feat: add fetching flags from bazel (#18)

diff --git a/WORKSPACE b/WORKSPACE
index 1e8310f..6156665 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -43,6 +43,19 @@
     ],
 )
 
+http_archive(
+    name = "rules_proto",
+    sha256 = "9fc210a34f0f9e7cc31598d109b5d069ef44911a82f507d5a88716db171615a8",
+    strip_prefix = "rules_proto-f7a30f6f80006b591fa7c437fe5a951eb10bcbcf",
+    urls = ["https://github.com/bazelbuild/rules_proto/archive/f7a30f6f80006b591fa7c437fe5a951eb10bcbcf.tar.gz"],
+)
+
+load("@rules_proto//proto:repositories.bzl", "rules_proto_dependencies", "rules_proto_toolchains")
+
+rules_proto_dependencies()
+
+rules_proto_toolchains()
+
 load("@bazel_gazelle//:deps.bzl", "gazelle_dependencies")
 load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
 load("//:go.bzl", _go_repositories = "deps")
diff --git a/go.mod b/go.mod
index 2805070..95b6572 100644
--- a/go.mod
+++ b/go.mod
@@ -25,5 +25,6 @@
 	golang.org/x/text v0.3.7 // indirect
 	google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013
 	google.golang.org/grpc v1.27.0
+	google.golang.org/protobuf v1.26.0
 	gopkg.in/ini.v1 v1.62.0 // indirect
 )
diff --git a/pkg/bazel/BUILD.bazel b/pkg/bazel/BUILD.bazel
index f1ad4b9..21a9a6f 100644
--- a/pkg/bazel/BUILD.bazel
+++ b/pkg/bazel/BUILD.bazel
@@ -1,16 +1,25 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
+load("@rules_proto//proto:defs.bzl", "proto_library")
 
 go_library(
     name = "bazel",
     srcs = [
         "bazel.go",
+        "bazelisk.go",
         "flags.go",
     ],
+    embed = [":bazel_go_proto"],
     importpath = "aspect.build/cli/pkg/bazel",
     visibility = ["//:__subpackages__"],
     deps = [
         "@com_github_bazelbuild_bazelisk//core:go_default_library",
+        "@com_github_bazelbuild_bazelisk//httputil:go_default_library",
+        "@com_github_bazelbuild_bazelisk//platforms:go_default_library",
         "@com_github_bazelbuild_bazelisk//repositories:go_default_library",
+        "@com_github_bazelbuild_bazelisk//versions:go_default_library",
+        "@com_github_mitchellh_go_homedir//:go-homedir",
+        "@org_golang_google_protobuf//proto",
     ],
 )
 
@@ -22,3 +31,16 @@
         "@com_github_onsi_gomega//:gomega",
     ],
 )
+
+proto_library(
+    name = "bazel_proto",
+    srcs = ["flags.proto"],
+    visibility = ["//visibility:public"],
+)
+
+go_proto_library(
+    name = "bazel_go_proto",
+    importpath = "aspect.build/cli/pkg/bazel",
+    proto = ":bazel_proto",
+    visibility = ["//visibility:public"],
+)
diff --git a/pkg/bazel/bazel.go b/pkg/bazel/bazel.go
index b45948c..894cbd6 100644
--- a/pkg/bazel/bazel.go
+++ b/pkg/bazel/bazel.go
@@ -9,6 +9,7 @@
 import (
 	"github.com/bazelbuild/bazelisk/core"
 	"github.com/bazelbuild/bazelisk/repositories"
+	"io"
 )
 
 type Spawner interface {
@@ -22,15 +23,22 @@
 	return &Bazel{}
 }
 
-// Spawn is similar to the main() function of bazelisk
-// see https://github.com/bazelbuild/bazelisk/blob/7c3d9d5/bazelisk.go
-func (*Bazel) Spawn(command []string) (int, error) {
+func (*Bazel) createRepositories() *core.Repositories {
 	gcs := &repositories.GCSRepo{}
 	gitHub := repositories.CreateGitHubRepo(core.GetEnvOrConfig("BAZELISK_GITHUB_TOKEN"))
 	// Fetch LTS releases, release candidates and Bazel-at-commits from GCS, forks and rolling releases from GitHub.
 	// TODO(https://github.com/bazelbuild/bazelisk/issues/228): get rolling releases from GCS, too.
