[RFC-0010] Add gcp auth library

auth/gcp/gke_metadata.go

@@ -0,0 +1,127 @@
+/*
+Copyright 2025 The Flux authors
+
+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.
+*/
+
+package gcp
+
+import (
+	"context"
+	"fmt"
+	"sync"
+
+	"cloud.google.com/go/compute/metadata"
+)
+
+type gkeMetadataLoader struct {
+	projectID string
+	location  string
+	name      string
+
+	mu     sync.RWMutex
+	loaded bool
+}
+
+var gkeMetadata gkeMetadataLoader
+
+func (g *gkeMetadataLoader) getAudience(ctx context.Context) (string, error) {
+	if err := g.load(ctx); err != nil {
+		return "", err
+	}
+	wiPool, _ := g.workloadIdentityPool(ctx)
+	wiProvider, _ := g.workloadIdentityProvider(ctx)
+	return fmt.Sprintf("identitynamespace:%s:%s", wiPool, wiProvider), nil
+}
+
+func (g *gkeMetadataLoader) workloadIdentityPool(ctx context.Context) (string, error) {
+	if err := g.load(ctx); err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("%s.svc.id.goog", g.projectID), nil
+}
+
+func (g *gkeMetadataLoader) workloadIdentityProvider(ctx context.Context) (string, error) {
+	if err := g.load(ctx); err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("https://container.googleapis.com/v1/projects/%s/locations/%s/clusters/%s",
+		g.projectID,
+		g.location,
+		g.name), nil
+}
+
+// load loads the GKE cluster metadata from the metadata service, assuming the
+// pod is running on a GKE node/pod. It will fail otherwise, and this
+// is the reason why this method should be called lazily. If this code ran on any
+// other cluster that is not GKE it would fail consistently and throw the pods
+// in crash loop if running on startup. This method is thread-safe and will
+// only load the metadata successfully once.
+//
+// Technically we could receive options here to use a custom HTTP client with
+// a proxy, but this proxy is configured at the object level and here we are
+// loading cluster-level metadata that doesn't change during the lifetime of
+// the pod. So we can't use an object-level proxy here. Furthermore, this
+// implementation targets specifically GKE clusters, and in such clusters the
+// metadata server is usually a DaemonSet pod that serves only node-local
+// traffic, so a proxy doesn't make sense here anyway.
+func (g *gkeMetadataLoader) load(ctx context.Context) error {
+	// Bail early if the metadata was already loaded.
+	g.mu.RLock()
+	loaded := g.loaded
+	g.mu.RUnlock()
+	if loaded {
+		return nil
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	// Check again if the metadata was loaded while we were waiting for the lock.
+	if g.loaded {
+		return nil
+	}
+
+	client := metadata.NewClient(nil)
+
+	projectID, err := client.GetWithContext(ctx, "project/project-id")
+	if err != nil {
+		return fmt.Errorf("failed to get GKE cluster project ID from the metadata service: %w", err)
+	}
+	if projectID == "" {
+		return fmt.Errorf("failed to get GKE cluster project ID from the metadata service: empty value")
+	}
+
+	location, err := client.GetWithContext(ctx, "instance/attributes/cluster-location")
+	if err != nil {
+		return fmt.Errorf("failed to get GKE cluster location from the metadata service: %w", err)
+	}
+	if location == "" {
+		return fmt.Errorf("failed to get GKE cluster location from the metadata service: empty value")
+	}
+
+	name, err := client.GetWithContext(ctx, "instance/attributes/cluster-name")
+	if err != nil {
+		return fmt.Errorf("failed to get GKE cluster name from the metadata service: %w", err)
+	}
+	if name == "" {
+		return fmt.Errorf("failed to get GKE cluster name from the metadata service: empty value")
+	}
+
+	g.projectID = projectID
+	g.location = location
+	g.name = name
+	g.loaded = true
+
+	return nil
+}

auth/gcp/options.go

@@ -0,0 +1,40 @@
+/*
+Copyright 2025 The Flux authors
+
+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.
+*/
+
+package gcp
+
+import (
+	"fmt"
+	"regexp"
+
+	corev1 "k8s.io/api/core/v1"
+)
+
+const serviceAccountEmailPattern = `^[a-zA-Z0-9-]{1,100}@[a-zA-Z0-9-]{1,100}\.iam\.gserviceaccount\.com$`
+
+var serviceAccountEmailRegex = regexp.MustCompile(serviceAccountEmailPattern)
+
+func getServiceAccountEmail(serviceAccount corev1.ServiceAccount) (string, error) {
+	email := serviceAccount.Annotations["iam.gke.io/gcp-service-account"]
+	if email == "" {
+		return "", nil
+	}
+	if !serviceAccountEmailRegex.MatchString(email) {
+		return "", fmt.Errorf("invalid GCP service account email: '%s'. must match %s",
+			email, serviceAccountEmailPattern)
+	}
+	return email, nil
+}

