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
test for old histedit issue #6:
editing a changeset without any actual change would corrupt the repository
$ . "$TESTDIR/histedit-helpers.sh"
$ cat >> $HGRCPATH <<EOF
> [extensions]
> histedit=
> EOF
$ initrepo ()
> {
> dir="$1"
> comment="$2"
> if [ -n "${comment}" ]; then
> echo % ${comment}
> echo % ${comment} | sed 's:.:-:g'
> fi
> hg init ${dir}
> cd ${dir}
> for x in a b c d e f ; do
> echo $x > $x
> hg add $x
> hg ci -m $x
> done
> cd ..
> }
$ geneditor ()
> {
> # generate an editor script for selecting changesets to be edited
> choice=$1 # changesets that should be edited (using sed line ranges)
> cat <<EOF | sed 's:^....::'
> # editing the rules, replacing 'pick' with 'edit' for the chosen lines
> sed '${choice}s:^pick:edit:' "\$1" > "\${1}.tmp"
> mv "\${1}.tmp" "\$1"
> # displaying the resulting rules, minus comments and empty lines
> sed '/^#/d;/^$/d;s:^:| :' "\$1" >&2
> EOF
> }
$ startediting ()
> {
> # begin an editing session
> choice="$1" # changesets that should be edited
> number="$2" # number of changesets considered (from tip)
> comment="$3"
> geneditor "${choice}" > edit.sh
> echo % start editing the history ${comment}
> HGEDITOR="sh ./edit.sh" hg histedit -- -${number} 2>&1 | fixbundle
> }
$ continueediting ()
> {
> # continue an edit already in progress
> editor="$1" # message editor when finalizing editing
> comment="$2"
> echo % finalize changeset editing ${comment}
> HGEDITOR=${editor} hg histedit --continue 2>&1 | fixbundle
> }
$ graphlog ()
> {
> comment="${1:-log}"
> echo % "${comment}"
> hg log -G --template '{rev} {node} \"{desc|firstline}\"\n'
> }
$ initrepo r1 "test editing with no change"
% test editing with no change
-----------------------------
$ cd r1
$ graphlog "log before editing"
% log before editing
@ 5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
|
o 4 e860deea161a2f77de56603b340ebbb4536308ae "e"
|
o 3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
|
o 2 177f92b773850b59254aa5e923436f921b55483b "c"
|
o 1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
|
o 0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
$ startediting 2 3 "(not changing anything)" # edit the 2nd of 3 changesets
% start editing the history (not changing anything)
| pick 055a42cdd887 3 d
| edit e860deea161a 4 e
| pick 652413bf663e 5 f
0 files updated, 0 files merged, 2 files removed, 0 files unresolved
Editing (e860deea161a), you may commit or record as needed now.
(hg histedit --continue to resume)
$ continueediting true "(leaving commit message unaltered)"
% finalize changeset editing (leaving commit message unaltered)
check state of working copy
$ hg id
794fe033d0a0 tip
$ graphlog "log after history editing"
% log after history editing
@ 5 794fe033d0a030f8df77c5de945fca35c9181c30 "f"
|
o 4 04d2fab980779f332dec458cc944f28de8b43435 "e"
|
o 3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
|
o 2 177f92b773850b59254aa5e923436f921b55483b "c"
|
o 1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
|
o 0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
$ cd ..
$ initrepo r2 "test editing with no change, then abort"
% test editing with no change, then abort
-----------------------------------------
$ cd r2
$ graphlog "log before editing"
% log before editing
@ 5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
|
o 4 e860deea161a2f77de56603b340ebbb4536308ae "e"
|
o 3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
|
o 2 177f92b773850b59254aa5e923436f921b55483b "c"
|
o 1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
|
o 0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
$ startediting 1,2 3 "(not changing anything)" # edit the 1st two of 3 changesets
% start editing the history (not changing anything)
| edit 055a42cdd887 3 d
| edit e860deea161a 4 e
| pick 652413bf663e 5 f
0 files updated, 0 files merged, 3 files removed, 0 files unresolved
Editing (055a42cdd887), you may commit or record as needed now.
(hg histedit --continue to resume)
$ continueediting true "(leaving commit message unaltered)"
% finalize changeset editing (leaving commit message unaltered)
Editing (e860deea161a), you may commit or record as needed now.
(hg histedit --continue to resume)
$ graphlog "log after first edit"
% log after first edit
@ 6 e5ae3ca2f1ffdbd89ec41ebc273a231f7c3022f2 "d"
|
| o 5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
| |
| o 4 e860deea161a2f77de56603b340ebbb4536308ae "e"
| |
| o 3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
|/
o 2 177f92b773850b59254aa5e923436f921b55483b "c"
|
o 1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
|
o 0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
abort editing session, after first forcibly updating away
$ hg up 0
abort: histedit in progress
(use 'hg histedit --continue' or 'hg histedit --abort')
[255]
$ mv .hg/histedit-state .hg/histedit-state-ignore
$ hg up 0
0 files updated, 0 files merged, 3 files removed, 0 files unresolved
$ mv .hg/histedit-state-ignore .hg/histedit-state
$ hg sum
parent: 0:cb9a9f314b8b
a
branch: default
commit: 1 added, 1 unknown (new branch head)
update: 6 new changesets (update)
phases: 7 draft
hist: 2 remaining (histedit --continue)
$ hg histedit --abort 2>&1 | fixbundle
modified files should survive the abort when we've moved away already
$ hg st
A e
? edit.sh
$ graphlog "log after abort"
% log after abort
o 5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
|
o 4 e860deea161a2f77de56603b340ebbb4536308ae "e"
|
o 3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
|
o 2 177f92b773850b59254aa5e923436f921b55483b "c"
|
o 1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
|
@ 0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
aborting and not changing files can skip mentioning updating (no) files
$ hg up
5 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg commit --close-branch -m 'closebranch'
$ startediting 1 1 "(not changing anything)" # edit the 3rd of 3 changesets
% start editing the history (not changing anything)
| edit 292aec348d9e 6 closebranch
Editing (292aec348d9e), you may commit or record as needed now.
(hg histedit --continue to resume)
$ hg histedit --abort
$ cd ..