implement singleton registry that knows about all helpers

bzl/private/source_files/BUILD.bazel

@@ -1,9 +1,9 @@
 release_files = [
     "//:all_files",
     "//agent:all_files",
-    "//agent/locate:all_files",
     "//agent/internal:all_files",
     "//agent/internal/lockfile:all_files",
+    "//agent/locate:all_files",
     "//api:all_files",
     "//authenticate/gcs:all_files",
     "//authenticate/github:all_files",
@@ -28,9 +28,9 @@ release_files = [
     "//config:all_files",
     "//helperfactory:all_files",
     "//helperfactory/fallback:all_files",
-    "//helperfactory/string:all_files",
     "//installer:all_files",
     "//logging:all_files",
+    "//registry:all_files",
 ]
 
 dev_files = [

config/BUILD.bazel

@@ -8,8 +8,8 @@ go_library(
     deps = [
         "//agent/locate",
         "//api",
-        "//helperfactory/string",
         "//logging",
+        "//registry",
     ],
 )
 

config/config.go

@@ -10,8 +10,8 @@ import (
 
 	"github.com/tweag/credential-helper/agent/locate"
 	"github.com/tweag/credential-helper/api"
-	helperstringfactory "github.com/tweag/credential-helper/helperfactory/string"
 	"github.com/tweag/credential-helper/logging"
+	"github.com/tweag/credential-helper/registry"
 )
 
 var ErrConfigNotFound = errors.New("config file not found")
@@ -53,7 +53,7 @@ func (c Config) FindHelper(uri string) (api.Helper, []byte, error) {
 		if len(urlConfig.Path) > 0 && !globMatch(urlConfig.Path, requested.Path) {
 			continue
 		}
-		helper := helperstringfactory.HelperFromString(urlConfig.Helper)
+		helper := registry.HelperFromString(urlConfig.Helper)
 		if helper != nil {
 			logging.Debugf("selected helper %s from config", urlConfig.Helper)
 			return helper, urlConfig.Config, nil
@@ -62,7 +62,7 @@ func (c Config) FindHelper(uri string) (api.Helper, []byte, error) {
 	}
 	// this is equivalent to null.Null{}
 	// but avoids the import of the null package
-	return helperstringfactory.HelperFromString("null"), nil, nil
+	return registry.HelperFromString("null"), nil, nil
 }
 
 type ConfigReader interface {

docs/plugins.md

@@ -27,6 +27,17 @@ You can find the built-in default implementations under [/authenticate][authenti
 
 The credential helper supports multiple authentication providers in a single helper binary. The factory function determines which `api.Helper` to use based on the request URI. You can find the default implementation in [github.com/tweag/credential-helper/helperfactory/fallback.FallbackHelperFactory][fallback-helper-factory].
 
+## Registering your helper
+
+Custom helpers need to be registered at program startup to work and be recognized correctly. For this, the `registry.Register` function can be called to add a globally known register (with a unique name) to the registry.
+Simply add an `init` function to the package that implements `api.Helper`:
+
+```go
+func init() {
+	registry.Register("foo", FooHelper{})
+}
+```
+
 ## (Optional) Replace the default in-memory cache
 
 By default, the agent process uses a simple in-memory key-value store to cache credentials. You can provide a custom implementation to persist credentials (on disk, in a database, in a shared key-value store, using a (hardware backed) secure storage, etc.), implement more selective caching (decide what to keep), or perform other custom logic.

examples/customized/helper/authenticate/BUILD.bazel

@@ -5,5 +5,8 @@ go_library(
     srcs = ["pathtoheader.go"],
     importpath = "github.com/tweag/credential-helper/examples/customized/helper/authenticate",
     visibility = ["//visibility:public"],
-    deps = ["@tweag-credential-helper//api"],
+    deps = [
+        "@tweag-credential-helper//api",
+        "@tweag-credential-helper//registry",
+    ],
 )

examples/customized/helper/authenticate/pathtoheader.go

@@ -9,8 +9,16 @@ import (
 	"time"
 
 	"github.com/tweag/credential-helper/api"
+	"github.com/tweag/credential-helper/registry"
 )
 
+func init() {
+	// This code runs when the program starts.
+	// It registers this helper with the registry under the name `pathtoheader`.
+	// The registry is used by the agent to look up helpers by name.
+	registry.Register("pathtoheader", PathToHeader{})
+}
+
 // PathToHeader is a credential helper that
 // takes a request path and returns
 // a custom header including it.

helperfactory/string/BUILD.bazel → registry/BUILD.bazel

@@ -1,9 +1,9 @@
 load("@rules_go//go:def.bzl", "go_library")
 
 go_library(
-    name = "string",
-    srcs = ["helperstring.go"],
-    importpath = "github.com/tweag/credential-helper/helperfactory/string",
+    name = "registry",
+    srcs = ["registry.go"],
+    importpath = "github.com/tweag/credential-helper/registry",
     visibility = ["//visibility:public"],
     deps = [
         "//api",

registry/registry.go

@@ -0,0 +1,53 @@
+package registry
+
+import (
+	"github.com/tweag/credential-helper/api"
+	authenticateGCS "github.com/tweag/credential-helper/authenticate/gcs"
+	authenticateGitHub "github.com/tweag/credential-helper/authenticate/github"
+	authenticateNull "github.com/tweag/credential-helper/authenticate/null"
+	authenticateOCI "github.com/tweag/credential-helper/authenticate/oci"
+	authenticateRemoteAPIs "github.com/tweag/credential-helper/authenticate/remoteapis"
+	authenticateS3 "github.com/tweag/credential-helper/authenticate/s3"
+)
+
+var singleton = Helpers{
+	Map: map[string]api.Helper{
+		"gcs":        &authenticateGCS.GCS{},
+		"github":     &authenticateGitHub.GitHub{},
+		"null":       &authenticateNull.Null{},
+		"oci":        authenticateOCI.NewFallbackOCI(),
+		"remoteapis": &authenticateRemoteAPIs.RemoteAPIs{},
+		"s3":         &authenticateS3.S3{},
+	},
+}
+
+// HelperFromString returns the helper corresponding to the given string.
+func HelperFromString(s string) api.Helper {
+	return singleton.Map[s]
+}
+
+// Register registers a new helper with the given name.
+func Register(name string, helper api.Helper) {
+	singleton.register(name, helper)
+}
+
+// Names returns the names of all registered helpers.
+func Names() []string {
+	return singleton.names()
+}
+
+type Helpers struct {
+	Map map[string]api.Helper
+}
+
+func (h *Helpers) register(name string, helper api.Helper) {
+	h.Map[name] = helper
+}
+
+func (h *Helpers) names() []string {
+	names := make([]string, 0, len(h.Map))
+	for name := range h.Map {
+		names = append(names, name)
+	}
+	return names
+}