Extract a method for locating Bazel (#635)

`bazelisk` can be thought of as two separate pieces. The first of these is a
mechanism for locating a particular `bazel` version, possibly downloading it
from the Web, or using a local install. The second piece is the additional
functionality for flag flipping and finding out where problems have occurred
within commits to the project.

This PR extracts the location of the `bazel` binary into its own method which
can be used by other tooling that also needs to locate the binary.

core/core.go

@@ -95,51 +95,17 @@ func RunBazeliskWithArgsFuncAndConfig(argsFunc ArgsFunc, repos *Repositories, co
 func RunBazeliskWithArgsFuncAndConfigAndOut(argsFunc ArgsFunc, repos *Repositories, config config.Config, out io.Writer) (int, error) {
 	httputil.UserAgent = getUserAgent(config)
 
-	bazeliskHome, err := getBazeliskHome(config)
-	if err != nil {
-		return -1, fmt.Errorf("could not determine Bazelisk home directory: %v", err)
-	}
-
-	err = os.MkdirAll(bazeliskHome, 0755)
-	if err != nil {
-		return -1, fmt.Errorf("could not create directory %s: %v", bazeliskHome, err)
-	}
-
-	bazelVersionString, err := GetBazelVersion(config)
-	if err != nil {
-		return -1, fmt.Errorf("could not get Bazel version: %v", err)
-	}
-
-	bazelPath, err := homedir.Expand(bazelVersionString)
+	bazelInstallation, err := GetBazelInstallation(repos, config)
 	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) {
-		bazelPath, err = downloadBazel(bazelVersionString, bazeliskHome, repos, config)
-		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("could not link local Bazel: %v", err)
-		}
+		return -1, err
 	}
 
-	args := argsFunc(resolvedBazelVersion)
+	args := argsFunc(bazelInstallation.Version)
 
 	// --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, config)
+		cmd := makeBazelCmd(bazelInstallation.Path, args, nil, config)
 		for _, val := range cmd.Env {
 			fmt.Println(val)
 		}
@@ -152,12 +118,12 @@ func RunBazeliskWithArgsFuncAndConfigAndOut(argsFunc ArgsFunc, repos *Repositori
 		if err != nil {
 			return -1, err
 		}
-		newFlags, err := getIncompatibleFlags(bazelPath, cmd, config)
+		newFlags, err := getIncompatibleFlags(bazelInstallation.Path, cmd, config)
 		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, config)
+			migrate(bazelInstallation.Path, args[1:], newFlags, config)
 		} else {
 			// When --strict is present, it expands to the list of --incompatible_ flags
 			// that should be enabled for the given Bazel version.
@@ -171,6 +137,11 @@ func RunBazeliskWithArgsFuncAndConfigAndOut(argsFunc ArgsFunc, repos *Repositori
 		value := args[0][len("--bisect="):]
 		commits := strings.Split(value, "..")
 		if len(commits) == 2 {
+			bazeliskHome, err := getBazeliskHome(config)
+			if err != nil {
+				return -1, fmt.Errorf("could not determine Bazelisk home directory: %v", err)
+			}
+
 			bisect(commits[0], commits[1], args[1:], bazeliskHome, repos, config)
 		} else {
 			return -1, fmt.Errorf("Error: Invalid format for --bisect. Expected format: '--bisect=[~]<good bazel commit>..<bad bazel commit>'")
@@ -197,13 +168,69 @@ func RunBazeliskWithArgsFuncAndConfigAndOut(argsFunc ArgsFunc, repos *Repositori
 		}
 	}
 
-	exitCode, err := runBazel(bazelPath, args, out, config)
+	exitCode, err := runBazel(bazelInstallation.Path, args, out, config)
 	if err != nil {
 		return -1, fmt.Errorf("could not run Bazel: %v", err)
 	}
 	return exitCode, nil
 }
 
+// BazelInstallation provides a summary of a single install of `bazel`
+type BazelInstallation struct {
+	Version string
+	Path    string
+}
+
+// GetBazelInstallation provides a mechanism to find the `bazel` binary to execute, as well as its version
+func GetBazelInstallation(repos *Repositories, config config.Config) (*BazelInstallation, error) {
+	bazeliskHome, err := getBazeliskHome(config)
+	if err != nil {
+		return nil, fmt.Errorf("could not determine Bazelisk home directory: %v", err)
+	}
+
+	err = os.MkdirAll(bazeliskHome, 0755)
+	if err != nil {
+		return nil, fmt.Errorf("could not create directory %s: %v", bazeliskHome, err)
+	}
+
+	bazelVersionString, err := GetBazelVersion(config)
+	if err != nil {
+		return nil, fmt.Errorf("could not get Bazel version: %v", err)
+	}
+
+	bazelPath, err := homedir.Expand(bazelVersionString)
+	if err != nil {
+		return nil, fmt.Errorf("could not expand home directory in path: %v", err)
+	}
+
+	var resolvedVersion string
+
+	// 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) {
+		resolvedVersion = bazelVersionString
+		bazelPath, err = downloadBazel(bazelVersionString, bazeliskHome, repos, config)
+		if err != nil {
+			return nil, fmt.Errorf("could not download Bazel: %v", err)
+		}
+	} else {
+		// 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.
+		resolvedVersion = "unknown"
+		baseDirectory := filepath.Join(bazeliskHome, "local")
+		bazelPath, err = linkLocalBazel(baseDirectory, bazelPath)
+		if err != nil {
+			return nil, fmt.Errorf("could not link local Bazel: %v", err)
+		}
+	}
+
+	return &BazelInstallation{
+			Version: resolvedVersion,
+			Path:    bazelPath,
+		},
+		nil
+}
+
 func getBazelCommand(args []string) (string, error) {
 	for _, a := range args {
 		if !strings.HasPrefix(a, "-") {