Extension not fully rebuild when `config.yml` changes

[Earlopain]

Ref #3926 (comment)

I can reproduce it with the following:

$ bundle exec rake clean
$ git checkout 7c16f777bd4708a513097d9e1f6c91639f9c5a6f
bundle exec rake compile

$ git checkout 27c24fdc0d8c9d14f819f83050d573315e8474ee # changes to config.yml
$ bundle exec rake # crashes/segfaults

I don't really know much about this but noticed that some object files don't get rebuild, which they should probably.

$ find tmp -name "*.o" | xargs ls -la
10:53 tmp/x86_64-linux/prism/4.0.0/api_node.o
10:53 tmp/x86_64-linux/prism/4.0.0/diagnostic.o
10:50 tmp/x86_64-linux/prism/4.0.0/encoding.o
10:50 tmp/x86_64-linux/prism/4.0.0/extension.o
10:53 tmp/x86_64-linux/prism/4.0.0/node.o
10:50 tmp/x86_64-linux/prism/4.0.0/options.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_buffer.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_char.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_constant_pool.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_integer.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_list.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_memchr.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_newline_list.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_string.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_strncasecmp.o
10:50 tmp/x86_64-linux/prism/4.0.0/pm_strpbrk.o
10:53 tmp/x86_64-linux/prism/4.0.0/prettyprint.o
10:53 tmp/x86_64-linux/prism/4.0.0/prism.o
10:50 tmp/x86_64-linux/prism/4.0.0/regexp.o
10:53 tmp/x86_64-linux/prism/4.0.0/serialize.o
10:50 tmp/x86_64-linux/prism/4.0.0/static_literals.o
10:53 tmp/x86_64-linux/prism/4.0.0/token_type.o

extension.o for example did not get rebuild, even though prism/ast.h changed. Probably a few more should also be updated.
In the generated Makefile, I can see that extension.h is the only prism-specific header that is referenced.

But I don't know what to change in extconf.rb to make this work.

[kddnewton]

@tenderlove any chance you can take a look at this?

[eregon]

I'm taking a look, will open a PR in a minute.

[eregon]

#3932 fixes this, I have tried the instructions before the depend file it fails as expected, and with it it recompiles as wanted when following the instructions.
It needs a clean to take effect (as the first instruction), because otherwise the Makefile is not re-generated.

Maybe it'd make sense for the Makefile to depend on extconf.rb (not sure why CRuby doesn't do that by default), but that's a separate problem and still wouldn't fix that "bootstrapping case". Might recompile needlessly for extconf.rb changes which don't affect .o files though.