-	repos := core.CreateRepositories(gcs, gcs, gitHub, gcs, gitHub, true)
+	return core.CreateRepositories(gcs, gcs, gitHub, gcs, gitHub, true)
+}
 
-	exitCode, err := core.RunBazelisk(command, repos)
+// Spawn is similar to the main() function of bazelisk
+// see https://github.com/bazelbuild/bazelisk/blob/7c3d9d5/bazelisk.go
+func (b *Bazel) Spawn(command []string) (int, error) {
+	return b.RunCommand(command, nil)
+}
+
+func (b *Bazel) RunCommand(command []string, out io.Writer) (int, error) {
+	repos := b.createRepositories()
+	exitCode, err := RunBazelisk(command, repos, out)
 	return exitCode, err
 }
diff --git a/pkg/bazel/bazelisk.go b/pkg/bazel/bazelisk.go
new file mode 100644
index 0000000..ced2bb9
--- /dev/null
+++ b/pkg/bazel/bazelisk.go
@@ -0,0 +1,600 @@
+package bazel
+
+import (
+	"bufio"
+	"fmt"
+	"github.com/bazelbuild/bazelisk/core"
+	"github.com/bazelbuild/bazelisk/httputil"
+	"github.com/bazelbuild/bazelisk/platforms"
+	"github.com/bazelbuild/bazelisk/versions"
+	"github.com/mitchellh/go-homedir"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"os/signal"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"sort"
+	"strings"
+	"sync"
+	"syscall"
+)
+
+const (
+	bazelReal      = "BAZEL_REAL"
+	skipWrapperEnv = "BAZELISK_SKIP_WRAPPER"
+	wrapperPath    = "./tools/bazel"
+)
+
+var (
+	// BazeliskVersion is filled in via x_defs when building a release.
+	BazeliskVersion = "development"
+
+	fileConfig     map[string]string
+	fileConfigOnce sync.Once
+)
+
+// RunBazelisk runs the main Bazelisk logic for the given arguments and Bazel repositories.
+func RunBazelisk(args []string, repos *core.Repositories, out io.Writer) (int, error) {
+	httputil.UserAgent = getUserAgent()
+
+	bazeliskHome := GetEnvOrConfig("BAZELISK_HOME")
+	if len(bazeliskHome) == 0 {
+		userCacheDir, err := os.UserCacheDir()
+		if err != nil {
+			return -1, fmt.Errorf("could not get the user's cache directory: %v", err)
+		}
+
+		bazeliskHome = filepath.Join(userCacheDir, "bazelisk")
+	}
+
+	err := os.MkdirAll(bazeliskHome, 0755)
+	if err != nil {
+		return -1, fmt.Errorf("could not create directory %s: %v", bazeliskHome, err)
+	}
+
+	bazelVersionString, err := getBazelVersion()
+	if err != nil {
+		return -1, fmt.Errorf("could not get Bazel version: %v", err)
+	}
+
+	bazelPath, err := homedir.Expand(bazelVersionString)
+	if err != nil {
+		return -1, fmt.Errorf("could not expand home directory in path: %v", err)
+	}
+
+	// If the Bazel version is an absolute path to a Bazel binary in the filesystem, we can
+	// use it directly. In that case, we don't know which exact version it is, though.
+	resolvedBazelVersion := "unknown"
+
+	// If we aren't using a local Bazel binary, we'll have to parse the version string and
+	// download the version that the user wants.
+	if !filepath.IsAbs(bazelPath) {
+		bazelFork, bazelVersion, err := parseBazelForkAndVersion(bazelVersionString)
+		if err != nil {
+			return -1, fmt.Errorf("could not parse Bazel fork and version: %v", err)
+		}
+
+		var downloader core.DownloadFunc
+		resolvedBazelVersion, downloader, err = repos.ResolveVersion(bazeliskHome, bazelFork, bazelVersion)
+		if err != nil {
+			return -1, fmt.Errorf("could not resolve the version '%s' to an actual version number: %v", bazelVersion, err)
+		}
+
+		bazelForkOrURL := dirForURL(GetEnvOrConfig(core.BaseURLEnv))
+		if len(bazelForkOrURL) == 0 {
+			bazelForkOrURL = bazelFork
+		}
+
+		baseDirectory := filepath.Join(bazeliskHome, "downloads", bazelForkOrURL)
+		bazelPath, err = downloadBazel(bazelFork, resolvedBazelVersion, baseDirectory, repos, downloader)
+		if err != nil {
+			return -1, fmt.Errorf("could not download Bazel: %v", err)
+		}
+	} else {
+		baseDirectory := filepath.Join(bazeliskHome, "local")
+		bazelPath, err = linkLocalBazel(baseDirectory, bazelPath)
+		if err != nil {
+			return -1, fmt.Errorf("cound not link local Bazel: %v", err)
+		}
+	}
+
+	// --print_env must be the first argument.
+	if len(args) > 0 && args[0] == "--print_env" {
+		// print environment variables for sub-processes
+		cmd := makeBazelCmd(bazelPath, args, nil)
+		for _, val := range cmd.Env {
+			fmt.Println(val)
+		}
+		return 0, nil
+	}
+
+	// --strict and --migrate must be the first argument.
+	if len(args) > 0 && (args[0] == "--strict" || args[0] == "--migrate") {
+		cmd, err := getBazelCommand(args)
+		if err != nil {
+			return -1, err
+		}
+		newFlags, err := getIncompatibleFlags(bazelPath, cmd)
+		if err != nil {
+			return -1, fmt.Errorf("could not get the list of incompatible flags: %v", err)
+		}
+
+		if args[0] == "--migrate" {
+			migrate(bazelPath, args[1:], newFlags)
+		} else {
+			// When --strict is present, it expands to the list of --incompatible_ flags
+			// that should be enabled for the given Bazel version.
