build: change how cmd/api is run in run.bash and run.bat

In prep for Robert's forthcoming cmd/api rewrite which
depends on the go.tools subrepo, we'll need to be more
careful about how and when we run cmd/api.

Rather than implement this policy in both run.bash and
run.bat, this change moves the policy and mechanism into
cmd/api/run.go, which will then evolve.

The plan is in a TODO in run.go.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/12482044

src/cmd/api/run.go

@@ -0,0 +1,65 @@
+// +build from_src_run
+
+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The run program is invoked via "go run" from src/run.bash or
+// src/run.bat conditionally builds and runs the cmd/api tool.
+//
+// TODO(bradfitz): the "conditional" condition is always true.
+// We should only do this if the user has the hg codereview extension
+// enabled and verifies that the go.tools subrepo is checked out with
+// a suitably recently version. In prep for the cmd/api rewrite.
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+)
+
+var goroot string
+
+func main() {
+	log.SetFlags(0)
+	goroot = os.Getenv("GOROOT") // should be set by run.{bash,bat}
+	if goroot == "" {
+		log.Fatal("No $GOROOT set.")
+	}
+	isGoDeveloper := exec.Command("hg", "pq").Run() == nil
+	if !isGoDeveloper && !forceAPICheck() {
+		fmt.Println("Skipping cmd/api checks; hg codereview extension not available and GO_FORCE_API_CHECK not set")
+		return
+	}
+
+	out, err := exec.Command("go", "install", "--tags=api_tool", "cmd/api").CombinedOutput()
+	if err != nil {
+		log.Fatalf("Error installing cmd/api: %v\n%s", err, out)
+	}
+	out, err = exec.Command("go", "tool", "api",
+		"-c", file("go1", "go1.1"),
+		"-next", file("next"),
+		"-except", file("except")).CombinedOutput()
+	if err != nil {
+		log.Fatalf("Error running API checker: %v\n%s", err, 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:]...)
+	}
+	return filepath.Join(goroot, "api", s[0]+".txt")
+}
+
+// GO_FORCE_API_CHECK is set by builders.
+func forceAPICheck() bool {
+	v, _ := strconv.ParseBool(os.Getenv("GO_FORCE_API_CHECK"))
+	return v
+}