cgen: guard shared-library GC divisor tuning behind GC_is_init_called (#27555)

In a `-shared -gc boehm` library loaded into a process that already initialized
Boehm (e.g. with GC_FREE_SPACE_DIVISOR=2), the library's GC_set_free_space_divisor
ran before its own GC_INIT(), which is a no-op once GC is up and so never re-reads
the env var. That silently clobbered the host's process-wide divisor and defeated
the advertised env override. Only tune the divisor when the library is the one
bringing up the collector. The main-executable path is unchanged: it owns its GC.

Author: medvednikov

vlib/v/gen/c/cmain.v

@@ -198,7 +198,13 @@ fn (mut g Gen) gen_shared_library_boehm_init() {
 	if g.pref.gc_mode in [.boehm_full_opt, .boehm_incr_opt] {
 		// See gen_boehm_gc_init: rarer stop-the-world pauses for the opt modes,
 		// overridable via the `GC_FREE_SPACE_DIVISOR` env var (issue #27555).
-		g.writeln('\tGC_set_free_space_divisor(1);')
+		// Only tune when this library is the one bringing up the collector. If the
+		// host process already initialized Boehm, the local GC_INIT() below is a
+		// no-op that will not re-read the env var, so setting the divisor here would
+		// silently override the host's process-wide value (and its env override).
+		g.writeln('\tif (!GC_is_init_called()) {')
+		g.writeln('\t\tGC_set_free_space_divisor(1);')
+		g.writeln('\t}')
 	}
 	g.writeln('\tGC_INIT();')
 	g.writeln('\tGC_register_displacement(sizeof(void*));')

vlib/v/gen/c/coutput_test.v

@@ -579,6 +579,10 @@ fn test_user_defined_windows_dllmain_disables_generated_entrypoint() {
 	ensure_compilation_succeeded(compilation, cmd)
 	assert compilation.output.contains('void _vinit_caller() {')
 	assert compilation.output.contains('GC_set_pages_executable(0);')
+	// The shared-library GC tuning (issue #27555) must stay guarded, so loading
+	// the library into an already-GC-initialized host does not clobber the host's
+	// process-wide free-space divisor (its local GC_INIT() would be a no-op).
+	assert compilation.output.contains('if (!GC_is_init_called()) {')
 	assert compilation.output.contains('GC_INIT();')
 	assert compilation.output.contains('DllMain(')
 	assert compilation.output.contains('_vinit_caller();')