+			args = insertArgs(args[1:], newFlags)
+		}
+	}
+
+	// print bazelisk version information if "version" is the first argument
+	// bazel version is executed after this command
+	if len(args) > 0 && args[0] == "version" {
+		// Check if the --gnu_format flag is set, if that is the case,
+		// the version is printed differently
+		var gnuFormat bool
+		for _, arg := range args {
+			if arg == "--gnu_format" {
+				gnuFormat = true
+				break
+			}
+		}
+
+		if gnuFormat {
+			fmt.Printf("Bazelisk %s\n", BazeliskVersion)
+		} else {
+			fmt.Printf("Bazelisk version: %s\n", BazeliskVersion)
+		}
+	}
+
+	exitCode, err := runBazel(bazelPath, args, out)
+	if err != nil {
+		return -1, fmt.Errorf("could not run Bazel: %v", err)
+	}
+	return exitCode, nil
+}
+
+func getBazelCommand(args []string) (string, error) {
+	for _, a := range args {
+		if !strings.HasPrefix(a, "-") {
+			return a, nil
+		}
+	}
+	return "", fmt.Errorf("could not find a valid Bazel command in %q. Please run `bazel help` if you need help on how to use Bazel.", strings.Join(args, " "))
+}
+
+func getUserAgent() string {
+	agent := GetEnvOrConfig("BAZELISK_USER_AGENT")
+	if len(agent) > 0 {
+		return agent
+	}
+	return fmt.Sprintf("Bazelisk/%s", BazeliskVersion)
+}
+
+// GetEnvOrConfig reads a configuration value from the environment, but fall back to reading it from .bazeliskrc in the workspace root.
+func GetEnvOrConfig(name string) string {
+	if val := os.Getenv(name); val != "" {
+		return val
+	}
+
+	// Parse .bazeliskrc in the workspace root, once, if it can be found.
+	fileConfigOnce.Do(func() {
+		workingDirectory, err := os.Getwd()
+		if err != nil {
+			return
+		}
+		workspaceRoot := findWorkspaceRoot(workingDirectory)
+		if workspaceRoot == "" {
+			return
+		}
+		rcFilePath := filepath.Join(workspaceRoot, ".bazeliskrc")
+		contents, err := ioutil.ReadFile(rcFilePath)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return
+			}
+			log.Fatal(err)
+		}
+		fileConfig = make(map[string]string)
+		for _, line := range strings.Split(string(contents), "\n") {
+			if strings.HasPrefix(line, "#") {
+				// comments
+				continue
+			}
+			parts := strings.SplitN(line, "=", 2)
+			if len(parts) < 2 {
+				continue
+			}
+			key := strings.TrimSpace(parts[0])
+			fileConfig[key] = strings.TrimSpace(parts[1])
+		}
+	})
+
+	return fileConfig[name]
+}
+
+// isValidWorkspace returns true iff the supplied path is the workspace root, defined by the presence of
+// a file named WORKSPACE or WORKSPACE.bazel
+// see https://github.com/bazelbuild/bazel/blob/8346ea4cfdd9fbd170d51a528fee26f912dad2d5/src/main/cpp/workspace_layout.cc#L37
+func isValidWorkspace(path string) bool {
+	info, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+
+	return !info.IsDir()
+}
+
+func findWorkspaceRoot(root string) string {
+	if isValidWorkspace(filepath.Join(root, "WORKSPACE")) {
+		return root
+	}
+
+	if isValidWorkspace(filepath.Join(root, "WORKSPACE.bazel")) {
+		return root
+	}
+
+	parentDirectory := filepath.Dir(root)
+	if parentDirectory == root {
+		return ""
+	}
+
+	return findWorkspaceRoot(parentDirectory)
+}
+
+func getBazelVersion() (string, error) {
+	// Check in this order:
+	// - env var "USE_BAZEL_VERSION" is set to a specific version.
+	// - env var "USE_NIGHTLY_BAZEL" or "USE_BAZEL_NIGHTLY" is set -> latest
+	//   nightly. (TODO)
+	// - env var "USE_CANARY_BAZEL" or "USE_BAZEL_CANARY" is set -> latest
+	//   rc. (TODO)
+	// - the file workspace_root/tools/bazel exists -> that version. (TODO)
+	// - workspace_root/.bazeliskrc exists and contains a 'USE_BAZEL_VERSION'
+	//   variable -> read contents, that version.
+	// - workspace_root/.bazelversion exists -> read contents, that version.
