patchGIT-VERSION-GEN: allow it to be run in parallel

From: Johannes Schindelin <johannes.schindelin@gmx.de>

"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, a38edab7c88b (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.

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/pull.216.git.gitgitgadget@gmail.com/

This commit fixes one of the issues that are currently delaying Git for Windows v2.48.0-rc2.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    GIT-VERSION-GEN: allow it to be run in parallel

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1850%2Fdscho%2Fasciidoctor-extensions-gen-race-work-around-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1850/dscho/asciidoctor-extensions-gen-race-work-around-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1850

 GIT-VERSION-GEN | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

On Thu, 9 Jan 2025 at 15:24, Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com> wrote:

Nicely described. Indeed, there's a reason recursive make is considered harmful. This is of course not the time or place for addressing that.

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

Martin Ågren <martin.agren@gmail.com> writes:

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.

From: Johannes Schindelin <johannes.schindelin@gmx.de>

"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, a38edab7c88b (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 f9bbaa384ef (Add intermediate build products to .gitignore, 2009-11-08).

Helped-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    GIT-VERSION-GEN: allow it to be run in parallel
    
    Changes since v1:
    
     * Appended + again, to get the benefit of the .gitignore pattern that
       prevents the temporary files from being committed.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1850%2Fdscho%2Fasciidoctor-extensions-gen-race-work-around-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1850/dscho/asciidoctor-extensions-gen-race-work-around-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1850

Range-diff vs v1:

 1:  7a1c39b9075 ! 1:  4057596dc16 GIT-VERSION-GEN: allow it to be run in parallel
     @@ Commit message
          guarantee that the two competing invocations cannot overwrite or remove
          each others' temporary files.
      
     -    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/pull.216.git.gitgitgadget@gmail.com/
     -
     -    This commit fixes one of the issues that are currently delaying Git for
     -    Windows v2.48.0-rc2.
     +    The filename suffix still ends in `+` to ensure that the temporary
     +    artifacts are matched by the `*+` pattern in `.gitignore` that was added
     +    in f9bbaa384ef (Add intermediate build products to .gitignore,
     +    2009-11-08).
      
     +    Helped-by: Martin Ågren <martin.agren@gmail.com>
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
      
       ## GIT-VERSION-GEN ##
     @@ GIT-VERSION-GEN: sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
       	-e "s|@GIT_USER_AGENT@|$GIT_USER_AGENT|" \
       	-e "s|@GIT_DATE@|$GIT_DATE|" \
      -	"$INPUT" >"$OUTPUT"+
     -+	"$INPUT" >"$OUTPUT".$$
     ++	"$INPUT" >"$OUTPUT".$$+
       
      -if ! test -f "$OUTPUT" || ! cmp "$OUTPUT"+ "$OUTPUT" >/dev/null
     -+if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$ "$OUTPUT" >/dev/null
     ++if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$+ "$OUTPUT" >/dev/null
       then
      -	mv "$OUTPUT"+ "$OUTPUT"
     -+	mv "$OUTPUT".$$ "$OUTPUT"
     ++	mv "$OUTPUT".$$+ "$OUTPUT"
       else
      -	rm "$OUTPUT"+
     -+	rm "$OUTPUT".$$
     ++	rm "$OUTPUT".$$+
       fi

 GIT-VERSION-GEN | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com> writes:

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.