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64f3ff3ffc
"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> Signed-off-by: Junio C Hamano <gitster@pobox.com>
97 lines
2.3 KiB
Bash
Executable File
97 lines
2.3 KiB
Bash
Executable File
#!/bin/sh
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DEF_VER=v2.48.0-rc2
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LF='
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'
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if test "$#" -ne 3
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then
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echo >&2 "USAGE: $0 <SOURCE_DIR> <INPUT> <OUTPUT>"
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exit 1
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fi
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SOURCE_DIR="$1"
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INPUT="$2"
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OUTPUT="$3"
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if ! test -f "$INPUT"
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then
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echo >&2 "Input is not a file: $INPUT"
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exit 1
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fi
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# Protect us from reading Git version information outside of the Git directory
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# in case it is not a repository itself, but embedded in an unrelated
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# repository.
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GIT_CEILING_DIRECTORIES="$SOURCE_DIR/.."
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export GIT_CEILING_DIRECTORIES
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if test -z "$GIT_VERSION"
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then
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# First see if there is a version file (included in release tarballs),
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# then try git-describe, then default.
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if test -f "$SOURCE_DIR"/version
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then
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VN=$(cat "$SOURCE_DIR"/version) || VN="$DEF_VER"
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elif {
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test -d "$SOURCE_DIR/.git" ||
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test -d "${GIT_DIR:-.git}" ||
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test -f "$SOURCE_DIR"/.git;
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} &&
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VN=$(git -C "$SOURCE_DIR" describe --match "v[0-9]*" HEAD 2>/dev/null) &&
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case "$VN" in
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*$LF*) (exit 1) ;;
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v[0-9]*)
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git -C "$SOURCE_DIR" update-index -q --refresh
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test -z "$(git -C "$SOURCE_DIR" diff-index --name-only HEAD --)" ||
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VN="$VN-dirty" ;;
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esac
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then
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VN=$(echo "$VN" | sed -e 's/-/./g');
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else
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VN="$DEF_VER"
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fi
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GIT_VERSION=$(expr "$VN" : v*'\(.*\)')
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fi
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if test -z "$GIT_BUILT_FROM_COMMIT"
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then
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GIT_BUILT_FROM_COMMIT=$(git -C "$SOURCE_DIR" rev-parse -q --verify HEAD 2>/dev/null)
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fi
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if test -z "$GIT_DATE"
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then
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GIT_DATE=$(git -C "$SOURCE_DIR" show --quiet --format='%as' 2>/dev/null)
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fi
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if test -z "$GIT_USER_AGENT"
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then
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GIT_USER_AGENT="git/$GIT_VERSION"
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fi
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# While released Git versions only have three numbers, development builds also
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# have a fourth number that corresponds to the number of patches since the last
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# release.
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read GIT_MAJOR_VERSION GIT_MINOR_VERSION GIT_MICRO_VERSION GIT_PATCH_LEVEL trailing <<EOF
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$(echo "$GIT_VERSION" 0 0 0 0 | tr '.a-zA-Z-' ' ')
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EOF
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sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
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-e "s|@GIT_MAJOR_VERSION@|$GIT_MAJOR_VERSION|" \
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-e "s|@GIT_MINOR_VERSION@|$GIT_MINOR_VERSION|" \
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-e "s|@GIT_MICRO_VERSION@|$GIT_MICRO_VERSION|" \
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-e "s|@GIT_PATCH_LEVEL@|$GIT_PATCH_LEVEL|" \
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-e "s|@GIT_BUILT_FROM_COMMIT@|$GIT_BUILT_FROM_COMMIT|" \
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-e "s|@GIT_USER_AGENT@|$GIT_USER_AGENT|" \
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-e "s|@GIT_DATE@|$GIT_DATE|" \
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"$INPUT" >"$OUTPUT".$$+
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if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$+ "$OUTPUT" >/dev/null
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then
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mv "$OUTPUT".$$+ "$OUTPUT"
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else
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rm "$OUTPUT".$$+
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fi
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