GIT-VERSION-GEN: allow it to be run in parallel #1850
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On the Git mailing list, Martin Ågren wrote (reply to this): On Thu, 9 Jan 2025 at 15:24, Johannes Schindelin via GitGitGadget
<[email protected]> wrote:
> And this is how that race surfaces: When calling `make -j2 html man`
> from the top-level directory (a variant of which is invoked in Git for
> Windows' release process), two sub-processes are spawned, a `make -C
> Documentation html` one and a `make -C Documentation man` one. Both run
> the rule to (re-)generate `asciidoctor-extensions.rb` or
> `asciidoc.conf`, invoking `GIT-VERSION-GEN` to do so.
Nicely described. Indeed, there's a reason recursive make is considered
harmful. This is of course not the time or place for addressing that.
> Incidentally, this also fixes something else: The `+` character is
> not even a valid filename character on Windows. The only reason why Git
> for Windows did not need this is that above-mentioned POSIX emulation
> layer also plays a couple of tricks with filenames (tricks that are not
> interoperable with regular Windows programs, though), and previous
> attempts to remedy this in git/git were unsuccessful, see e.g.
> https://lore.kernel.org/git/[email protected]/
> - "$INPUT" >"$OUTPUT"+
> + "$INPUT" >"$OUTPUT".$$
>
> -if ! test -f "$OUTPUT" || ! cmp "$OUTPUT"+ "$OUTPUT" >/dev/null
> +if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$ "$OUTPUT" >/dev/null
> then
> - mv "$OUTPUT"+ "$OUTPUT"
> + mv "$OUTPUT".$$ "$OUTPUT"
> else
> - rm "$OUTPUT"+
> + rm "$OUTPUT".$$
> fi
Our `.gitignore` contains an entry "*+" to ignore this sort of temporary
files. Yes, they're supposed to disappear within a second or so, but
according to f9bbaa384e (Add intermediate build products to .gitignore,
2009-11-08), they can linger after interrupted builds. Maybe separate
tooling built around git could pick up these as untracked files for a
second, causing them to come and go in whatever GUI.
You could use "$OUTPUT"."$$"+ to restore this. That of course
invalidates your remark about "Incidentally, ..." above, but might give
this fix a tiny bit less chance of regressing something somewhere?
Martin |
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User |
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On the Git mailing list, Junio C Hamano wrote (reply to this): Martin Ågren <[email protected]> writes:
>> ... attempts to remedy this in git/git were unsuccessful, see e.g.
>> https://lore.kernel.org/git/[email protected]/
> ...
> You could use "$OUTPUT"."$$"+ to restore this. That of course
> invalidates your remark about "Incidentally, ..." above, but might give
> this fix a tiny bit less chance of regressing something somewhere?
Thanks for being careful.
My reading of that old thread cited there tells me that the reason
that previous one failed was mostly because it wasn't being self
consistent and only touched the use of "+" in the Documentation
directory but not what the top-level Makefile did, and also because
it did not adjust .gitignore patterns, so it is good that somebody
actually read the cited thread and made sure this time we do better.
Again, I was not opposed to moving from "+" to something else that
is equally short-and-sweet, and I still am not ("~" is a fine suffix
for this kind of thing, for example). But if we are aiming for a
short-term fix, I think your ".$$+" may make the most sense.
Thanks.
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This patch series was integrated into seen via git@a883025. |
"Why would one want to run it in parallel?" I hear you ask. I am glad you are curious, because a curious story is what it is, indeed. The `GIT-VERSION-GEN` script is quite a pillar of Git's source code, with most lines being unchanged for the past 15 years. Until the v2.48.0 release candidate cycle. Its original purpose was to generate the version string and store it in the `GIT-VERSION-FILE`. This paradigm changed quite dramatically when support for building with Meson was introduced. Most crucially, a38edab (Makefile: generate doc versions via GIT-VERSION-GEN, 2024-12-06) changed the way the documentation is built by using the `GIT-VERSION-GEN` file to write out the `asciidocor-extensions.rb` and `asciidoc.conf` files with now hard-coded version strings. Crucially, the Makefile rule to generate those files needs to be run in every build because `GIT_VERSION` could have been specified in the `make` command-line, which would require these files to be modified. This introduced a surprising race condition! And this is how that race surfaces: When calling `make -j2 html man` from the top-level directory (a variant of which is invoked in Git for Windows' release process), two sub-processes are spawned, a `make -C Documentation html` one and a `make -C Documentation man` one. Both run the rule to (re-)generate `asciidoctor-extensions.rb` or `asciidoc.conf`, invoking `GIT-VERSION-GEN` to do so. That script first generates a temporary file (appending the `+` character to the filename), then looks whether it contains something different than the already existing file (if it exists, that is), and either replaces it if needed, or removes the temporary file. If one of the two parallel invocations removes that temporary file before the other can compare it, or even worse: if one tries to replace the target file just after the other _started_ writing the temporary file (but did not finish writing