Enable DTrace USDT GC probes in the modular default GC

When Ruby is built with `--with-modular-gc`, the default GC module is
built with `BUILDING_MODULAR_GC` defined.

The existing code stubbed out `probes.h` and `RUBY_DTRACE_GC_HOOK` in
this build path, so any DTrace script attached against the modular
default GC saw zero GC events.

This PR enables the GC probes in the modular GC library so they fire
when Ruby is built with `--enable-dtrace --with-modular-gc`

To test: build with `./configure --enable-dtrace
--with-modular-gc=/tmp/modgc`, then `make install-modular-gc
MODULAR_GC=default`.

Probes are present in the shared library:

```console
$ sudo bpftrace -l 'usdt:/tmp/modgc/librubygc.default.so:ruby:*'
usdt:/tmp/modgc/librubygc.default.so:ruby:gc__mark__begin
usdt:/tmp/modgc/librubygc.default.so:ruby:gc__mark__end
usdt:/tmp/modgc/librubygc.default.so:ruby:gc__sweep__begin
usdt:/tmp/modgc/librubygc.default.so:ruby:gc__sweep__end
```

They fire under a GC-heavy workload (`test.rb` contains `100000.times { Object.new }; GC.start`):

```console
$ sudo bpftrace -e '
  usdt:/tmp/modgc/librubygc.default.so:ruby:gc__mark__begin  { @mark_begin++ }
  usdt:/tmp/modgc/librubygc.default.so:ruby:gc__mark__end    { @mark_end++ }
  usdt:/tmp/modgc/librubygc.default.so:ruby:gc__sweep__begin { @sweep_begin++ }
  usdt:/tmp/modgc/librubygc.default.so:ruby:gc__sweep__end   { @sweep_end++ }
' -c 'env RUBY_GC_LIBRARY=default ./miniruby --disable=gems test.rb'

@mark_begin: 2
@mark_end: 2
@sweep_begin: 18
@sweep_end: 18
```

common.mk

@@ -1975,7 +1975,9 @@ gc/distclean gc/realclean::
 
 modular-gc-precheck:
 modular-gc: probes.h gc/Makefile
-	$(Q) $(RUNRUBY) $(srcdir)/ext/extmk.rb \
+	$(Q) DTRACE='$(DTRACE)' DTRACE_EXT='$(DTRACE_EXT)' \
+		DTRACE_OBJ='$(DTRACE_OBJ)' DTRACE_REBUILD='$(DTRACE_REBUILD)' \
+		$(RUNRUBY) $(srcdir)/ext/extmk.rb \
 		$(SCRIPT_ARGS) \
 		--make='$(MAKE)' --make-flags="V=$(V) MINIRUBY='$(MINIRUBY)'" \
 		--gnumake=$(gnumake) --extflags="$(EXTLDFLAGS)" \

gc/default/default.c

@@ -34,9 +34,7 @@
 #include "gc/gc.h"
 #include "gc/gc_impl.h"
 
-#ifndef BUILDING_MODULAR_GC
-# include "probes.h"
-#endif
+#include "probes.h"
 
 #ifdef BUILDING_MODULAR_GC
 # define RB_DEBUG_COUNTER_INC(_name) ((void)0)
@@ -8940,12 +8938,8 @@ gc_prof_timer_stop(rb_objspace_t *objspace)
     }
 }
 
-#ifdef BUILDING_MODULAR_GC
-# define RUBY_DTRACE_GC_HOOK(name)
-#else
-# define RUBY_DTRACE_GC_HOOK(name) \
+#define RUBY_DTRACE_GC_HOOK(name) \
     do {if (RUBY_DTRACE_GC_##name##_ENABLED()) RUBY_DTRACE_GC_##name();} while (0)
-#endif
 
 static inline void
 gc_prof_mark_timer_start(rb_objspace_t *objspace)

gc/extconf_base.rb

@@ -4,11 +4,49 @@ require "mkmf"
 
 srcdir = File.join(__dir__, "..")
 $INCFLAGS << " -I#{srcdir}"
-
 $CPPFLAGS << " -DBUILDING_MODULAR_GC"
 
 append_cflags("-fPIC")
 
