implement USE_BAZEL_VERSION for m

This allows USE_BAZEL_VERSION to be set for m builds, which will use
Bazelisk for any Bazel invocations during those builds.

This should be used only for manual debugging, typically to either test
new Bazel features, verify Bazel compatibility with Android, or culprit
find new Bazel breakages.

Test: Manually run builds with USE_BAZEL_VERSION, toggled off and on to
ensure the build was rerun. Tested with a broken commit, a working
commit, and 'last_green' special term

Change-Id: I8b475dca5c8d4bd849ee3724a8c3aca9b631bcb8

bazel/bazel_proxy.go

@@ -26,10 +26,11 @@ import (
 	"time"
 )
 
-// Logs fatal events of ProxyServer.
+// Logs events of ProxyServer.
 type ServerLogger interface {
 	Fatal(v ...interface{})
 	Fatalf(format string, v ...interface{})
+	Println(v ...interface{})
 }
 
 // CmdRequest is a request to the Bazel Proxy server.
@@ -71,9 +72,10 @@ type ProxyClient struct {
 // The ProxyServer will only live as long as soong_ui does; the
 // underlying Bazel server will live past the duration of the build.
 type ProxyServer struct {
-	logger       ServerLogger
-	outDir       string
-	workspaceDir string
+	logger          ServerLogger
+	outDir          string
+	workspaceDir    string
+	bazeliskVersion string
 	// The server goroutine will listen on this channel and stop handling requests
 	// once it is written to.
 	done chan struct{}
@@ -119,12 +121,17 @@ func (b *ProxyClient) IssueCommand(req CmdRequest) (CmdResponse, error) {
 }
 
 // NewProxyServer is a constructor for a ProxyServer.
-func NewProxyServer(logger ServerLogger, outDir string, workspaceDir string) *ProxyServer {
+func NewProxyServer(logger ServerLogger, outDir string, workspaceDir string, bazeliskVersion string) *ProxyServer {
+	if len(bazeliskVersion) > 0 {
+		logger.Println("** Using Bazelisk for this build, due to env var USE_BAZEL_VERSION=" + bazeliskVersion + " **")
+	}
+
 	return &ProxyServer{
-		logger:       logger,
-		outDir:       outDir,
-		workspaceDir: workspaceDir,
-		done:         make(chan struct{}),
+		logger:          logger,
+		outDir:          outDir,
+		workspaceDir:    workspaceDir,
+		done:            make(chan struct{}),
+		bazeliskVersion: bazeliskVersion,
 	}
 }
 
@@ -155,6 +162,9 @@ func (b *ProxyServer) handleRequest(conn net.Conn) error {
 		return fmt.Errorf("Error decoding request: %s", err)
 	}
 
+	if len(b.bazeliskVersion) > 0 {
+		req.Env = append(req.Env, "USE_BAZEL_VERSION="+b.bazeliskVersion)
+	}
 	stdout, stderr, cmdErr := ExecBazel("./build/bazel/bin/bazel", b.workspaceDir, req)
 	errorString := ""
 	if cmdErr != nil {