go/tools/builders/pack: preserve file extensions (#1899)

The .a format limits file names to 16 characters. There are various
extensions to this format which support longer names, but Go doesn't
recognize them and generally doesn't need them. Before Go 1.12, the
linker passed all extra files (other than the compiled Go code and the
export data) to the external link.

In Go 1.12, the linker only passes on .o files. The GoPack action
simply truncated long file names, which meant .o files were sometimes
ignored. GoPack will now preserve file extensions.

Updates #1896

go/private/rules/cgo.bzl

@@ -188,7 +188,6 @@ def _cgo_codegen_impl(ctx):
         mangled_stem, src_ext = _mangle(src, stems)
         gen_file = go.declare_file(go, path = mangled_stem + ".cgo1." + src_ext)
         transformed_go_outs.append(gen_file)
-        transformed_go_map[gen_go_file] = src
         builder_args.add("-src", gen_file.path + "=" + src.path)
     for src in source.c:
         mangled_stem, src_ext = _mangle(src, stems)

go/tools/builders/pack.go

@@ -140,8 +140,10 @@ func extractFiles(archive, dir string, names map[string]struct{}) (files []strin
 			}
 			continue
 		}
-		name = simpleName(name, names)
-		names[name] = struct{}{}
+		name, err = simpleName(name, names)
+		if err != nil {
+			return nil, err
+		}
 		name = filepath.Join(dir, name)
 		if err := extractFile(r, name, size); err != nil {
 			return nil, err
@@ -281,30 +283,36 @@ func isObjectFile(name string) bool {
 
 // simpleName returns a file name which is at most 15 characters
 // and doesn't conflict with other names. If it is not possible to choose
-// such a name, simpleName will truncate the given name to 15 characters
-func simpleName(name string, names map[string]struct{}) string {
+// such a name, simpleName will truncate the given name to 15 characters.
+// The original file extension will be preserved.
+func simpleName(name string, names map[string]struct{}) (string, error) {
 	if _, ok := names[name]; !ok && len(name) < 16 {
-		return name
+		names[name] = struct{}{}
+		return name, nil
 	}
 	var stem, ext string
-	if i := strings.LastIndexByte(name, '.'); i < 0 || len(name)-i >= 10 {
+	if i := strings.LastIndexByte(name, '.'); i < 0 {
 		stem = name
 	} else {
-		stem = name[:i]
+		stem = strings.Replace(name[:i], ".", "_", -1)
 		ext = name[i:]
 	}
-	for n := 0; n < len(names); n++ {
+	for n := 0; n < len(names)+1; n++ {
 		ns := strconv.Itoa(n)
 		stemLen := 15 - len(ext) - len(ns)
+		if stemLen < 0 {
+			break
+		}
 		if stemLen > len(stem) {
 			stemLen = len(stem)
 		}
 		candidate := stem[:stemLen] + ns + ext
 		if _, ok := names[candidate]; !ok {
-			return candidate
+			names[candidate] = struct{}{}
+			return candidate, nil
 		}
 	}
-	return name[:15]
+	return "", fmt.Errorf("cannot shorten file name: %q", name)
 }
 
 func appendFiles(goenv *env, archive string, files []string) error {