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52 lines
2.5 KiB
CMake
52 lines
2.5 KiB
CMake
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# 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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