eval=`pwd`/eval.py" >> $HGRCPATH

Arbitraryfilectx.cmp does not follow symlinks:
  $ mkdir case1
  $ cd case1
  $ hg init
#if symlink
  $ printf "A" > real_A
  $ printf "foo" > A
  $ printf "foo" > B
  $ ln -s A sym_A
  $ hg add .
  adding A
  adding B
  adding real_A
  adding sym_A
  $ hg commit -m "base"
#else
  $ hg import -q --bypass - <<EOF
  > # HG changeset patch
  > # User test
  > # Date 0 0
  > base
  > 
  > diff --git a/A b/A
  > new file mode 100644
  > --- /dev/null
  > +++ b/A
  > @@ -0,0 +1,1 @@
  > +foo
  > \ No newline at end of file
  > diff --git a/B b/B
  > new file mode 100644
  > --- /dev/null
  > +++ b/B
  > @@ -0,0 +1,1 @@
  > +foo
  > \ No newline at end of file
  > diff --git a/real_A b/real_A
  > new file mode 100644
  > --- /dev/null
  > +++ b/real_A
  > @@ -0,0 +1,1 @@
  > +A
  > \ No newline at end of file
  > diff --git a/sym_A b/sym_A
  > new file mode 120000
  > --- /dev/null
  > +++ b/sym_A
  > @@ -0,0 +1,1 @@
  > +A
  > \ No newline at end of file
  > EOF
  $ hg up -q
#endif

These files are different and should return True (different):
(Note that filecmp.cmp's return semantics are inverted from ours, so we invert
for simplicity):
  $ hg eval "context.arbitraryfilectx(b'A', repo).cmp(repo[None][b'real_A'])"
  True (no-eol)
  $ hg eval "not filecmp.cmp(b'A', b'real_A')"
  True (no-eol)

These files are identical and should return False (same):
  $ hg eval "context.arbitraryfilectx(b'A', repo).cmp(repo[None][b'A'])"
  False (no-eol)
  $ hg eval "context.arbitraryfilectx(b'A', repo).cmp(repo[None][b'B'])"
  False (no-eol)
  $ hg eval "not filecmp.cmp(b'A', b'B')"
  False (no-eol)

This comparison should also return False, since A and sym_A are substantially
the same in the eyes of ``filectx.cmp``, which looks at data only.
  $ hg eval "context.arbitraryfilectx(b'real_A', repo).cmp(repo[None][b'sym_A'])"
  False (no-eol)

A naive use of filecmp on those two would wrongly return True, since it follows
the symlink to "A", which has different contents.
#if symlink
  $ hg eval "not filecmp.cmp(b'real_A', b'sym_A')"
  True (no-eol)
#else
  $ hg eval "not filecmp.cmp(b'real_A', b'sym_A')"
  False (no-eol)
#endif