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d09210c08b
This untangles the CMake versioning issues (I hope) as discussed in #4626. Note most of the advice found on the Internet about how to inject git versions into CMake is just wrong. The behavior we want is to unconditionally run the script build_tools/git_version_gen.sh at build time (i.e. when you invoke ninja or make, and not when you invoke cmake, which is build system generation time). This script is careful to only update the FISH-BUILD-VERSION-FILE if the contents have changed, to avoid spurious rebuilding dependencies of FISH-BUILD-VERSION-FILE. Assuming the git version hasn't changed, the script will run, but not update FISH-BUILD-VERSION-FILE, and therefore fish_version.o will not have to be rebuilt. This might normally rebuild more than is necessary even if the timestamp is not updated, because ninja computes the dependency chain ahead of time. But Ninja also supports the 'restat' option for just this case, and CMake is rad and exposes this via BYPRODUCTS. So mark FISH-BUILD-VERSION-FILE as a byproduct and make the script always update a dummy file (fish-build-version-witness.txt). Note this is the use case for which BYPRODUCTS is designed. We also have fish_version.cpp #include "FISH-BUILD-VERSION-FILE", and do a semi-silly thing and make FISH-BUILD-VERSION-FILE valid C++ (so there's just one version file). This means we have to filter out the quotes in other cases..
52 lines
2.5 KiB
CMake
52 lines
2.5 KiB
CMake
# This file adds commands to manage the FISH-BUILD-VERSION-FILE (hereafter
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# FBVF). This file exists in the build directory and is used to populate the
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# documentation and also the version string in fish_version.o (printed with
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# `echo $version` and also fish --version). The essential idea is that we are
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# going to invoke git_version_gen.sh, which will update the
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# FISH-BUILD-VERSION-FILE only if it needs to change; this is what makes
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# incremental rebuilds fast.
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#
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# This code is delicate, with the chief subtlety revolving around Ninja. A
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# natural and naive approach would tell the generated build system that FBVF is
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# a dependency of fish_version.o, and that git_version_gen.sh updates it. Make
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# will then invoke the script, check the timestamp on fish_version.o and FBVF,
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# see that FBVF is earlier, and then not rebuild fish_version.o. Ninja,
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# however, decides what to build up-front and will unconditionally rebuild
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# fish_version.o.
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#
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# To avoid this with Ninja, we want to hook into its 'restat' option which we
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# can do through the BYPRODUCTS feature of CMake. See
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# https://cmake.org/cmake/help/latest/policy/CMP0058.html
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#
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# Unfortunately BYPRODUCTS behaves strangely with the Makefile generator: it
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# marks FBVF as generated and then CMake itself will `touch` it on every build,
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# meaning that using BYPRODUCTS will cause fish_version.o to be rebuilt
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# unconditionally with the Makefile generator. Thus we want to use the
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# natural-and-naive approach for Makefiles.
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# **IMPORTANT** If you touch these build rules, please test both Ninja and
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# Makefile generators with both a clean and dirty git tree. Verify that both
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# generated build systems rebuild fish when the git tree goes from dirty to
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# clean (and vice versa), and verify they do NOT rebuild it when the git tree
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# stays the same (incremental builds must be fast).
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# Just a handy abbreviation.
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SET(FBVF FISH-BUILD-VERSION-FILE)
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# TODO: find a cleaner way to do this.
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IF (${CMAKE_GENERATOR} STREQUAL Ninja)
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SET(FBVF-OUTPUT fish-build-version-witness.txt)
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SET(CFBVF-BYPRODUCTS ${FBVF})
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ELSE(${CMAKE_GENERATOR} STREQUAL Ninja)
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SET(FBVF-OUTPUT ${FBVF})
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SET(CFBVF-BYPRODUCTS)
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ENDIF(${CMAKE_GENERATOR} STREQUAL Ninja)
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# Set up the version targets
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ADD_CUSTOM_TARGET(CHECK-FISH-BUILD-VERSION-FILE
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COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/build_tools/git_version_gen.sh
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BYPRODUCTS ${CFBVF-BYPRODUCTS})
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ADD_CUSTOM_COMMAND(OUTPUT ${FBVF-OUTPUT}
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DEPENDS CHECK-FISH-BUILD-VERSION-FILE)
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