tests/test-pull-pull-corruption.t

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

Corrupt an hg repo with two pulls.
create one repo with a long history

  $ hg init source1
  $ cd source1
  $ touch foo
  $ hg add foo
  $ for i in 1 2 3 4 5 6 7 8 9 10; do
  >     echo $i >> foo
  >     hg ci -m $i
  > done
  $ cd ..

create one repo with a shorter history

  $ hg clone -r 0 source1 source2
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  new changesets 495a0ec48aaf
  updating to branch default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd source2
  $ echo a >> foo
  $ hg ci -m a
  $ cd ..

create a third repo to pull both other repos into it

  $ hg init corrupted
  $ cd corrupted

use a hook to make the second pull start while the first one is still running

  $ echo '[hooks]' >> .hg/hgrc
  $ echo 'prechangegroup = sleep 5' >> .hg/hgrc

start a pull...

  $ hg pull ../source1 > pull.out 2>&1 &

... and start another pull before the first one has finished

  $ sleep 1
  $ hg pull ../source2 2>/dev/null
  pulling from ../source2
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files (+1 heads)
  new changesets ca3c05af513e
  (run 'hg heads' to see heads, 'hg merge' to merge)
  $ cat pull.out
  pulling from ../source1
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 10 changesets with 10 changes to 1 files
  new changesets 495a0ec48aaf:1e7b6c812ca8
  (run 'hg update' to get a working copy)

see the result

  $ wait
  $ hg verify
  checking changesets
  checking manifests
  crosschecking files in changesets and manifests
  checking files
  checked 11 changesets with 11 changes to 1 files

  $ cd ..