cmd/api: don’t rely on hardcoded go versions

Instead of requiring that cmd/api/run.go be edited upon each
release to include the next Go version number, look in $GOROOT/api
for files with the prefix go1* and use those instead to perform
API checks.

Change-Id: I5d9407f2bd368ff5e62f487cccdd245641ca9c9b
Reviewed-on: https://go-review.googlesource.com/83355Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/api/run.go

@@ -15,6 +15,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"strings"
 )
 
 func goCmd() string {
@@ -38,21 +39,34 @@ func main() {
 		log.Fatal("No $GOROOT set.")
 	}
 
+	apiDir := filepath.Join(goroot, "api")
 	out, err := exec.Command(goCmd(), "tool", "api",
-		"-c", file("go1", "go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7", "go1.8", "go1.9", "go1.10"),
-		"-next", file("next"),
-		"-except", file("except")).CombinedOutput()
+		"-c", findAPIDirFiles(apiDir),
+		"-next", filepath.Join(apiDir, "next.txt"),
+		"-except", filepath.Join(apiDir, "except.txt")).CombinedOutput()
 	if err != nil {
 		log.Fatalf("Error running API checker: %v\n%s", err, out)
 	}
 	fmt.Print(string(out))
 }
 
-// file expands s to $GOROOT/api/s.txt.
-// If there are more than 1, they're comma-separated.
-func file(s ...string) string {
-	if len(s) > 1 {
-		return file(s[0]) + "," + file(s[1:]...)
+// findAPIDirFiles returns a comma-separated list of Go API files
+// (go1.txt, go1.1.txt, etc.) located in apiDir.
+func findAPIDirFiles(apiDir string) string {
+	dir, err := os.Open(apiDir)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer dir.Close()
+	fs, err := dir.Readdirnames(-1)
+	if err != nil {
+		log.Fatal(err)
+	}
+	var apiFiles []string
+	for _, fn := range fs {
+		if strings.HasPrefix(fn, "go1") {
+			apiFiles = append(apiFiles, filepath.Join(apiDir, fn))
+		}
 	}
-	return filepath.Join(goroot, "api", s[0]+".txt")
+	return strings.Join(apiFiles, ",")
 }