feat: server-level GitHub machine credential for authenticated requests (#109)

## Why

Anonymous GitHub is capped at 60 requests/hour and cannot read private repositories. A machine credential usable by a free (non-enterprise) account is needed to lift the request budget to ~5000/hr and to read private-repo release assets.

Builds on the background syncer (#108, now merged to `master`); this diff is the auth changes only.

## How

- Add `internal/githubauth`: a process-wide GitHub credential delivered via env/secret, applied by default to every outbound GitHub request (releases scan, ranged asset-header GETs, and the generic-github byte proxy for private assets).
- Support two modes:
  - **PAT** — `GITHUB_TOKEN` sent as `Authorization: Bearer <token>`.
  - **GitHub App** — `GITHUB_APP_ID` + `GITHUB_APP_INSTALLATION_ID` + private key (`GITHUB_APP_PRIVATE_KEY` inline PEM or `GITHUB_APP_PRIVATE_KEY_PATH`). Mint a short-lived RS256 JWT with stdlib `crypto/rsa` (no new dependency), exchange it at `POST /app/installations/{id}/access_tokens` for a ~1h installation token, cache it, and single-flight a refresh a few minutes before expiry.
- Inject at the two GitHub call paths: the rpm github provider header builder (releases + ranged fetches) and the generic provider `AuthHeaders` (byte proxy, github.com hosts only; the pre-signed `objects.githubusercontent.com` redirect deliberately gets no Authorization).
- Honor precedence: a remote's own `username`/`password` overrides the server credential; no credential configured stays anonymous (current behavior).
- Fail closed at startup on partial App configuration (e.g. App id without a private key); a token-and-App conflict is also rejected.
- Never persist the credential to the DB, return it from an API, or log it (token-exchange failures never echo the response body).
- Read config via the existing `getenv` convention; document PAT vs App setup, the free-account fine-grained PAT scopes (Contents:read + Metadata:read), precedence, and the rate-limit implication.

## Rate limit

Authenticated requests share the syncer's single global limiter — no second limiter is added. A token raises the effective GitHub ceiling (~5000/hr vs ~60/hr), so the limiter defaults stay safe.

## Tests

`internal/githubauth` and `internal/provider/{rpm,generic}`:
- PAT attaches the correct `Authorization` header to releases + asset-header requests.
- App mints a valid RS256 JWT (verified against the app public key), exchanges it at a mocked endpoint, reuses the cached token without re-exchanging, refreshes near expiry, and single-flights concurrent callers.
- Per-remote credential overrides the server credential (rpm + generic).
- No credential → no `Authorization` header, requests still succeed anonymously.
- ETag/304 flow still works with auth attached.
- The credential does not appear in a remote's serialized JSON.
- Config validation: no-config is anonymous; partial App config and token/App conflict both error.

Verified fail-before/pass-after for the injection tests. `gofmt -l`, `go build ./...`, `go vet ./...`, `go test ./...` all clean (26 packages).

Reviewed-on: #109
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/githubauth/app.go

