build: Allow overriden functions calls

Before this commit, every overriden standard library function was renamed '_'. Now, if there is the following directive in the function comment: "//gopherjs:keep_overridden", they are prefixed with "_gopherjs_overridden_" so that they can be called from the overriding function. This is typically useful to use the standard library implementation as a fallback.
gopherjs · r-l-x · Dec 21, 2016 · Dec 21, 2016 · Dec 28, 2016 · Dec 29, 2016
commit 24a2d2dc0863bf03db1cdf15941a8f6e2635f787
diff --git a/build/build.go b/build/build.go
@@ -187,6 +187,23 @@ func ImportDir(dir string, mode build.ImportMode, installSuffix string, buildTag
 	return &PackageData{Package: pkg, JSFiles: jsFiles}, nil
 }
 
+// Test if we find the '//gopherjs:keep_overridden' comment
+func findKeepOverridenComment(doc *ast.CommentGroup) bool {
+	if doc == nil {
+		return false
+	}
+	for _, comment := range doc.List {
+		text := comment.Text
+		if i := strings.Index(text, " "); i >= 0 {
+			text = text[:i]
+		}
+		if text == "//gopherjs:keep_overridden" {
+			return true
+		}
+	}
+	return false
+}
+
 // parseAndAugment parses and returns all .go files of given pkg.
 // Standard Go library packages are augmented with files in compiler/natives folder.
 // If isTest is true and pkg.ImportPath has no _test suffix, package is built for running internal tests.
@@ -195,12 +212,19 @@ func ImportDir(dir string, mode build.ImportMode, installSuffix string, buildTag
 // The native packages are augmented by the contents of natives.FS in the following way.
 // The file names do not matter except the usual `_test` suffix. The files for
 // native overrides get added to the package (even if they have the same name
-// as an existing file from the standard library). For all identifiers that exist
-// in the original AND the overrides, the original identifier in the AST gets
-// replaced by `_`. New identifiers that don't exist in original package get added.
+// as an existing file from the standard library). For function identifiers that exist
+// in the original AND the overrides AND that include the following directive in their comment:
+// //gopherjs:keep_overridden, the original identifier in the AST gets prefixed by
+// `_gopherjs_overridden_`. For other identifiers that exist in the original AND the overrides,
+// the original identifier gets replaced by `_`. New identifiers that don't exist in original
+// package get added.
 func parseAndAugment(pkg *build.Package, isTest bool, fileSet *token.FileSet) ([]*ast.File, error) {
 	var files []*ast.File
-	replacedDeclNames := make(map[string]bool)
+
+	type overrideInfo struct {
+		keepOverriden bool
+	}
+	replacedDeclNames := make(map[string]overrideInfo)
 	funcName := func(d *ast.FuncDecl) string {
 		if d.Recv == nil || len(d.Recv.List) == 0 {
 			return d.Name.Name
@@ -279,17 +303,17 @@ func parseAndAugment(pkg *build.Package, isTest bool, fileSet *token.FileSet) ([
 			for _, decl := range file.Decls {
 				switch d := decl.(type) {
 				case *ast.FuncDecl:
-					replacedDeclNames[funcName(d)] = true
+					replacedDeclNames[funcName(d)] = overrideInfo{keepOverriden: findKeepOverridenComment(d.Doc)}
 				case *ast.GenDecl:
 					switch d.Tok {
 					case token.TYPE:
 						for _, spec := range d.Specs {
-							replacedDeclNames[spec.(*ast.TypeSpec).Name.Name] = true
+							replacedDeclNames[spec.(*ast.TypeSpec).Name.Name] = overrideInfo{}
 						}
 					case token.VAR, token.CONST:
 						for _, spec := range d.Specs {
 							for _, name := range spec.(*ast.ValueSpec).Names {
-								replacedDeclNames[name.Name] = true
+								replacedDeclNames[name.Name] = overrideInfo{}
 							}
 						}
 					}
@@ -341,23 +365,29 @@ func parseAndAugment(pkg *build.Package, isTest bool, fileSet *token.FileSet) ([
 		for _, decl := range file.Decls {
 			switch d := decl.(type) {
 			case *ast.FuncDecl:
-				if replacedDeclNames[funcName(d)] {
-					d.Name = ast.NewIdent("_")
+				if info, ok := replacedDeclNames[funcName(d)]; ok {
+					if info.keepOverriden {
+						// Allow overridden function calls
+						// The standard library implementation of foo() becomes _gopherjs_overridden_foo()
+						d.Name.Name = "_gopherjs_overridden_" + d.Name.Name
+					} else {
+						d.Name = ast.NewIdent("_")
+					}
 				}
 			case *ast.GenDecl:
 				switch d.Tok {
 				case token.TYPE:
 					for _, spec := range d.Specs {
 						s := spec.(*ast.TypeSpec)
-						if replacedDeclNames[s.Name.Name] {
+						if _, ok := replacedDeclNames[s.Name.Name]; ok {
 							s.Name = ast.NewIdent("_")
 						}
 					}
 				case token.VAR, token.CONST:
 					for _, spec := range d.Specs {
 						s := spec.(*ast.ValueSpec)
 						for i, name := range s.Names {
-							if replacedDeclNames[name.Name] {
+							if _, ok := replacedDeclNames[name.Name]; ok {
 								s.Names[i] = ast.NewIdent("_")
 							}
 						}