it yet), that race condition now causes bad builds. This may sound highly theoretical, but due to the design of Git's build process, Git for Windows is forced to use a (slow) POSIX emulation layer to run that script and in the blink of an eye it becomes very much not theoretical at all. See Exhibit A: These GitHub workflow runs failed because one of the two competing `make` processes tried to remove the temporary file when the other process had already done so: https://github.com/git-for-windows/git-sdk-32/actions/runs/12663456654 https://github.com/git-for-windows/git-sdk-32/actions/runs/12683174970 https://github.com/git-for-windows/git-sdk-64/actions/runs/12649348496 While it is undesirable to run this script over and over again, certainly when this involves above-mentioned slow POSIX emulation layer, the stage of the release cycle in which we are presently finding ourselves does not lend itself to a re-design where this script could be run once, and once only, but instead dictates that a quick and reliable work-around be implemented that prevents the race condition without changing the overall architecture of the build process. This patch does that: By using a filename suffix for the temporary file which is based on the currently-executing script's process ID, We guarantee that the two competing invocations cannot overwrite or remove each others' temporary files. The filename suffix still ends in `+` to ensure that the temporary artifacts are matched by the `*+` pattern in `.gitignore` that was added in f9bbaa3 (Add intermediate build products to .gitignore, 2009-11-08). Helped-by: Martin Ågren <[email protected]> Signed-off-by: Johannes Schindelin <[email protected]>
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This patch series was integrated into seen via git@631d441. |
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Submitted as [email protected] To fetch this version into To fetch this version to local tag |
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This patch series was integrated into seen via git@262a185. |
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This patch series was integrated into seen via git@0d844af. |
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On the Git mailing list, Junio C Hamano wrote (reply to this): "Johannes Schindelin via GitGitGadget" <[email protected]>
writes:
> * Appended + again, to get the benefit of the .gitignore pattern that
> prevents the temporary files from being committed.
I had queued an incremental fix-up in my tree on top, which may have
been easier to read, but you did the testing on a system that showed
the problem earlier, so I'll drop it (together with the v1) and
replace them with this one.
Thanks.
--- >8 ---
Subject: [PATCH] Make sure the name of the temporary file ends with "+"
To avoid regression by leaving untracked cruft that is not covered
by the .gitignore file, match the convention to generate into
$name$suffix and then move to $name where $suffix ends with "+",
which is used everywhere else in the system.
diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN
index 5b49e2d72f..bc11258d9b 100755
--- a/GIT-VERSION-GEN
+++ b/GIT-VERSION-GEN
@@ -78,6 +78,8 @@ read GIT_MAJOR_VERSION GIT_MINOR_VERSION GIT_MICRO_VERSION GIT_PATCH_LEVEL trail
$(echo "$GIT_VERSION" 0 0 0 0 | tr '.a-zA-Z-' ' ')
EOF
+OUTPUT_TMP="$OUTPUT.$$+"
+
sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
-e "s|@GIT_MAJOR_VERSION@|$GIT_MAJOR_VERSION|" \
-e "s|@GIT_MINOR_VERSION@|$GIT_MINOR_VERSION|" \
@@ -86,11 +88,11 @@ sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
-e "s|@GIT_BUILT_FROM_COMMIT@|$GIT_BUILT_FROM_COMMIT|" \
-e "s|@GIT_USER_AGENT@|$GIT_USER_AGENT|" \
-e "s|@GIT_DATE@|$GIT_DATE|" \
- "$INPUT" >"$OUTPUT".$$
+ "$INPUT" >"$OUTPUT_TMP"
-if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$ "$OUTPUT" >/dev/null
+if ! test -f "$OUTPUT" || ! cmp "$OUTPUT_TMP" "$OUTPUT" >/dev/null
then
- mv "$OUTPUT".$$ "$OUTPUT"
+ mv "$OUTPUT_TMP" "$OUTPUT"
else
- rm "$OUTPUT".$$
+ rm "$OUTPUT_TMP"
fi
--- 8< ---
> GIT-VERSION-GEN | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN
> index 6d1cb66d69a..2e2d0811581 100755
> --- a/GIT-VERSION-GEN
> +++ b/GIT-VERSION-GEN
> @@ -86,11 +86,11 @@ sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
> -e "s|@GIT_BUILT_FROM_COMMIT@|$GIT_BUILT_FROM_COMMIT|" \
> -e "s|@GIT_USER_AGENT@|$GIT_USER_AGENT|" \
> -e "s|@GIT_DATE@|$GIT_DATE|" \
> - "$INPUT" >"$OUTPUT"+
> + "$INPUT" >"$OUTPUT".$$+
>
> -if ! test -f "$OUTPUT" || ! cmp "$OUTPUT"+ "$OUTPUT" >/dev/null
> +if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$+ "$OUTPUT" >/dev/null
> then
> - mv "$OUTPUT"+ "$OUTPUT"
> + mv "$OUTPUT".$$+ "$OUTPUT"
> else
> - rm "$OUTPUT"+
> + rm "$OUTPUT".$$+
> fi
>
> base-commit: a60673e9252b08d4eca90543b3729f4798b9aafd |
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This patch series was integrated into seen via git@3ae3564. |
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This patch series was integrated into master via git@3ae3564. |
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This patch series was integrated into next via git@3ae3564. |
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Closed via 3ae3564. |
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Changes since v1:
+again, to get the benefit of the.gitignorepattern that prevents the temporary files from being committed.Cc: Patrick Steinhardt [email protected]
cc: Martin Ågren [email protected]