@@ -0,0 +1,199 @@
+package githubauth
+
+import (
+	"context"
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+)
+
+const (
+	defaultAPIBase = "https://api.github.com"
+
+	// jwtLifetime is how long the app JWT is valid. GitHub caps it at 10 minutes;
+	// 9 leaves headroom for clock skew.
+	jwtLifetime = 9 * time.Minute
+	// jwtBackdate backdates iat to tolerate the app server's clock running behind
+	// GitHub's, which otherwise rejects the JWT.
+	jwtBackdate = 60 * time.Second
+	// refreshSkew refreshes the installation token this long before it expires so
+	// a request never races an expiry.
+	refreshSkew = 5 * time.Minute
+)
+
+type httpDoer interface {
+	Do(*http.Request) (*http.Response, error)
+}
+
+// appCredential mints installation access tokens for a GitHub App. It signs a
+// short-lived RS256 JWT with the app private key, exchanges it for a ~1h
+// installation token, caches that token, and refreshes it shortly before expiry.
+// Refreshes are single-flighted by holding the mutex across the exchange, so
+// concurrent callers coalesce onto one HTTP request and reuse the cached token.
+type appCredential struct {
+	appID          string
+	installationID string
+	key            *rsa.PrivateKey
+	apiBase        string
+	client         httpDoer
+
+	mu     sync.Mutex
+	token  string
+	expiry time.Time
+}
+
+func newAppCredential(opts Options) (*appCredential, error) {
+	if opts.AppID == "" {
+		return nil, errors.New("github app: GITHUB_APP_ID is required")
+	}
+	if opts.InstallationID == "" {
+		return nil, errors.New("github app: GITHUB_APP_INSTALLATION_ID is required")
+	}
+	pemBytes, err := loadPrivateKeyPEM(opts)
+	if err != nil {
+		return nil, err
+	}
+	key, err := parseRSAPrivateKey(pemBytes)
+	if err != nil {
+		return nil, err
+	}
+
+	apiBase := opts.apiBaseURL
+	if apiBase == "" {
+		apiBase = defaultAPIBase
+	}
+	client := opts.httpClient
+	if client == nil {
+		client = &http.Client{Timeout: 30 * time.Second}
+	}
+
+	return &appCredential{
+		appID:          opts.AppID,
+		installationID: opts.InstallationID,
+		key:            key,
+		apiBase:        strings.TrimRight(apiBase, "/"),
+		client:         client,
+	}, nil
+}
+
+// Token returns a cached installation token, refreshing it under a single-flight
+// lock when it is missing or within refreshSkew of expiry.
+func (a *appCredential) Token(ctx context.Context) (string, error) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if a.token != "" && time.Now().Before(a.expiry.Add(-refreshSkew)) {
+		return a.token, nil
+	}
+	if err := a.refreshLocked(ctx); err != nil {
+		return "", err
+	}
+	return a.token, nil
+}
+
+func (a *appCredential) refreshLocked(ctx context.Context) error {
+	jwt, err := mintJWT(a.appID, a.key, time.Now())
+	if err != nil {
+		return fmt.Errorf("github app: mint jwt: %w", err)
+	}
+
+	u := fmt.Sprintf("%s/app/installations/%s/access_tokens", a.apiBase, a.installationID)
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, u, nil)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Authorization", "Bearer "+jwt)
+	req.Header.Set("Accept", "application/vnd.github+json")
+	req.Header.Set("X-GitHub-Api-Version", "2022-11-28")
+
+	resp, err := a.client.Do(req)
+	if err != nil {
+		return fmt.Errorf("github app: token exchange: %w", err)
+	}
+	defer resp.Body.Close()
+	body, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	if resp.StatusCode != http.StatusCreated && resp.StatusCode != http.StatusOK {
+		// Never echo the body verbatim — it can contain sensitive material.
+		return fmt.Errorf("github app: token exchange status %d", resp.StatusCode)
+	}
+
+	var out struct {
+		Token     string    `json:"token"`
+		ExpiresAt time.Time `json:"expires_at"`
+	}
+	if err := json.Unmarshal(body, &out); err != nil {
+		return fmt.Errorf("github app: decode token response: %w", err)
+	}
+	if out.Token == "" {
+		return errors.New("github app: token exchange returned an empty token")
+	}
+	a.token = out.Token
+	a.expiry = out.ExpiresAt
+	if a.expiry.IsZero() {
+		// Defensive: assume the documented ~1h lifetime if GitHub omits it.
+		a.expiry = time.Now().Add(time.Hour)
+	}
+	return nil
+}
+
+// mintJWT builds and RS256-signs a GitHub App JWT (iss=app id, backdated iat,
+// ≤10m exp) using stdlib crypto — no third-party JWT dependency.
+func mintJWT(appID string, key *rsa.PrivateKey, now time.Time) (string, error) {
+	header := map[string]string{"alg": "RS256", "typ": "JWT"}
+	claims := map[string]any{
+		"iat": now.Add(-jwtBackdate).Unix(),
+		"exp": now.Add(jwtLifetime).Unix(),
+		"iss": appID,
+	}
+	hb, err := json.Marshal(header)
+	if err != nil {
+		return "", err
+	}
+	cb, err := json.Marshal(claims)
+	if err != nil {
+		return "", err
+	}
+	signingInput := b64url(hb) + "." + b64url(cb)
+	digest := sha256.Sum256([]byte(signingInput))
+	sig, err := rsa.SignPKCS1v15(rand.Reader, key, crypto.SHA256, digest[:])
+	if err != nil {
+		return "", err
+	}
+	return signingInput + "." + b64url(sig), nil
+}
+
+func b64url(b []byte) string {
+	return base64.RawURLEncoding.EncodeToString(b)
+}
+
+// parseRSAPrivateKey accepts PKCS#1 ("RSA PRIVATE KEY") and PKCS#8 ("PRIVATE
+// KEY") PEM, covering both GitHub App key export formats.
+func parseRSAPrivateKey(pemBytes []byte) (*rsa.PrivateKey, error) {
+	block, _ := pem.Decode(pemBytes)
+	if block == nil {
+		return nil, errors.New("github app: private key is not valid PEM")
+	}
+	if key, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
+		return key, nil
+	}
+	keyAny, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		return nil, errors.New("github app: private key is not a supported RSA PKCS#1/PKCS#8 key")
+	}
+	rsaKey, ok := keyAny.(*rsa.PrivateKey)
+	if !ok {
+		return nil, errors.New("github app: private key is not an RSA key")
+	}
+	return rsaKey, nil
+}