+DTRACE_RULES = <<~MAKE
+
+  probes.stamp: $(GC_DTRACE_OBJS)
+  	$(Q) if test -f $@ -o -f $(GC_DTRACE_OBJ); then \
+  	  $(RM) $(GC_DTRACE_OBJS) $@; \
+  	  $(ECHO) rebuilding objects which were modified by "dtrace -G"; \
+  	  $(MAKE) $(GC_DTRACE_OBJS); \
+  	fi
+  	$(Q) touch $@
+
+  $(GC_DTRACE_OBJ): $(top_srcdir)/probes.d $(GC_DTRACE_REBUILD:yes=probes.stamp)
+  	$(ECHO) processing GC probes in object files
+  	$(Q) CC="$(CC)" CFLAGS="$(CFLAGS) $(INCFLAGS) $(CPPFLAGS)" $(GC_DTRACE) -G -C $(INCFLAGS) -s $(top_srcdir)/probes.d -o $@ $(GC_DTRACE_OBJS)
+MAKE
+
 def create_gc_makefile(name, &block)
-  create_makefile("librubygc.#{name}", &block)
+  dtrace_obj = ENV.fetch("DTRACE_OBJ", "")
+  dtrace_enabled = (name == "default" && !dtrace_obj.empty?)
+
+  if name == "default"
+    $INCFLAGS << " -I$(topdir)"
+    $headers << "$(topdir)/probes.h"
+    $cleanfiles << "probes.stamp"
+  end
+
+  create_makefile("librubygc.#{name}") do |conf|
+    conf = block.call(conf) if block
+    next conf unless dtrace_enabled
+
+    gc_objs = $objs.join(" ")
+    conf = Array(conf).join.sub(/^OBJS = .*$/, "OBJS = #{gc_objs} #{dtrace_obj}")
+    conf + <<~MAKE
+
+      GC_DTRACE = #{ENV.fetch("DTRACE")}
+      GC_DTRACE_OBJ = #{dtrace_obj}
+      GC_DTRACE_OBJS = #{gc_objs}
+      GC_DTRACE_REBUILD = #{ENV.fetch("DTRACE_REBUILD", "")}
+    MAKE
+  end
+
+  File.write("Makefile", DTRACE_RULES, mode: "ab") if dtrace_enabled
 end

test/dtrace/helper.rb

@@ -115,7 +115,7 @@ module DTrace
       IO.popen(cmd, err: [:child, :out], &:readlines)
     end
 
-    def trap_probe d_program, ruby_program
+    def trap_probe d_program, ruby_program, env: {}
       if Hash === d_program
         d_program = d_program[IMPL] or
           omit "#{d_program} not implemented for #{IMPL}"
@@ -132,15 +132,17 @@ module DTrace
 
       d_path  = d.path
       rb_path = rb.path
-      cmd = "#{RUBYBIN} --disable=gems -I#{INCLUDE} #{rb_path}"
+      ruby_env = env
+      env_prefix = ruby_env.map {|key, val| "#{key}=#{val}" }
+      cmd = [*env_prefix, "#{RUBYBIN} --disable=gems -I#{INCLUDE} #{rb_path}"].join(" ")
       if IMPL == :stap
         cmd = %W(stap #{d_path} -c #{cmd})
       else
         cmd = [*DTRACE_CMD, "-q", "-s", d_path, "-c", cmd ]
       end
       if sudo = @@sudo
-        NEEDED_ENVS.each do |name|
-          if val = ENV[name]
+        (NEEDED_ENVS + ruby_env.keys).uniq.each do |name|
+          if val = ruby_env[name] || ENV[name]
             cmd.unshift("#{name}=#{val}")
           end
         end

test/dtrace/test_gc_modular.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: false
+require_relative 'helper'
+
+module DTrace
+  class TestModularGC < TestCase
+    MODULAR_GC_DIR = RbConfig::CONFIG["modular_gc_dir"]
+    DLEXT = RbConfig::CONFIG["DLEXT"]
+    DEFAULT_GC = MODULAR_GC_DIR && File.join(MODULAR_GC_DIR, "librubygc.default.#{DLEXT}")
+
+    def setup
+      super
+      omit "Ruby was not configured with --with-modular-gc" if MODULAR_GC_DIR.nil? || MODULAR_GC_DIR.empty?
+      omit "default modular GC is not installed" unless DEFAULT_GC && File.file?(DEFAULT_GC)
+    end
+
+    %w[
+      gc-mark-begin
+      gc-mark-end
+      gc-sweep-begin
+      gc-sweep-end
+    ].each do |probe_name|
+      define_method(:"test_modular_#{probe_name.gsub(/-/, '_')}") do
+        probe = "ruby$target:::#{probe_name} { printf(\"#{probe_name}\\n\"); }"
+
+        trap_probe(probe, ruby_program, env: {"RUBY_GC_LIBRARY" => "default"}) do |_, _, saw|
+          assert_operator saw.length, :>, 0
+        end
+      end
+    end
+
+    private
+
+    def ruby_program
+      "100000.times { Object.new }"
+    end
+  end
+end if defined?(DTrace::TestCase)