+	// - workspace_root/WORKSPACE contains a version -> that version. (TODO)
+	// - fallback: latest release
+	bazelVersion := GetEnvOrConfig("USE_BAZEL_VERSION")
+	if len(bazelVersion) != 0 {
+		return bazelVersion, nil
+	}
+
+	workingDirectory, err := os.Getwd()
+	if err != nil {
+		return "", fmt.Errorf("could not get working directory: %v", err)
+	}
+
+	workspaceRoot := findWorkspaceRoot(workingDirectory)
+	if len(workspaceRoot) != 0 {
+		bazelVersionPath := filepath.Join(workspaceRoot, ".bazelversion")
+		if _, err := os.Stat(bazelVersionPath); err == nil {
+			f, err := os.Open(bazelVersionPath)
+			if err != nil {
+				return "", fmt.Errorf("could not read %s: %v", bazelVersionPath, err)
+			}
+			defer f.Close()
+
+			scanner := bufio.NewScanner(f)
+			scanner.Scan()
+			bazelVersion := scanner.Text()
+			if err := scanner.Err(); err != nil {
+				return "", fmt.Errorf("could not read version from file %s: %v", bazelVersion, err)
+			}
+
+			if len(bazelVersion) != 0 {
+				return bazelVersion, nil
+			}
+		}
+	}
+
+	return "latest", nil
+}
+
+func parseBazelForkAndVersion(bazelForkAndVersion string) (string, string, error) {
+	var bazelFork, bazelVersion string
+
+	versionInfo := strings.Split(bazelForkAndVersion, "/")
+
+	if len(versionInfo) == 1 {
+		bazelFork, bazelVersion = versions.BazelUpstream, versionInfo[0]
+	} else if len(versionInfo) == 2 {
+		bazelFork, bazelVersion = versionInfo[0], versionInfo[1]
+	} else {
+		return "", "", fmt.Errorf("invalid version \"%s\", could not parse version with more than one slash", bazelForkAndVersion)
+	}
+
+	return bazelFork, bazelVersion, nil
+}
+
+func downloadBazel(fork string, version string, baseDirectory string, repos *core.Repositories, downloader core.DownloadFunc) (string, error) {
+	pathSegment, err := platforms.DetermineBazelFilename(version, false)
+	if err != nil {
+		return "", fmt.Errorf("could not determine path segment to use for Bazel binary: %v", err)
+	}
+
+	destFile := "bazel" + platforms.DetermineExecutableFilenameSuffix()
+	destinationDir := filepath.Join(baseDirectory, pathSegment, "bin")
+
+	if url := GetEnvOrConfig(core.BaseURLEnv); url != "" {
+		return repos.DownloadFromBaseURL(url, version, destinationDir, destFile)
+	}
+
+	return downloader(destinationDir, destFile)
+}
+
+func copyFile(src, dst string, perm os.FileMode) error {
+	srcFile, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer srcFile.Close()
+
+	dstFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE, perm)
+	if err != nil {
+		return err
+	}
+	defer dstFile.Close()
+
+	_, err = io.Copy(dstFile, srcFile)
+
+	return err
+}
+
+func linkLocalBazel(baseDirectory string, bazelPath string) (string, error) {
+	normalizedBazelPath := dirForURL(bazelPath)
+	destinationDir := filepath.Join(baseDirectory, normalizedBazelPath, "bin")
+	err := os.MkdirAll(destinationDir, 0755)
+	if err != nil {
+		return "", fmt.Errorf("could not create directory %s: %v", destinationDir, err)
+	}
+	destinationPath := filepath.Join(destinationDir, "bazel"+platforms.DetermineExecutableFilenameSuffix())
+	if _, err := os.Stat(destinationPath); err != nil {
+		err = os.Symlink(bazelPath, destinationPath)
+		// If can't create Symlink, fallback to copy
+		if err != nil {
+			err = copyFile(bazelPath, destinationPath, 0755)
+			if err != nil {
+				return "", fmt.Errorf("cound not copy file from %s to %s: %v", bazelPath, destinationPath, err)
+			}
+		}
+	}
+	return destinationPath, nil
+}
+
+func maybeDelegateToWrapper(bazel string) string {
+	if GetEnvOrConfig(skipWrapperEnv) != "" {
+		return bazel
+	}
+
+	wd, err := os.Getwd()
+	if err != nil {
+		return bazel
+	}
+
+	root := findWorkspaceRoot(wd)
+	wrapper := filepath.Join(root, wrapperPath)
+	if stat, err := os.Stat(wrapper); err != nil || stat.IsDir() || stat.Mode().Perm()&0001 == 0 {
+		return bazel
+	}
+
+	return wrapper
+}
+
+func prependDirToPathList(cmd *exec.Cmd, dir string) {
+	found := false
+	for idx, val := range cmd.Env {
+		splits := strings.Split(val, "=")
+		if len(splits) != 2 {
+			continue
+		}
+		if strings.EqualFold(splits[0], "PATH") {
+			found = true
+			cmd.Env[idx] = fmt.Sprintf("PATH=%s%s%s", dir, string(os.PathListSeparator), splits[1])
+			break
+		}
+	}
+
+	if !found {
+		cmd.Env = append(cmd.Env, fmt.Sprintf("PATH=%s", dir))
+	}
+}
+
+func makeBazelCmd(bazel string, args []string, out io.Writer) *exec.Cmd {
+	execPath := maybeDelegateToWrapper(bazel)
+
+	cmd := exec.Command(execPath, args...)
