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feat: add -impersonate-service-account flag #53

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09beb20
fix: update ADC fallback error message check
pokutuna Jan 28, 2026
933f23b
feat: add service account impersonation support
pokutuna Jan 30, 2026
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24 changes: 20 additions & 4 deletions main.go
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,7 @@ import (
"golang.org/x/oauth2"
"golang.org/x/oauth2/google"
"google.golang.org/api/idtoken"
"google.golang.org/api/impersonate"
)

type contextKey string
Expand Down Expand Up @@ -66,6 +67,8 @@ var (
"handle http2 requests (allows grpc calls)")
flagAuthorizationHeader = flag.String("authorization-header", "X-Serverless-Authorization",
"header to provide the bearer token")
flagImpersonateServiceAccount = flag.String("impersonate-service-account", "",
"impersonate the specified service account to obtain ID tokens")
)

func main() {
Expand Down Expand Up @@ -130,7 +133,7 @@ func realMain(ctx context.Context) error {

// Get the best token source. Cloud Run expects the audience parameter to be
// the URL of the service.
tokenSource, err := findTokenSource(ctx, *flagToken, audience)
tokenSource, err := findTokenSource(ctx, *flagToken, audience, *flagImpersonateServiceAccount)
if err != nil {
return fmt.Errorf("failed to find token source: %w", err)
}
Expand Down Expand Up @@ -299,15 +302,28 @@ func createServer(bind *url.URL, proxy *httputil.ReverseProxy, enableHttp2 bool)

// findTokenSource fetches the reusable/cached oauth2 token source. If rawToken
// is provided, that token is used as a static value and the audience parameter
// is ignored. Othwerise, this attempts to get the renewable token from the
// is ignored. Otherwise, this attempts to get the renewable token from the
// environment (via Application Default Credentials).
func findTokenSource(ctx context.Context, rawToken, audience string) (oauth2.TokenSource, error) {
func findTokenSource(ctx context.Context, rawToken, audience, impersonateServiceAccount string) (oauth2.TokenSource, error) {
// Prefer supplied value, usually from the flag.
if rawToken != "" {
token := &oauth2.Token{AccessToken: rawToken}
return oauth2.StaticTokenSource(token), nil
}

// If impersonation is requested, use the impersonate package to get ID tokens.
if impersonateServiceAccount != "" {
tokenSource, err := impersonate.IDTokenSource(ctx, impersonate.IDTokenConfig{
TargetPrincipal: impersonateServiceAccount,
Audience: audience,
IncludeEmail: true,
})
if err != nil {
return nil, fmt.Errorf("failed to create impersonated token source: %w", err)
}
return tokenSource, nil
}

// Try to use the idtoken package, which will use the metadata service.
// However, the idtoken package does not work with gcloud's ADC, so we need to
// handle that case by falling back to default ADC search. However, the
Expand All @@ -316,7 +332,7 @@ func findTokenSource(ctx context.Context, rawToken, audience string) (oauth2.Tok
tokenSource, err := idtoken.NewTokenSource(ctx, audience)
if err != nil {
// Return any unexpected error.
if !strings.Contains(err.Error(), "credential must be service_account") {
if !strings.Contains(err.Error(), "unsupported credentials type") {
return nil, fmt.Errorf("failed to get idtoken source: %w", err)
}

Expand Down
11 changes: 5 additions & 6 deletions main_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -67,7 +67,7 @@ func TestBuildProxy(t *testing.T) {
Host: fmt.Sprintf("localhost:%d", testRandomPort(t)),
}

src, err := findTokenSource(ctx, "mytoken", "aud")
src, err := findTokenSource(ctx, "mytoken", "aud", "")
if err != nil {
t.Fatal(err)
}
Expand All @@ -80,7 +80,7 @@ func TestBuildProxy(t *testing.T) {
w := httptest.NewRecorder()
r := httptest.NewRequest(http.MethodGet, "/", nil)
proxy.ServeHTTP(w, r)
if !called{
if !called {
t.Errorf("handler not called")
}
})
Expand All @@ -94,7 +94,7 @@ func TestFindTokenSource(t *testing.T) {
t.Run("static", func(t *testing.T) {
t.Parallel()

src, err := findTokenSource(ctx, "mytoken", "aud")
src, err := findTokenSource(ctx, "mytoken", "aud", "")
if err != nil {
t.Fatal(err)
}
Expand Down Expand Up @@ -184,7 +184,6 @@ func TestHttp2(t *testing.T) {
called = true
})


srv := httptest.NewUnstartedServer(mux)
srv.EnableHTTP2 = true
srv.StartTLS()
Expand All @@ -199,7 +198,7 @@ func TestHttp2(t *testing.T) {
Host: fmt.Sprintf("localhost:%d", testRandomPort(t)),
}

src, err := findTokenSource(ctx, "mytoken", "aud")
src, err := findTokenSource(ctx, "mytoken", "aud", "")
if err != nil {
t.Fatal(err)
}
Expand All @@ -214,7 +213,7 @@ func TestHttp2(t *testing.T) {

proxy.ServeHTTP(w, r)

if !called{
if !called {
t.Errorf("handler not called")
}
defer srv.Close()
Expand Down