builder: support appending to non-existant archive

go/tools/builders/compilepkg.go

@@ -404,11 +404,11 @@ func compileArchive(
 	// nogo facts file (if there is one). This allows compile actions to depend
 	// on .x files only, so we don't need to recompile a package when one of its
 	// imports changes in a way that doesn't affect export data.
-	// TODO(golang/go#33820): Ideally, we would use -linkobj to tell the compiler
-	// to create separate .a and .x files for compiled code and export data, then
-	// copy the nogo facts into the .x file. Unfortunately, when building a plugin,
-	// the linker needs export data in the .a file. To work around this, we copy
-	// the export data into the .x file ourselves.
+	// TODO(golang/go#33820): After Go 1.16 is the minimum supported version,
+	// use -linkobj to tell the compiler to create separate .a and .x files for
+	// compiled code and export data. Before that version, the linker needed
+	// export data in the .a file when building a plugin. To work around that,
+	// we copy the export data into .x ourselves.
 	if err = extractFileFromArchive(outPath, workDir, pkgDef); err != nil {
 		return err
 	}

go/tools/builders/pack.go

@@ -347,6 +347,23 @@ func simpleName(name string, names map[string]struct{}) (string, error) {
 }
 
 func appendFiles(goenv *env, archive string, files []string) error {
+	// Create an empty archive if one doesn't already exist.
+	// In Go 1.16, 'go tool pack r' reports an error if the archive doesn't exist.
+	// 'go tool pack c' copies export data in addition to creating the archive,
+	// so we don't want to use that directly.
+	_, err := os.Stat(archive)
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+	if os.IsNotExist(err) {
+		if err := ioutil.WriteFile(archive, []byte(arHeader), 0666); err != nil {
+			return err
+		}
+	}
+
+	// Append files to the archive.
+	// TODO(jayconrod): copy cmd/internal/archive and use that instead of
+	// shelling out to cmd/pack.
 	args := goenv.goTool("pack", "r", archive)
 	args = append(args, files...)
 	return goenv.runCommand(args)