+	cmd.Env = append(os.Environ(), skipWrapperEnv+"=true")
+	if execPath != bazel {
+		cmd.Env = append(cmd.Env, fmt.Sprintf("%s=%s", bazelReal, bazel))
+	}
+	prependDirToPathList(cmd, filepath.Dir(execPath))
+	cmd.Stdin = os.Stdin
+	if out == nil {
+		cmd.Stdout = os.Stdout
+	} else {
+		cmd.Stdout = out
+	}
+	cmd.Stderr = os.Stderr
+	return cmd
+}
+
+func runBazel(bazel string, args []string, out io.Writer) (int, error) {
+	cmd := makeBazelCmd(bazel, args, out)
+	err := cmd.Start()
+	if err != nil {
+		return 1, fmt.Errorf("could not start Bazel: %v", err)
+	}
+
+	c := make(chan os.Signal)
+	signal.Notify(c, os.Interrupt, syscall.SIGTERM)
+	go func() {
+		s := <-c
+		if runtime.GOOS != "windows" {
+			cmd.Process.Signal(s)
+		} else {
+			cmd.Process.Kill()
+		}
+	}()
+
+	err = cmd.Wait()
+	if err != nil {
+		if exitError, ok := err.(*exec.ExitError); ok {
+			waitStatus := exitError.Sys().(syscall.WaitStatus)
+			return waitStatus.ExitStatus(), nil
+		}
+		return 1, fmt.Errorf("could not launch Bazel: %v", err)
+	}
+	return 0, nil
+}
+
+// getIncompatibleFlags returns all incompatible flags for the current Bazel command in alphabetical order.
+func getIncompatibleFlags(bazelPath, cmd string) ([]string, error) {
+	out := strings.Builder{}
+	if _, err := runBazel(bazelPath, []string{"help", cmd, "--short"}, &out); err != nil {
+		return nil, fmt.Errorf("unable to determine incompatible flags with binary %s: %v", bazelPath, err)
+	}
+
+	re := regexp.MustCompile(`(?m)^\s*--\[no\](incompatible_\w+)$`)
+	flags := make([]string, 0)
+	for _, m := range re.FindAllStringSubmatch(out.String(), -1) {
+		flags = append(flags, fmt.Sprintf("--%s", m[1]))
+	}
+	sort.Strings(flags)
+	return flags, nil
+}
+
+// insertArgs will insert newArgs in baseArgs. If baseArgs contains the
+// "--" argument, newArgs will be inserted before that. Otherwise, newArgs
+// is appended.
+func insertArgs(baseArgs []string, newArgs []string) []string {
+	var result []string
+	inserted := false
+	for _, arg := range baseArgs {
+		if !inserted && arg == "--" {
+			result = append(result, newArgs...)
+			inserted = true
+		}
+		result = append(result, arg)
+	}
+
+	if !inserted {
+		result = append(result, newArgs...)
+	}
+	return result
+}
+
+func shutdownIfNeeded(bazelPath string) {
+	bazeliskClean := GetEnvOrConfig("BAZELISK_SHUTDOWN")
+	if len(bazeliskClean) == 0 {
+		return
+	}
+
+	fmt.Printf("bazel shutdown\n")
+	exitCode, err := runBazel(bazelPath, []string{"shutdown"}, nil)
+	fmt.Printf("\n")
+	if err != nil {
+		log.Fatalf("failed to run bazel shutdown: %v", err)
+	}
+	if exitCode != 0 {
+		fmt.Printf("Failure: shutdown command failed.\n")
+		os.Exit(exitCode)
+	}
+}
+
+func cleanIfNeeded(bazelPath string) {
+	bazeliskClean := GetEnvOrConfig("BAZELISK_CLEAN")
+	if len(bazeliskClean) == 0 {
+		return
+	}
+
+	fmt.Printf("bazel clean --expunge\n")
+	exitCode, err := runBazel(bazelPath, []string{"clean", "--expunge"}, nil)
+	fmt.Printf("\n")
+	if err != nil {
+		log.Fatalf("failed to run clean: %v", err)
+	}
+	if exitCode != 0 {
+		fmt.Printf("Failure: clean command failed.\n")
+		os.Exit(exitCode)
+	}
+}
+
+// migrate will run Bazel with each flag separately and report which ones are failing.
+func migrate(bazelPath string, baseArgs []string, flags []string) {
+	// 1. Try with all the flags.
+	args := insertArgs(baseArgs, flags)
+	fmt.Printf("\n\n--- Running Bazel with all incompatible flags\n\n")
+	shutdownIfNeeded(bazelPath)
+	cleanIfNeeded(bazelPath)
+	fmt.Printf("bazel %s\n", strings.Join(args, " "))
+	exitCode, err := runBazel(bazelPath, args, nil)
+	if err != nil {
+		log.Fatalf("could not run Bazel: %v", err)
+	}
+	if exitCode == 0 {
+		fmt.Printf("Success: No migration needed.\n")
+		os.Exit(0)
+	}
+
+	// 2. Try with no flags, as a sanity check.
+	args = baseArgs
+	fmt.Printf("\n\n--- Running Bazel with no incompatible flags\n\n")
+	shutdownIfNeeded(bazelPath)
+	cleanIfNeeded(bazelPath)
+	fmt.Printf("bazel %s\n", strings.Join(args, " "))
+	exitCode, err = runBazel(bazelPath, args, nil)
+	if err != nil {
+		log.Fatalf("could not run Bazel: %v", err)
+	}
+	if exitCode != 0 {
+		fmt.Printf("Failure: Command failed, even without incompatible flags.\n")
+		os.Exit(exitCode)
+	}
+
+	// 3. Try with each flag separately.