internal/githubauth/app_test.go

@@ -0,0 +1,207 @@
+package githubauth
+
+import (
+	"context"
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"encoding/pem"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+func testRSAKeyPEM(t *testing.T) string {
+	t.Helper()
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatalf("generate key: %v", err)
+	}
+	der := x509.MarshalPKCS1PrivateKey(key)
+	return string(pem.EncodeToMemory(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: der}))
+}
+
+// appFixture serves the installation-token exchange endpoint, records requests,
+// verifies the presented JWT against the app public key, and returns tokens with
+// a controllable expiry.
+type appFixture struct {
+	srv       *httptest.Server
+	pub       *rsa.PublicKey
+	mu        sync.Mutex
+	exchanges int
+	lastJWT   string
+	expiresAt func() time.Time
+	tokenSeq  int
+}
+
+func newAppFixture(t *testing.T, pemKey string) *appFixture {
+	t.Helper()
+	block, _ := pem.Decode([]byte(pemKey))
+	key, err := x509.ParsePKCS1PrivateKey(block.Bytes)
+	if err != nil {
+		t.Fatalf("parse test key: %v", err)
+	}
+	f := &appFixture{
+		pub:       &key.PublicKey,
+		expiresAt: func() time.Time { return time.Now().Add(time.Hour) },
+	}
+	mux := http.NewServeMux()
+	mux.HandleFunc("/app/installations/456/access_tokens", func(w http.ResponseWriter, r *http.Request) {
+		auth := r.Header.Get("Authorization")
+		jwt := strings.TrimPrefix(auth, "Bearer ")
+		f.mu.Lock()
+		f.exchanges++
+		f.lastJWT = jwt
+		f.tokenSeq++
+		seq := f.tokenSeq
+		exp := f.expiresAt()
+		f.mu.Unlock()
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusCreated)
+		json.NewEncoder(w).Encode(map[string]any{
+			"token":      fmt.Sprintf("ghs_installation_%d", seq),
+			"expires_at": exp.UTC().Format(time.RFC3339),
+		})
+	})
+	f.srv = httptest.NewServer(mux)
+	t.Cleanup(f.srv.Close)
+	return f
+}
+
+func (f *appFixture) verifyJWT(t *testing.T) {
+	t.Helper()
+	f.mu.Lock()
+	jwt := f.lastJWT
+	f.mu.Unlock()
+	parts := strings.Split(jwt, ".")
+	if len(parts) != 3 {
+		t.Fatalf("jwt not three-part: %q", jwt)
+	}
+	signingInput := parts[0] + "." + parts[1]
+	sig, err := base64.RawURLEncoding.DecodeString(parts[2])
+	if err != nil {
+		t.Fatalf("decode sig: %v", err)
+	}
+	digest := sha256.Sum256([]byte(signingInput))
+	if err := rsa.VerifyPKCS1v15(f.pub, crypto.SHA256, digest[:], sig); err != nil {
+		t.Fatalf("jwt signature invalid: %v", err)
+	}
+	var claims struct {
+		Iss string `json:"iss"`
+		Iat int64  `json:"iat"`
+		Exp int64  `json:"exp"`
+	}
+	cb, _ := base64.RawURLEncoding.DecodeString(parts[1])
+	if err := json.Unmarshal(cb, &claims); err != nil {
+		t.Fatalf("decode claims: %v", err)
