cmd/api: add exception file

Fixes build.

R=golang-dev, adg, bradfitz, dsymonds, dave
CC=golang-dev
https://golang.org/cl/6586074

api/README

@@ -5,6 +5,9 @@ Each file is a list of of API features, one per line.
 go1.txt (and similarly named files) are frozen once a version has been
 shipped. Each file adds new lines but does not remove any.
 
+except.txt lists features that may disappear without breaking
+true compatibility. The only package there is text/template/parse.
+
 next.txt is the only file intended to be mutated. It's a list of
 features that may be added to the next version. It only affects
 warning output from the go api tool.

api/except.txt

@@ -0,0 +1,2 @@
+pkg text/template/parse, type DotNode bool
+pkg text/template/parse, type Node interface { Copy, String, Type }

src/cmd/api/goapi.go

@@ -40,6 +40,7 @@ var (
 	// list of files, rather than just one.
 	checkFile  = flag.String("c", "", "optional filename to check API against")
 	allowNew   = flag.Bool("allow_new", true, "allow API additions")
+	exceptFile = flag.String("except", "", "optional filename of packages that are allowed to change without triggering a failure in the tool")
 	nextFile   = flag.String("next", "", "optional filename of tentative upcoming API features for the next release. This file can be lazily maintained. It only affects the delta warnings from the -c file printed on success.")
 	verbose    = flag.Bool("v", false, "verbose debugging")
 	forceCtx   = flag.String("contexts", "", "optional comma-separated list of <goos>-<goarch>[-cgo] to override default contexts.")
@@ -198,6 +199,13 @@ func main() {
 		}
 	}
 
+	var exception = make(map[string]bool) // exception => true
+	if *exceptFile != "" {
+		for _, feature := range fileFeatures(*exceptFile) {
+			exception[feature] = true
+		}
+	}
+
 	take := func(sl *[]string) string {
 		s := (*sl)[0]
 		*sl = (*sl)[1:]
@@ -207,8 +215,13 @@ func main() {
 	for len(required) > 0 || len(features) > 0 {
 		switch {
 		case len(features) == 0 || required[0] < features[0]:
-			fmt.Fprintf(bw, "-%s\n", take(&required))
+			feature := take(&required)
+			if exception[feature] {
+				fmt.Fprintf(bw, "~%s\n", feature)
+			} else {
+				fmt.Fprintf(bw, "-%s\n", feature)
 				fail = true // broke compatibility
+			}
 		case len(required) == 0 || required[0] > features[0]:
 			newFeature := take(&features)
 			if optional[newFeature] {

src/run.bash

@@ -112,7 +112,7 @@ time go run run.go
 
 echo
 echo '# Checking API compatibility.'
-go tool api -c $GOROOT/api/go1.txt -next $GOROOT/api/next.txt
+go tool api -c $GOROOT/api/go1.txt -next $GOROOT/api/next.txt -except $GOROOT/api/except.txt
 
 echo
 echo ALL TESTS PASSED