build: keep a generated source that a new mrbc writes the same - #7254

Stacked on #7253 (the test objects through the rules); the first commit is that PR.

mrblib.c, the gem_init.c of every gem and the three sources of
mrbtest (assert.c, mrbtest.c, the gem_test.c of every gem) are
written by mrbc, and each of them has bin/mrbc among its
prerequisites. Any compile of the compiler relinks mrbc, and every one
of them is then written again, preprocessed again for presym and compiled
again, whether the bytecode changed or not:

$ rake                        # built once
$ touch mrbgems/mruby-compiler/src/diagnostic.c
$ rake                        # 43 GEN, 44 CPP, 45 CC: diagnostic.o twice, and 43 loaders
$ rake test                   # built once
$ touch mrbgems/mruby-compiler/src/diagnostic.c
$ rake test                   # 95 GEN, 44 CPP, 97 CC

The bytecode is the same in all of them; nothing about it changed.

Why

Comparing the text before writing does not do it alone. Rake holds an
output out of date when anything behind its prerequisites is newer
(Rake::FileTask#out_of_date? walks all_prerequisite_tasks), so an
object whose source has mrbc behind it is compiled again the moment
mrbc is, whatever the source says. tasks/presym.rake meets the same
wall (#6721) and puts a task without prerequisites between the objects
and the presym list, with the list's timestamp; active_gems.txt is
written by a task that always runs and writes only when the text differs.

What this does

generated_file in lib/mruby/core_ext.rb is a file task for a
generated source that takes both:

• The source task has no prerequisites of its own and runs every time. It
  invokes a stamp task, <source>.stamp beside the source, which has the
  prerequisites (mrbc, the Ruby files, the generator) and holds the time
  of the last generation.
• The stamp task writes the text to <source>.tmp and moves it over the
  source only when it differs from what is there. A source that is missing
  is written again whatever the stamp says.

The five generators go through it. mrbtest's wrote to a .tmp and
renamed already, so a failed mrbc leaves the source as it was; the
others do now too. The compile lines do not change: the flags record of
every object is the same as on master.

mrbc still runs for every source when it changed; only the write, the
preprocess and the compile are skipped when the text is the same. A change
to what mrbc writes still reaches every object it reaches.

Verified

rake -m -j16 (-j1 where noted), CCACHE_DISABLE=1, gcc; default
gembox with enable_bintest and enable_test; the build directory built
once (rake, or rake test where the row says so) before each row, and
the run after each row does nothing in both columns. Times are the median
of three alternating runs (two for -j1); the rake test rows include running the tests.

The last row is why this is stacked: once the sources stop being written,
the test objects have to answer to their .d and flags records themselves.

Environment

gcc -MMD -c -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DMRB_USE_COMPLEX -DMRB_USE_BIGINT -DMRB_USE_DEBUG_HOOK -I"include" -I"build/host/include" -o "build/host/mrblib/mrblib.o" "build/host/mrblib/mrblib.c"

No C source changes and the compile lines are the same, so .text is
unaffected in every build.

Summary by CodeRabbit

• Bug Fixes

  • Improved generated-file handling so build outputs are updated only when their contents change.
  • Ensured generated sources are consistently refreshed when their inputs change.
  • Preserved existing gem initialization, test generation, and compilation behavior.

• Developer Experience

  • Added clearer warnings to generated source files.
  • Improved reliability and efficiency of repeated builds by avoiding unnecessary file rewrites.