+	}
+	if claims.Iss != "123" {
+		t.Fatalf("iss = %q, want 123", claims.Iss)
+	}
+	if claims.Exp-claims.Iat > int64((10*time.Minute)/time.Second) {
+		t.Fatalf("jwt lifetime exceeds 10m: iat=%d exp=%d", claims.Iat, claims.Exp)
+	}
+	if claims.Iat > time.Now().Unix() {
+		t.Fatalf("iat not backdated: %d", claims.Iat)
+	}
+}
+
+func newAppCred(t *testing.T, f *appFixture, pemKey string) *appCredential {
+	t.Helper()
+	c, err := newAppCredential(Options{
+		AppID:          "123",
+		InstallationID: "456",
+		PrivateKeyPEM:  pemKey,
+		apiBaseURL:     f.srv.URL,
+		httpClient:     f.srv.Client(),
+	})
+	if err != nil {
+		t.Fatalf("newAppCredential: %v", err)
+	}
+	return c
+}
+
+func TestApp_MintsJWTAndExchangesForInstallationToken(t *testing.T) {
+	pemKey := testRSAKeyPEM(t)
+	f := newAppFixture(t, pemKey)
+	c := newAppCred(t, f, pemKey)
+
+	tok, err := c.Token(context.Background())
+	if err != nil {
+		t.Fatalf("token: %v", err)
+	}
+	if tok != "ghs_installation_1" {
+		t.Fatalf("token = %q, want ghs_installation_1", tok)
+	}
+	if f.exchanges != 1 {
+		t.Fatalf("exchanges = %d, want 1", f.exchanges)
+	}
+	f.verifyJWT(t)
+}
+
+func TestApp_CachesInstallationToken(t *testing.T) {
+	pemKey := testRSAKeyPEM(t)
+	f := newAppFixture(t, pemKey)
+	c := newAppCred(t, f, pemKey)
+
+	for i := 0; i < 5; i++ {
+		if _, err := c.Token(context.Background()); err != nil {
+			t.Fatalf("token: %v", err)
+		}
+	}
+	if f.exchanges != 1 {
+		t.Fatalf("exchanges = %d, want 1 (token should be cached)", f.exchanges)
+	}
+}
+
+func TestApp_RefreshesNearExpiry(t *testing.T) {
+	pemKey := testRSAKeyPEM(t)
+	f := newAppFixture(t, pemKey)
+	// Token expires within refreshSkew, so every call must re-exchange.
+	f.expiresAt = func() time.Time { return time.Now().Add(2 * time.Minute) }
+	c := newAppCred(t, f, pemKey)
+
+	t1, err := c.Token(context.Background())
+	if err != nil {
+		t.Fatalf("token 1: %v", err)
+	}
+	t2, err := c.Token(context.Background())
+	if err != nil {
+		t.Fatalf("token 2: %v", err)
+	}
+	if f.exchanges != 2 {
+		t.Fatalf("exchanges = %d, want 2 (near-expiry token must refresh)", f.exchanges)
+	}
+	if t1 == t2 {
+		t.Fatalf("expected a fresh token after refresh, both = %q", t1)
+	}
+}
+
+func TestApp_ConcurrentTokenSingleFlights(t *testing.T) {
+	pemKey := testRSAKeyPEM(t)
+	f := newAppFixture(t, pemKey)
+	c := newAppCred(t, f, pemKey)
+
+	var wg sync.WaitGroup
+	for i := 0; i < 20; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, err := c.Token(context.Background()); err != nil {
+				t.Errorf("token: %v", err)
+			}
+		}()
+	}
+	wg.Wait()
+	if f.exchanges != 1 {
+		t.Fatalf("exchanges = %d, want 1 (concurrent calls must coalesce)", f.exchanges)
+	}
+}