+	var passList []string
+	var failList []string
+	for _, arg := range flags {
+		args = insertArgs(baseArgs, []string{arg})
+		fmt.Printf("\n\n--- Running Bazel with %s\n\n", arg)
+		shutdownIfNeeded(bazelPath)
+		cleanIfNeeded(bazelPath)
+		fmt.Printf("bazel %s\n", strings.Join(args, " "))
+		exitCode, err = runBazel(bazelPath, args, nil)
+		if err != nil {
+			log.Fatalf("could not run Bazel: %v", err)
+		}
+		if exitCode == 0 {
+			passList = append(passList, arg)
+		} else {
+			failList = append(failList, arg)
+		}
+	}
+
+	print := func(l []string) {
+		for _, arg := range l {
+			fmt.Printf("  %s\n", arg)
+		}
+	}
+
+	// 4. Print report
+	fmt.Printf("\n\n+++ Result\n\n")
+	fmt.Printf("Command was successful with the following flags:\n")
+	print(passList)
+	fmt.Printf("\n")
+	fmt.Printf("Migration is needed for the following flags:\n")
+	print(failList)
+
+	os.Exit(1)
+}
+
+func dirForURL(url string) string {
+	// Replace all characters that might not be allowed in filenames with "-".
+	return regexp.MustCompile("[[:^alnum:]]").ReplaceAllString(url, "-")
+}
diff --git a/pkg/bazel/flags.go b/pkg/bazel/flags.go
index 4ff1cfc..9051e67 100644
--- a/pkg/bazel/flags.go
+++ b/pkg/bazel/flags.go
@@ -5,3 +5,45 @@
 */
 
 package bazel
+
+import (
+	"encoding/base64"
+	"fmt"
+	"io"
+	"io/ioutil"
+
+	"google.golang.org/protobuf/proto"
+)
+
+func (b *Bazel) Flags() (map[string]*FlagInfo, error) {
+	r, w := io.Pipe()
+	decoder := base64.NewDecoder(base64.StdEncoding, r)
+	bazelErrs := make(chan error, 1)
+	defer close(bazelErrs)
+	go func() {
+		defer w.Close()
+		_, err := b.RunCommand([]string{"help", "flags-as-proto"}, w)
+		bazelErrs <- err
+	}()
+
+	helpProtoBytes, err := ioutil.ReadAll(decoder)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get Bazel flags: %w", err)
+	}
+
+	if err := <-bazelErrs; err != nil {
+		return nil, fmt.Errorf("failed to get Bazel flags: %w", err)
+	}
+
+	flagCollection := &FlagCollection{}
+	if err := proto.Unmarshal(helpProtoBytes, flagCollection); err != nil {
+		return nil, fmt.Errorf("failed to get Bazel flags: %w", err)
+	}
+
+	flags := make(map[string]*FlagInfo)
+	for i := range flagCollection.FlagInfos {
+		flags[*flagCollection.FlagInfos[i].Name] = flagCollection.FlagInfos[i]
+	}
+
+	return flags, nil
+}
diff --git a/pkg/bazel/flags.pb.go b/pkg/bazel/flags.pb.go
new file mode 100755
index 0000000..576e280
--- /dev/null
+++ b/pkg/bazel/flags.pb.go
@@ -0,0 +1,269 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.26.0
+// 	protoc        v3.13.0
+// source: pkg/bazel/flags.proto
+
+package bazel
+
+import (
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+type FlagInfo struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Name            *string  `protobuf:"bytes,1,req,name=name" json:"name,omitempty"`
+	HasNegativeFlag *bool    `protobuf:"varint,2,opt,name=has_negative_flag,json=hasNegativeFlag,def=0" json:"has_negative_flag,omitempty"`
+	Documentation   *string  `protobuf:"bytes,3,opt,name=documentation" json:"documentation,omitempty"`
+	Commands        []string `protobuf:"bytes,4,rep,name=commands" json:"commands,omitempty"`
+	Abbreviation    *string  `protobuf:"bytes,5,opt,name=abbreviation" json:"abbreviation,omitempty"`
+	AllowsMultiple  *bool    `protobuf:"varint,6,opt,name=allows_multiple,json=allowsMultiple,def=0" json:"allows_multiple,omitempty"`
+}
+
+// Default values for FlagInfo fields.
+const (
+	Default_FlagInfo_HasNegativeFlag = bool(false)
+	Default_FlagInfo_AllowsMultiple  = bool(false)
+)
+
+func (x *FlagInfo) Reset() {
+	*x = FlagInfo{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_pkg_bazel_flags_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *FlagInfo) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*FlagInfo) ProtoMessage() {}
+
+func (x *FlagInfo) ProtoReflect() protoreflect.Message {
+	mi := &file_pkg_bazel_flags_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use FlagInfo.ProtoReflect.Descriptor instead.