auth/gcp/provider.go

@@ -0,0 +1,150 @@
+/*
+Copyright 2025 The Flux authors
+
+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.
+*/
+
+package gcp
+
+import (
+	"context"
+	"fmt"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/google"
+	"golang.org/x/oauth2/google/externalaccount"
+	corev1 "k8s.io/api/core/v1"
+
+	auth "github.com/fluxcd/pkg/auth"
+)
+
+// ProviderName is the name of the GCP authentication provider.
+const ProviderName = "gcp"
+
+var scopes = []string{
+	"https://www.googleapis.com/auth/cloud-platform",
+	"https://www.googleapis.com/auth/userinfo.email",
+}
+
+// Provider implements the auth.Provider interface for GCP authentication.
+type Provider struct{}
+
+// GetName implements auth.Provider.
+func (Provider) GetName() string {
+	return ProviderName
+}
+
+// NewDefaultToken implements auth.Provider.
+func (Provider) NewDefaultToken(ctx context.Context, opts ...auth.Option) (auth.Token, error) {
+	var o auth.Options
+	o.Apply(opts...)
+
+	if hc := o.GetHTTPClient(); hc != nil {
+		ctx = context.WithValue(ctx, oauth2.HTTPClient, hc)
+	}
+
+	src, err := google.DefaultTokenSource(ctx, scopes...)
+	if err != nil {
+		return nil, err
+	}
+	token, err := src.Token()
+	if err != nil {
+		return nil, err
+	}
+
+	return &Token{*token}, nil
+}
+
+// GetAudience implements auth.Provider.
+func (Provider) GetAudience(ctx context.Context) (string, error) {
+	return gkeMetadata.workloadIdentityPool(ctx)
+}
+
+// GetIdentity implements auth.Provider.
+func (Provider) GetIdentity(serviceAccount corev1.ServiceAccount) (string, error) {
+	email, err := getServiceAccountEmail(serviceAccount)
+	if err != nil {
+		return "", err
+	}
+	return email, nil
+}
+
+// NewTokenForServiceAccount implements auth.Provider.
+func (Provider) NewTokenForServiceAccount(ctx context.Context, oidcToken string,
+	serviceAccount corev1.ServiceAccount, opts ...auth.Option) (auth.Token, error) {
+
+	var o auth.Options
+	o.Apply(opts...)
+
+	audience, err := gkeMetadata.getAudience(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	conf := externalaccount.Config{
+		UniverseDomain:       "googleapis.com",
+		Audience:             audience,
+		SubjectTokenType:     "urn:ietf:params:oauth:token-type:jwt",
+		TokenURL:             "https://sts.googleapis.com/v1/token",
+		SubjectTokenSupplier: tokenSupplier(oidcToken),
+		Scopes:               scopes,
+	}
+
+	email, err := getServiceAccountEmail(serviceAccount)
+	if err != nil {
+		return nil, err
+	}
+
+	if email != "" { // impersonation
+		conf.ServiceAccountImpersonationURL = fmt.Sprintf(
+			"https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/%s:generateAccessToken",
+			email)
+	} else { // direct access
+		conf.TokenInfoURL = "https://sts.googleapis.com/v1/introspect"
+	}
+
+	if hc := o.GetHTTPClient(); hc != nil {
+		ctx = context.WithValue(ctx, oauth2.HTTPClient, hc)
+	}
+
+	src, err := externalaccount.NewTokenSource(ctx, conf)
+	if err != nil {
+		return nil, err
+	}
+	token, err := src.Token()
+	if err != nil {
+		return nil, err
+	}
+
+	return &Token{*token}, nil
+}
+
+// GetArtifactCacheKey implements auth.Provider.
+func (Provider) GetArtifactCacheKey(artifactRepository string) string {
+	// The artifact repository is irrelevant for GCP registry credentials.
+	return ProviderName
+}
+
+// NewArtifactRegistryToken implements auth.Provider.
+func (Provider) NewArtifactRegistryToken(ctx context.Context, artifactRepository string,
+	accessToken auth.Token, opts ...auth.Option) (auth.Token, error) {
+
+	t := accessToken.(*Token)
+
+	// The artifact repository is irrelevant for GCP registry credentials.
+	return &auth.ArtifactRegistryCredentials{
+		Username:  "oauth2accesstoken",
+		Password:  t.AccessToken,
+		ExpiresAt: t.Expiry,
+	}, nil
+}

auth/gcp/token.go

@@ -0,0 +1,36 @@
+/*
+Copyright 2025 The Flux authors
+
+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.
+*/
+
+package gcp
+
+import (
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+// Token is the GCP token.
+type Token struct{ oauth2.Token }
+
+// GetDuration implements auth.Token.
+func (t *Token) GetDuration() time.Duration {
+	return time.Until(t.Expiry)
+}
+
+// Source gets a token source for the token to use with GCP libraries.
+func (t *Token) Source() oauth2.TokenSource {
+	return oauth2.StaticTokenSource(&t.Token)
+}