internal/githubauth/credential.go

@@ -0,0 +1,106 @@
+// Package githubauth provides the process-wide GitHub machine credential used to
+// authenticate every outbound GitHub request (releases scan, ranged asset header
+// fetches, and the generic-github byte proxy for private assets). The credential
+// is delivered via env/secret only — it is never stored per-remote in the DB,
+// never returned by any API, and never logged.
+package githubauth
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"sync"
+)
+
+// Credential yields a bearer token for GitHub requests. Token may block to mint
+// or refresh (the GitHub App path); an empty string means "no auth", which only
+// happens when no credential is configured.
+type Credential interface {
+	Token(ctx context.Context) (string, error)
+}
+
+// Options is the raw, env-sourced auth configuration. Exactly one mode may be
+// configured: a static token, or a GitHub App (id + installation id + private
+// key). Partial App configuration is an error (fail closed); no fields at all is
+// fine and yields a nil credential (anonymous, current behavior).
+type Options struct {
+	// Token is a Personal Access Token (fine-grained or classic) sent verbatim
+	// as "Authorization: Bearer <token>".
+	Token string
+
+	// GitHub App fields. PrivateKeyPEM and PrivateKeyPath are alternatives; the
+	// inline PEM wins when both are set.
+	AppID          string
+	InstallationID string
+	PrivateKeyPEM  string
+	PrivateKeyPath string
+
+	// apiBaseURL overrides https://api.github.com for tests. Empty uses the real
+	// endpoint. httpClient likewise overrides the default client for tests.
+	apiBaseURL string
+	httpClient httpDoer
+}
+
+// New builds the process credential from options, validating that auth is either
+// fully configured or fully absent. It returns (nil, nil) when nothing is set.
+func New(opts Options) (Credential, error) {
+	hasToken := opts.Token != ""
+	hasAppField := opts.AppID != "" || opts.InstallationID != "" ||
+		opts.PrivateKeyPEM != "" || opts.PrivateKeyPath != ""
+
+	switch {
+	case !hasToken && !hasAppField:
+		return nil, nil // no auth configured — anonymous is fine
+	case hasToken && hasAppField:
+		return nil, errors.New("github auth: both a token and GitHub App fields are set; configure exactly one")
+	case hasToken:
+		return staticToken{token: opts.Token}, nil
+	default:
+		return newAppCredential(opts)
+	}
+}
+
+// staticToken is a fixed PAT credential.
+type staticToken struct{ token string }
+
+func (s staticToken) Token(context.Context) (string, error) { return s.token, nil }
+
+// server is the process-wide credential set once at startup. A nil value means
+// no server credential (anonymous). Access is guarded so a late SetServer in a
+// test is race-free.
+var (
+	serverMu sync.RWMutex
+	server   Credential
+)
+
+// SetServer installs the process credential. Call once during startup.
+func SetServer(c Credential) {
+	serverMu.Lock()
+	server = c
+	serverMu.Unlock()
+}
+
+// Server returns the process credential, or nil if none is configured.
+func Server() Credential {
+	serverMu.RLock()
+	defer serverMu.RUnlock()
+	return server
+}
+
+// loadPrivateKeyPEM resolves the App private key bytes from the inline PEM or a
+// file path, without ever returning the key material in an error message.
+func loadPrivateKeyPEM(opts Options) ([]byte, error) {
+	if strings.TrimSpace(opts.PrivateKeyPEM) != "" {
+		return []byte(opts.PrivateKeyPEM), nil
+	}
+	if opts.PrivateKeyPath != "" {
+		b, err := os.ReadFile(opts.PrivateKeyPath)
+		if err != nil {
+			return nil, fmt.Errorf("github app: read private key file: %w", err)
+		}
+		return b, nil
+	}
+	return nil, errors.New("github app: no private key configured")
+}

internal/githubauth/credential_test.go

@@ -0,0 +1,77 @@
+package githubauth
+
+import (
+	"context"
+	"testing"
+)
+
+func TestNew_NoConfigIsAnonymous(t *testing.T) {
+	c, err := New(Options{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if c != nil {
+		t.Fatalf("expected nil credential when nothing configured, got %T", c)
+	}
+}
+
+func TestNew_TokenMode(t *testing.T) {
+	c, err := New(Options{Token: "ghp_example"})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	tok, err := c.Token(context.Background())
+	if err != nil {
+		t.Fatalf("token: %v", err)
+	}
+	if tok != "ghp_example" {
+		t.Fatalf("token = %q, want ghp_example", tok)
+	}
+}
+
+func TestNew_TokenAndAppConflict(t *testing.T) {
+	_, err := New(Options{Token: "ghp_example", AppID: "123"})
+	if err == nil {
+		t.Fatal("expected error when both token and app fields are set")
+	}
+}
+
+func TestNew_PartialAppFailsClosed(t *testing.T) {
+	cases := map[string]Options{
+		"app id without key":  {AppID: "123", InstallationID: "456"},
+		"key without app id":  {InstallationID: "456", PrivateKeyPEM: testRSAKeyPEM(t)},
+		"app id without inst": {AppID: "123", PrivateKeyPEM: testRSAKeyPEM(t)},
+	}
+	for name, opts := range cases {
+		t.Run(name, func(t *testing.T) {
+			if _, err := New(opts); err == nil {
+				t.Fatalf("expected fail-closed error for %q", name)
+			}
+		})
+	}
+}
+
+func TestNew_AppModeParsesKey(t *testing.T) {
+	c, err := New(Options{
+		AppID:          "123",
+		InstallationID: "456",
+		PrivateKeyPEM:  testRSAKeyPEM(t),
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if _, ok := c.(*appCredential); !ok {
+		t.Fatalf("expected *appCredential, got %T", c)
+	}
+}
+
+func TestNew_AppModeRejectsBadKey(t *testing.T) {
+	_, err := New(Options{
+		AppID:          "123",
+		InstallationID: "456",
+		PrivateKeyPEM:  "-----BEGIN RSA PRIVATE KEY-----\nnope\n-----END RSA PRIVATE KEY-----",
+	})
+	if err == nil {
+		t.Fatal("expected error for malformed private key")
+	}
+}