+func (*FlagInfo) Descriptor() ([]byte, []int) {
+	return file_pkg_bazel_flags_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *FlagInfo) GetName() string {
+	if x != nil && x.Name != nil {
+		return *x.Name
+	}
+	return ""
+}
+
+func (x *FlagInfo) GetHasNegativeFlag() bool {
+	if x != nil && x.HasNegativeFlag != nil {
+		return *x.HasNegativeFlag
+	}
+	return Default_FlagInfo_HasNegativeFlag
+}
+
+func (x *FlagInfo) GetDocumentation() string {
+	if x != nil && x.Documentation != nil {
+		return *x.Documentation
+	}
+	return ""
+}
+
+func (x *FlagInfo) GetCommands() []string {
+	if x != nil {
+		return x.Commands
+	}
+	return nil
+}
+
+func (x *FlagInfo) GetAbbreviation() string {
+	if x != nil && x.Abbreviation != nil {
+		return *x.Abbreviation
+	}
+	return ""
+}
+
+func (x *FlagInfo) GetAllowsMultiple() bool {
+	if x != nil && x.AllowsMultiple != nil {
+		return *x.AllowsMultiple
+	}
+	return Default_FlagInfo_AllowsMultiple
+}
+
+type FlagCollection struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	FlagInfos []*FlagInfo `protobuf:"bytes,1,rep,name=flag_infos,json=flagInfos" json:"flag_infos,omitempty"`
+}
+
+func (x *FlagCollection) Reset() {
+	*x = FlagCollection{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_pkg_bazel_flags_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *FlagCollection) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*FlagCollection) ProtoMessage() {}
+
+func (x *FlagCollection) ProtoReflect() protoreflect.Message {
+	mi := &file_pkg_bazel_flags_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use FlagCollection.ProtoReflect.Descriptor instead.
+func (*FlagCollection) Descriptor() ([]byte, []int) {
+	return file_pkg_bazel_flags_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *FlagCollection) GetFlagInfos() []*FlagInfo {
+	if x != nil {
+		return x.FlagInfos
+	}
+	return nil
+}
+
+var File_pkg_bazel_flags_proto protoreflect.FileDescriptor
+
+var file_pkg_bazel_flags_proto_rawDesc = []byte{
+	0x0a, 0x15, 0x70, 0x6b, 0x67, 0x2f, 0x62, 0x61, 0x7a, 0x65, 0x6c, 0x2f, 0x66, 0x6c, 0x61, 0x67,
+	0x73, 0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x12, 0x05, 0x62, 0x61, 0x7a, 0x65, 0x6c, 0x22, 0xe7,
+	0x01, 0x0a, 0x08, 0x46, 0x6c, 0x61, 0x67, 0x49, 0x6e, 0x66, 0x6f, 0x12, 0x12, 0x0a, 0x04, 0x6e,
+	0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x02, 0x28, 0x09, 0x52, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x12,
+	0x31, 0x0a, 0x11, 0x68, 0x61, 0x73, 0x5f, 0x6e, 0x65, 0x67, 0x61, 0x74, 0x69, 0x76, 0x65, 0x5f,
+	0x66, 0x6c, 0x61, 0x67, 0x18, 0x02, 0x20, 0x01, 0x28, 0x08, 0x3a, 0x05, 0x66, 0x61, 0x6c, 0x73,
+	0x65, 0x52, 0x0f, 0x68, 0x61, 0x73, 0x4e, 0x65, 0x67, 0x61, 0x74, 0x69, 0x76, 0x65, 0x46, 0x6c,
+	0x61, 0x67, 0x12, 0x24, 0x0a, 0x0d, 0x64, 0x6f, 0x63, 0x75, 0x6d, 0x65, 0x6e, 0x74, 0x61, 0x74,
+	0x69, 0x6f, 0x6e, 0x18, 0x03, 0x20, 0x01, 0x28, 0x09, 0x52, 0x0d, 0x64, 0x6f, 0x63, 0x75, 0x6d,
+	0x65, 0x6e, 0x74, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x12, 0x1a, 0x0a, 0x08, 0x63, 0x6f, 0x6d, 0x6d,
+	0x61, 0x6e, 0x64, 0x73, 0x18, 0x04, 0x20, 0x03, 0x28, 0x09, 0x52, 0x08, 0x63, 0x6f, 0x6d, 0x6d,
+	0x61, 0x6e, 0x64, 0x73, 0x12, 0x22, 0x0a, 0x0c, 0x61, 0x62, 0x62, 0x72, 0x65, 0x76, 0x69, 0x61,
+	0x74, 0x69, 0x6f, 0x6e, 0x18, 0x05, 0x20, 0x01, 0x28, 0x09, 0x52, 0x0c, 0x61, 0x62, 0x62, 0x72,
+	0x65, 0x76, 0x69, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x12, 0x2e, 0x0a, 0x0f, 0x61, 0x6c, 0x6c, 0x6f,
+	0x77, 0x73, 0x5f, 0x6d, 0x75, 0x6c, 0x74, 0x69, 0x70, 0x6c, 0x65, 0x18, 0x06, 0x20, 0x01, 0x28,
+	0x08, 0x3a, 0x05, 0x66, 0x61, 0x6c, 0x73, 0x65, 0x52, 0x0e, 0x61, 0x6c, 0x6c, 0x6f, 0x77, 0x73,
+	0x4d, 0x75, 0x6c, 0x74, 0x69, 0x70, 0x6c, 0x65, 0x22, 0x40, 0x0a, 0x0e, 0x46, 0x6c, 0x61, 0x67,
+	0x43, 0x6f, 0x6c, 0x6c, 0x65, 0x63, 0x74, 0x69, 0x6f, 0x6e, 0x12, 0x2e, 0x0a, 0x0a, 0x66, 0x6c,
