Extract config fetching into its own package (#462)

My goal here is to be able to pass my own config implementation when
calling RunBazelisk from code without setting environment variables,
because they cause repository rule cache invalidations.

As a side-effect, it helps towards the TODO in core.go to split
functionality into packages.

bazelisk.go

@@ -25,11 +25,12 @@ import (
 
 func main() {
 	gcs := &repositories.GCSRepo{}
-	gitHub := repositories.CreateGitHubRepo(core.GetEnvOrConfig("BAZELISK_GITHUB_TOKEN"))
+	config := core.MakeDefaultConfig()
+	gitHub := repositories.CreateGitHubRepo(config.Get("BAZELISK_GITHUB_TOKEN"))
 	// Fetch LTS releases, release candidates, rolling releases and Bazel-at-commits from GCS, forks from GitHub.
 	repos := core.CreateRepositories(gcs, gcs, gitHub, gcs, gcs, true)
 
-	exitCode, err := core.RunBazelisk(os.Args[1:], repos)
+	exitCode, err := core.RunBazeliskWithArgsFuncAndConfig(func(string) []string { return os.Args[1:] }, repos, config)
 	if err != nil {
 		log.Fatal(err)
 	}

config/BUILD

@@ -0,0 +1,12 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["config.go"],
+    importpath = "github.com/bazelbuild/bazelisk/config",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//ws:go_default_library",
+        "@com_github_mitchellh_go_homedir//:go_default_library",
+    ],
+)

config/config.go

@@ -0,0 +1,130 @@
+package config
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/bazelbuild/bazelisk/ws"
+)
+
+const rcFileName = ".bazeliskrc"
+
+// Config allows getting Bazelisk configuration values.
+type Config interface {
+	Get(name string) string
+}
+
+// FromEnv returns a Config which gets config values from environment variables.
+func FromEnv() Config {
+	return &fromEnv{}
+}
+
+type fromEnv struct{}
+
+func (c *fromEnv) Get(name string) string {
+	return os.Getenv(name)
+}
+
+// FromFile returns a Config which gets config values from a Bazelisk config file.
+func FromFile(path string) (Config, error) {
+	values, err := parseFileConfig(path)
+	if err != nil {
+		return nil, err
+	}
+	return &static{
+		values: values,
+	}, nil
+}
+
+type static struct {
+	values map[string]string
+}
+
+func (c *static) Get(name string) string {
+	return c.values[name]
+}
+
+// parseFileConfig parses a .bazeliskrc file as a map of key-value configuration values.
+func parseFileConfig(rcFilePath string) (map[string]string, error) {
+	config := make(map[string]string)
+
+	contents, err := ioutil.ReadFile(rcFilePath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			// Non-critical error.
+			return config, nil
+		}
+		return nil, err
+	}
+
+	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])
+		config[key] = strings.TrimSpace(parts[1])
+	}
+
+	return config, nil
+}
+
+// LocateUserConfigFile locates a .bazeliskrc file in the user's home directory.
+func LocateUserConfigFile() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, rcFileName), nil
+}
+
+// LocateWorkspaceConfigFile locates a .bazeliskrc file in the current workspace root.
+func LocateWorkspaceConfigFile() (string, error) {
+	workingDirectory, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+	workspaceRoot := ws.FindWorkspaceRoot(workingDirectory)
+	if workspaceRoot == "" {
+		return "", err
+	}
+	return filepath.Join(workspaceRoot, rcFileName), nil
+}
+
+// Layered returns a Config which gets config values from the first of a series of other Config values which sets the config.
+func Layered(configs ...Config) Config {
+	return &layered{
+		configs: configs,
+	}
+}
+
+type layered struct {
+	configs []Config
+}
+
+func (c *layered) Get(name string) string {
+	for _, config := range c.configs {
+		if value := config.Get(name); value != "" {
+			return value
+		}
+	}
+	return ""
+}
+
+// Null returns a Config with no config values.
+func Null() Config {
+	return &static{}
+}
+
+// Static returns a Config with static values.
+func Static(values map[string]string) Config {
+	return &static{
+		values: values,
+	}
+}

core/BUILD

@@ -10,9 +10,11 @@ go_library(
     visibility = ["//visibility:public"],
     x_defs = {"BazeliskVersion": "{STABLE_VERSION}"},
     deps = [
+        "//config:go_default_library",
         "//httputil:go_default_library",
         "//platforms:go_default_library",
         "//versions:go_default_library",
+        "//ws:go_default_library",
         "@com_github_mitchellh_go_homedir//:go_default_library",
     ],
 )
@@ -24,4 +26,7 @@ go_test(
         "repositories_test.go",
     ],
     embed = [":go_default_library"],
+    deps = [
+        "//config:go_default_library",
+    ],
 )