revlog: automatically read from opened file handles
The revlog reading code commonly opens a new file handle for
reading on demand. There is support for passing a file handle
to revlog.revision(). But it is marked as an internal argument.
When revlogs are written, we write() data as it is available. But
we don't flush() data until all revisions are written.
Putting these two traits together, it is possible for an in-process
revlog reader during active writes to trigger the opening of a new
file handle on a file with unflushed writes. The reader won't have
access to all "available" revlog data (as it hasn't been flushed).
And with the introduction of the previous patch, this can lead to
the revlog raising an error due to a partial read.
I witnessed this behavior when applying changegroup data (via
`hg pull`) before issue6006 was fixed via different means. Having
this and the previous patch in play would have helped cause errors
earlier rather than manifesting as hash verification failures.
While this has been a long-standing issue, I believe the relatively
new delta computation code has tickled it into being more common.
This is because the new delta computation code will compute deltas
in more scenarios. This can lead to revlog reading. While the delta
computation code is probably supposed to reuse file handles, it
appears it isn't doing so in all circumstances.
But the issue runs deeper than that. Theoretically, any code can
access revision data during revlog writes. It appears we were just
getting lucky that it wasn't. (The "add revision callback" passed to
addgroup() provides an avenue to do this.)
If I changed the revlog's behavior to not cache the full revision
text or to clear caches after revision insertion during addgroup(),
I was able to produce crashes 100% of the time when writing changelog
revisions. This is because changelog's add revision callback attempts
to resolve the revision data to access the changed files list. And
without the revision's fulltext being cached, we performed a revlog
read, which required opening a new file handle. This attempted to read
unflushed data, leading to a partial read and a crash.
This commit teaches the revlog to store the file handles used for
writing multiple revisions during addgroup(). It also teaches the
code for resolving a file handle when reading to use these handles,
if available. This ensures that *any* reads (regardless of their
source) use the active writing file handles, if available. These
file handles have access to the unflushed data because they wrote it.
This allows reads to complete without issue.
Differential Revision: https://phab.mercurial-scm.org/D5267
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('A', repo).cmp(repo[None]['real_A'])"
True (no-eol)
$ hg eval "not filecmp.cmp('A', 'real_A')"
True (no-eol)
These files are identical and should return False (same):
$ hg eval "context.arbitraryfilectx('A', repo).cmp(repo[None]['A'])"
False (no-eol)
$ hg eval "context.arbitraryfilectx('A', repo).cmp(repo[None]['B'])"
False (no-eol)
$ hg eval "not filecmp.cmp('A', '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('real_A', repo).cmp(repo[None]['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('real_A', 'sym_A')"
True (no-eol)
#else
$ hg eval "not filecmp.cmp('real_A', 'sym_A')"
False (no-eol)
#endif