+	0x61, 0x67, 0x5f, 0x69, 0x6e, 0x66, 0x6f, 0x73, 0x18, 0x01, 0x20, 0x03, 0x28, 0x0b, 0x32, 0x0f,
+	0x2e, 0x62, 0x61, 0x7a, 0x65, 0x6c, 0x2e, 0x46, 0x6c, 0x61, 0x67, 0x49, 0x6e, 0x66, 0x6f, 0x52,
+	0x09, 0x66, 0x6c, 0x61, 0x67, 0x49, 0x6e, 0x66, 0x6f, 0x73, 0x42, 0x47, 0x0a, 0x34, 0x63, 0x6f,
+	0x6d, 0x2e, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x2e, 0x64, 0x65, 0x76, 0x74, 0x6f, 0x6f, 0x6c,
+	0x73, 0x2e, 0x62, 0x75, 0x69, 0x6c, 0x64, 0x2e, 0x6c, 0x69, 0x62, 0x2e, 0x72, 0x75, 0x6e, 0x74,
+	0x69, 0x6d, 0x65, 0x2e, 0x63, 0x6f, 0x6d, 0x6d, 0x61, 0x6e, 0x64, 0x73, 0x2e, 0x70, 0x72, 0x6f,
+	0x74, 0x6f, 0x42, 0x0f, 0x42, 0x61, 0x7a, 0x65, 0x6c, 0x46, 0x6c, 0x61, 0x67, 0x73, 0x50, 0x72,
+	0x6f, 0x74, 0x6f,
+}
+
+var (
+	file_pkg_bazel_flags_proto_rawDescOnce sync.Once
+	file_pkg_bazel_flags_proto_rawDescData = file_pkg_bazel_flags_proto_rawDesc
+)
+
+func file_pkg_bazel_flags_proto_rawDescGZIP() []byte {
+	file_pkg_bazel_flags_proto_rawDescOnce.Do(func() {
+		file_pkg_bazel_flags_proto_rawDescData = protoimpl.X.CompressGZIP(file_pkg_bazel_flags_proto_rawDescData)
+	})
+	return file_pkg_bazel_flags_proto_rawDescData
+}
+
+var file_pkg_bazel_flags_proto_msgTypes = make([]protoimpl.MessageInfo, 2)
+var file_pkg_bazel_flags_proto_goTypes = []interface{}{
+	(*FlagInfo)(nil),       // 0: bazel.FlagInfo
+	(*FlagCollection)(nil), // 1: bazel.FlagCollection
+}
+var file_pkg_bazel_flags_proto_depIdxs = []int32{
+	0, // 0: bazel.FlagCollection.flag_infos:type_name -> bazel.FlagInfo
+	1, // [1:1] is the sub-list for method output_type
+	1, // [1:1] is the sub-list for method input_type
+	1, // [1:1] is the sub-list for extension type_name
+	1, // [1:1] is the sub-list for extension extendee
+	0, // [0:1] is the sub-list for field type_name
+}
+
+func init() { file_pkg_bazel_flags_proto_init() }
+func file_pkg_bazel_flags_proto_init() {
+	if File_pkg_bazel_flags_proto != nil {
+		return
+	}
+	if !protoimpl.UnsafeEnabled {
+		file_pkg_bazel_flags_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*FlagInfo); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_pkg_bazel_flags_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*FlagCollection); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_pkg_bazel_flags_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   2,
+			NumExtensions: 0,
+			NumServices:   0,
+		},
+		GoTypes:           file_pkg_bazel_flags_proto_goTypes,
+		DependencyIndexes: file_pkg_bazel_flags_proto_depIdxs,
+		MessageInfos:      file_pkg_bazel_flags_proto_msgTypes,
+	}.Build()
+	File_pkg_bazel_flags_proto = out.File
+	file_pkg_bazel_flags_proto_rawDesc = nil
+	file_pkg_bazel_flags_proto_goTypes = nil
+	file_pkg_bazel_flags_proto_depIdxs = nil
+}
diff --git a/pkg/bazel/flags.proto b/pkg/bazel/flags.proto
new file mode 100644
index 0000000..e8736ae
--- /dev/null
+++ b/pkg/bazel/flags.proto
@@ -0,0 +1,42 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file contains the protocol buffer representation of a list of supported
+// flags for Bazel commands.
+syntax = "proto2";
+
+option java_outer_classname = "BazelFlagsProto";
+option java_package = "com.google.devtools.build.lib.runtime.commands.proto";
+// option java_api_version = 2;
+
+package bazel;
+
+message FlagInfo {
+    // Name of the flag, without leading dashes.
+    required string name = 1;
+    // True if --noname exists, too.
+    optional bool has_negative_flag = 2 [default = false];
+    // Help text of the flag.
+    optional string documentation = 3;
+    // List of supported Bazel commands, e.g. ['build', 'test']
+    repeated string commands = 4;
+    // Flag name abbreviation, without leading dash.
+    optional string abbreviation = 5;
+    // True if a flag is allowed to occur multiple times in a single arg list.
+    optional bool allows_multiple = 6 [default = false];
+}
+
+message FlagCollection {
+    repeated FlagInfo flag_infos = 1;
+}