cmd/link: actually generate .debug_gdb_scripts section on windows

Adjust finddebugruntimepath to look for runtime/debug.go file
instead of runtime/runtime.go. This actually finds runtime.GOMAXPROCS
in every Go executable (including windows).

I also included "-Wl,-T,fix_debug_gdb_scripts.ld" parameter to gcc
invocation on windows to work around gcc bug (see #20183 for details).

This CL only fixes windows -buildmode=exe, buildmode=c-archive
is still broken.

Thanks to Egon Elbre and Nick Clifton for investigation.

Fixes #20183
Fixes #20218

Change-Id: I5369a4db3913226aef3d9bd6317446856b0a1c34
Reviewed-on: https://go-review.googlesource.com/43331Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/dwarf.go

@@ -872,7 +872,7 @@ func finddebugruntimepath(s *Symbol) {
 
 	for i := range s.FuncInfo.File {
 		f := s.FuncInfo.File[i]
-		if i := strings.Index(f.Name, "runtime/runtime.go"); i >= 0 {
+		if i := strings.Index(f.Name, "runtime/debug.go"); i >= 0 {
 			gdbscript = f.Name[:i] + "runtime/runtime-gdb.py"
 			break
 		}
@@ -1450,9 +1450,13 @@ func writearanges(ctxt *Link, syms []*Symbol) []*Symbol {
 }
 
 func writegdbscript(ctxt *Link, syms []*Symbol) []*Symbol {
-	if Linkmode == LinkExternal && Headtype == objabi.Hwindows {
+	if Linkmode == LinkExternal && Headtype == objabi.Hwindows && Buildmode == BuildmodeCArchive {
 		// gcc on Windows places .debug_gdb_scripts in the wrong location, which
 		// causes the program not to run. See https://golang.org/issue/20183
+		// Non c-archives can avoid this issue via a linker script
+		// (see fix near writeGDBLinkerScript).
+		// c-archive users would need to specify the linker script manually.
+		// For UX it's better not to deal with this.
 		return syms
 	}
 

src/cmd/link/internal/ld/lib.go

@@ -986,6 +986,29 @@ func hostobjCopy() (paths []string) {
 	return paths
 }
 
+// writeGDBLinkerScript creates gcc linker script file in temp
+// directory. writeGDBLinkerScript returns created file path.
+// The script is used to work around gcc bug
+// (see https://golang.org/issue/20183 for details).
+func writeGDBLinkerScript() string {
+	name := "fix_debug_gdb_scripts.ld"
+	path := filepath.Join(*flagTmpdir, name)
+	src := `SECTIONS
+{
+  .debug_gdb_scripts BLOCK(__section_alignment__) (NOLOAD) :
+  {
+    *(.debug_gdb_scripts)
+  }
+}
+INSERT AFTER .debug_types;
+`
+	err := ioutil.WriteFile(path, []byte(src), 0666)
+	if err != nil {
+		Errorf(nil, "WriteFile %s failed: %v", name, err)
+	}
+	return path
+}
+
 // archive builds a .a archive from the hostobj object files.
 func (ctxt *Link) archive() {
 	if Buildmode != BuildmodeCArchive {
@@ -1247,6 +1270,10 @@ func (l *Link) hostlink() {
 		}
 	}
 	if Headtype == objabi.Hwindows {
+		// use gcc linker script to work around gcc bug
+		// (see https://golang.org/issue/20183 for details).
+		p := writeGDBLinkerScript()
+		argv = append(argv, "-Wl,-T,"+p)
 		// libmingw32 and libmingwex have some inter-dependencies,
 		// so must use linker groups.
 		argv = append(argv, "-Wl,--start-group", "-lmingwex", "-lmingw32", "-Wl,--end-group")

src/debug/pe/file_test.go

@@ -363,6 +363,16 @@ func testDWARF(t *testing.T, linktype int) {
 	}
 	defer f.Close()
 
+	var foundDebugGDBScriptsSection bool
+	for _, sect := range f.Sections {
+		if sect.Name == ".debug_gdb_scripts" {
+			foundDebugGDBScriptsSection = true
+		}
+	}
+	if !foundDebugGDBScriptsSection {
+		t.Error(".debug_gdb_scripts section is not found")
+	}
+
 	d, err := f.DWARF()
 	if err != nil {
 		t.Fatal(err)