godoc: make ?m=src mode deterministic

Merge package files in the go/ast MergePackageFiles
function always	    in the same order (sorted by filename)
instead	 of map iteration order to obtain the same
package	 file each time.  This functionality is used
by godoc when displaying packages in ?m=src mode.

Also: minor cleanup in godoc.go.

R=rsc
CC=golang-dev
https://golang.org/cl/5540054

src/cmd/godoc/godoc.go

@@ -1086,18 +1086,18 @@ func (h *httpHandler) getPageInfo(abspath, relpath, pkgname string, mode PageInf
 	var past *ast.File
 	var pdoc *doc.Package
 	if pkg != nil {
-		var docMode doc.Mode
-		if mode&noFiltering != 0 {
-			docMode = doc.AllDecls
-		}
 		if mode&showSource == 0 {
 			// show extracted documentation
-			pdoc = doc.New(pkg, path.Clean(relpath), docMode) // no trailing '/' in importpath
+			var m doc.Mode
+			if mode&noFiltering != 0 {
+				m = doc.AllDecls
+			}
+			pdoc = doc.New(pkg, path.Clean(relpath), m) // no trailing '/' in importpath
 		} else {
 			// show source code
 			// TODO(gri) Consider eliminating export filtering in this mode,
 			//           or perhaps eliminating the mode altogether.
-			if docMode&doc.AllDecls == 0 {
+			if mode&noFiltering == 0 {
 				ast.PackageExports(pkg)
 			}
 			past = ast.MergePackageFiles(pkg, ast.FilterUnassociatedComments)

src/pkg/go/ast/filter.go

@@ -4,7 +4,10 @@
 
 package ast
 
-import "go/token"
+import (
+	"go/token"
+	"sort"
+)
 
 // ----------------------------------------------------------------------------
 // Export filtering
@@ -291,29 +294,35 @@ var separator = &Comment{noPos, "//"}
 //
 func MergePackageFiles(pkg *Package, mode MergeMode) *File {
 	// Count the number of package docs, comments and declarations across
-	// all package files.
+	// all package files. Also, compute sorted list of filenames, so that
+	// subsequent iterations can always iterate in the same order.
 	ndocs := 0
 	ncomments := 0
 	ndecls := 0
-	for _, f := range pkg.Files {
+	filenames := make([]string, len(pkg.Files))
+	i := 0
+	for filename, f := range pkg.Files {
+		filenames[i] = filename
+		i++
 		if f.Doc != nil {
 			ndocs += len(f.Doc.List) + 1 // +1 for separator
 		}
 		ncomments += len(f.Comments)
 		ndecls += len(f.Decls)
 	}
+	sort.Strings(filenames)
 
 	// Collect package comments from all package files into a single
-	// CommentGroup - the collected package documentation. The order
-	// is unspecified. In general there should be only one file with
-	// a package comment; but it's better to collect extra comments
-	// than drop them on the floor.
+	// CommentGroup - the collected package documentation. In general
+	// there should be only one file with a package comment; but it's
+	// better to collect extra comments than drop them on the floor.
 	var doc *CommentGroup
 	var pos token.Pos
 	if ndocs > 0 {
 		list := make([]*Comment, ndocs-1) // -1: no separator before first group
 		i := 0
-		for _, f := range pkg.Files {
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
 			if f.Doc != nil {
 				if i > 0 {
 					// not the first group - add separator
@@ -342,7 +351,8 @@ func MergePackageFiles(pkg *Package, mode MergeMode) *File {
 		funcs := make(map[string]int) // map of global function name -> decls index
 		i := 0                        // current index
 		n := 0                        // number of filtered entries
-		for _, f := range pkg.Files {
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
 			for _, d := range f.Decls {
 				if mode&FilterFuncDuplicates != 0 {
 					// A language entity may be declared multiple
@@ -398,7 +408,8 @@ func MergePackageFiles(pkg *Package, mode MergeMode) *File {
 	var imports []*ImportSpec
 	if mode&FilterImportDuplicates != 0 {
 		seen := make(map[string]bool)
-		for _, f := range pkg.Files {
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
 			for _, imp := range f.Imports {
 				if path := imp.Path.Value; !seen[path] {
 					// TODO: consider handling cases where: