Add a persistent bazel server between builds

This feature is toggled on with USE_PERSISTENT_BAZEL, which is off by
default. Those that opt-in will have a bazel server running between
builds (with a 3hr default TTL) which will greatly improve analysis on
subsequent builds. (As Bazel maintains a cache of analysis results).

Bug: 266983462
Test: Manual `m nothing` runs (timing with and without the feature)
Test: New integration test
Test: Presubmits
Change-Id: I3af4948baa0c490e9b87c48ffdbe9f67732586c7
diff --git a/android/bazel_handler.go b/android/bazel_handler.go
index e7ff08f..e7b84e3 100644
--- a/android/bazel_handler.go
+++ b/android/bazel_handler.go
@@ -607,7 +607,7 @@
 	dclaEnabledModules := map[string]bool{}
 	addToStringSet(dclaEnabledModules, dclaMixedBuildsEnabledList)
 	return &mixedBuildBazelContext{
-		bazelRunner:             &builtinBazelRunner{},
+		bazelRunner:             &builtinBazelRunner{c.UseBazelProxy, absolutePath(c.outDir)},
 		paths:                   &paths,
 		modulesDefaultToBazel:   c.BuildMode == BazelDevMode,
 		bazelEnabledModules:     enabledModules,
@@ -684,23 +684,46 @@
 	return "", "", nil
 }
 
-type builtinBazelRunner struct{}
+type builtinBazelRunner struct {
+	useBazelProxy bool
+	outDir        string
+}
 
 // Issues the given bazel command with given build label and additional flags.
 // Returns (stdout, stderr, error). The first and second return values are strings
 // containing the stdout and stderr of the run command, and an error is returned if
 // the invocation returned an error code.
 func (r *builtinBazelRunner) issueBazelCommand(bazelCmd *exec.Cmd, eventHandler *metrics.EventHandler) (string, string, error) {
-	eventHandler.Begin("bazel command")
-	defer eventHandler.End("bazel command")
-	stderr := &bytes.Buffer{}
-	bazelCmd.Stderr = stderr
-	if output, err := bazelCmd.Output(); err != nil {
-		return "", string(stderr.Bytes()),
-			fmt.Errorf("bazel command failed: %s\n---command---\n%s\n---env---\n%s\n---stderr---\n%s---",
-				err, bazelCmd, strings.Join(bazelCmd.Env, "\n"), stderr)
+	if r.useBazelProxy {
+		eventHandler.Begin("client_proxy")
+		defer eventHandler.End("client_proxy")
+		proxyClient := bazel.NewProxyClient(r.outDir)
+		// Omit the arg containing the Bazel binary, as that is handled by the proxy
+		// server.
+		bazelFlags := bazelCmd.Args[1:]
+		// TODO(b/270989498): Refactor these functions to not take exec.Cmd, as its
+		// not actually executed for client proxying.
+		resp, err := proxyClient.IssueCommand(bazel.CmdRequest{bazelFlags, bazelCmd.Env})
+
+		if err != nil {
+			return "", "", err
+		}
+		if len(resp.ErrorString) > 0 {
+			return "", "", fmt.Errorf(resp.ErrorString)
+		}
+		return resp.Stdout, resp.Stderr, nil
 	} else {
-		return string(output), string(stderr.Bytes()), nil
+		eventHandler.Begin("bazel command")
+		defer eventHandler.End("bazel command")
+		stderr := &bytes.Buffer{}
+		bazelCmd.Stderr = stderr
+		if output, err := bazelCmd.Output(); err != nil {
+			return "", string(stderr.Bytes()),
+				fmt.Errorf("bazel command failed: %s\n---command---\n%s\n---env---\n%s\n---stderr---\n%s---",
+					err, bazelCmd, strings.Join(bazelCmd.Env, "\n"), stderr)
+		} else {
+			return string(output), string(stderr.Bytes()), nil
+		}
 	}
 }