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HACKING.BUILD.md

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Hacking on the build system

This list was previously maintained as issue #242, until we realized that this needs proper version-control. (Duh!)

Exhaustive list of requirements on the build system:

  1. commit.h must change whenever it disagrees with the current git commit
  2. commit.h must not change whenever it agrees with the current git commit
  3. Must call cd tgl && ./configure with appropriate options whenever tgl/Makefile is missing, or notices that ./configure was run. ("Too often" is acceptable as long as make && make isn't violated.)
  4. Must call make -C tgl whenever this might have any effect. ("Too often" is acceptable as long as make && make isn't violated.)
  5. Any changes due to make -C tgl must cause all necessary rebuilds in telegram-purple during the same invocation of make. ("Too often" is acceptable as long as make && make isn't violated.)
  6. If tgl/Makefile.in is missing, explain that we need submodules, and stop.
  7. make -j12 must work fine
  8. Must support out-of-CVS builds.

While ensuring that:

  • Must not -include tgl/Makefile as it doesn't really support that, and I'm not going to try and convince @vysheng to do that. Also, it's too fragile. You know how he ticks.
  • make && make must not create any files in the second run (otherwise, sudo make install would become hairy)
  • Compatibility with other makes

Approach:

  • commit.h is a regular file which depends on a .PHONY target commit. Building commit.h only touches commit.h if necessary and possible. (=> 1, 2, half of 8)
  • Have a target tgl/Makefile which depends on Makefile. (=> 3)
  • The central target ${PRPL_LIBNAME} shall depend on the .PHONY submade, which depends on tgl/Makefile, and essentially executes make -C tgl. (=> 4, time constraint of 5)
  • All objects of telegram-purple depend on tgl/libs/libtgl.a. Rationale: if tgl/libs/libtgl.a stays the same, then nothing in tgl changed. If tgl/libs/libtgl.a changes, then due to lots of black preprocessor magic within tgl, it can't be safely determined which, if any, objects of telegram-purple can be re-used. Also, if tgl got recompiled (~ 1 minute), then tgp can be recompiled, too (~ 5 seconds). (=> dependency constraint of 5)
  • Let tgl/Makefile depend on tgl/Makefile.in, and put the warning into the rule for the latter. (=> 6)
  • The above already implies a dependency DAG that is completely known to make; except at one point: let tgl/libs/libtgl.a depend on submade, without any own code. Now make ensure thread-safety on it's own. (=> 7)
  • Bundle commit.h into the origtar (=> other half of 8)

Side effects:

  • Should be ultimately robust. Then again, that's what I thought about the last few attempts, and they didn't last long.
  • This reduces the number of .PHONYs drastically, making it easier to read, and making it easier to prove that make && make doesn't change anything.
  • Even though submade should trigger once and only once, this can handle multiple invocations just fine, too.
  • All files dependent on commit.h need to be mentioned explicitly as dependents in the Makefile. This is ugly, but (as far as I can see) unavoidable. Thankfully, any errors here will get detected by automated testing.
  • We currently use only one GNU make specific extension. See majn#137 (comment)