From 1ff76f52cf1c606a029e72b40a34f3a8f004a352 Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Thu, 9 Jan 2025 14:24:23 +0000 Subject: [PATCH 1/2] GIT-VERSION-GEN: allow it to be run in parallel "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 Signed-off-by: Junio C Hamano --- GIT-VERSION-GEN | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN index 6d1cb66d69a6d0..5b49e2d72fbd07 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 From f62d870ff776532b8359fbd84b038924a85f95e5 Mon Sep 17 00:00:00 2001 From: Junio C Hamano Date: Thu, 9 Jan 2025 13:47:34 -0800 Subject: [PATCH 2/2] 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. The paragraph 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/ no longer applies. It is not like this cript was the only offender. --- GIT-VERSION-GEN | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN index 5b49e2d72fbd07..bc11258d9b732c 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