cmd/go: use alternate debug_modinfo recipe for gccgo

Use a different recipe for capturing debug modinfo if we're compiling
with the gccgo toolchain, to avoid applying a go:linkname directive to
a variable (not supported by gccgo).

Fixes #30344.

Change-Id: I9ce3d42c3bbb809fd68b140f56f9bbe3406c351b
Reviewed-on: https://go-review.googlesource.com/c/go/+/171768Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -40,7 +40,7 @@ var (
 	ModPackageModuleInfo func(path string) *modinfo.ModulePublic                                                  // return module info for Package struct
 	ModImportPaths       func(args []string) []*search.Match                                                      // expand import paths
 	ModPackageBuildInfo  func(main string, deps []string) string                                                  // return module info to embed in binary
-	ModInfoProg          func(info string) []byte                                                                 // wrap module info in .go code for binary
+	ModInfoProg          func(info string, isgccgo bool) []byte                                                   // wrap module info in .go code for binary
 	ModImportFromFiles   func([]string)                                                                           // update go.mod to add modules for imports in these files
 	ModDirImportPath     func(string) string                                                                      // return effective import path for directory
 )

src/cmd/go/internal/modload/build.go

@@ -249,16 +249,34 @@ func findModule(target, path string) module.Version {
 	panic("unreachable")
 }
 
-func ModInfoProg(info string) []byte {
+func ModInfoProg(info string, isgccgo bool) []byte {
 	// Inject a variable with the debug information as runtime.modinfo,
 	// but compile it in package main so that it is specific to the binary.
 	// The variable must be a literal so that it will have the correct value
 	// before the initializer for package main runs.
 	//
-	// The runtime startup code refers to the variable, which keeps it live in all binaries.
-	return []byte(fmt.Sprintf(`package main
+	// The runtime startup code refers to the variable, which keeps it live
+	// in all binaries.
+	//
+	// Note: we use an alternate recipe below for gccgo (based on an
+	// init function) due to the fact that gccgo does not support
+	// applying a "//go:linkname" directive to a variable. This has
+	// drawbacks in that other packages may want to look at the module
+	// info in their init functions (see issue 29628), which won't
+	// work for gccgo. See also issue 30344.
+
+	if !isgccgo {
+		return []byte(fmt.Sprintf(`package main
 import _ "unsafe"
 //go:linkname __debug_modinfo__ runtime.modinfo
 var __debug_modinfo__ = %q
 	`, string(infoStart)+info+string(infoEnd)))
+	} else {
+		return []byte(fmt.Sprintf(`package main
+import _ "unsafe"
+//go:linkname __set_debug_modinfo__ runtime..z2fdebug.setmodinfo
+func __set_debug_modinfo__(string)
+func init() { __set_debug_modinfo__(%q) }
+	`, string(infoStart)+info+string(infoEnd)))
+	}
 }

src/cmd/go/internal/work/exec.go

@@ -663,7 +663,7 @@ func (b *Builder) build(a *Action) (err error) {
 	}
 
 	if p.Internal.BuildInfo != "" && cfg.ModulesEnabled {
-		if err := b.writeFile(objdir+"_gomod_.go", load.ModInfoProg(p.Internal.BuildInfo)); err != nil {
+		if err := b.writeFile(objdir+"_gomod_.go", load.ModInfoProg(p.Internal.BuildInfo, cfg.BuildToolchainName == "gccgo")); err != nil {
 			return err
 		}
 		gofiles = append(gofiles, objdir+"_gomod_.go")