credentials: implement file-based JWT Call Credentials (part 1 for A97)

credentials/jwt/doc.go

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+/*
+ *
+ * Copyright 2025 gRPC 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 jwt implements JWT token file-based call credentials.
+//
+// This package provides support for A97 JWT Call Credentials, allowing gRPC
+// clients to authenticate using JWT tokens read from files. While originally
+// designed for xDS environments, these credentials are general-purpose.
+//
+// The credentials can be used directly in gRPC clients or configured via xDS.
+//
+// # Token Requirements
+//
+// JWT tokens must:
+//   - Be valid, well-formed JWT tokens with header, payload, and signature
+//   - Include an "exp" (expiration) claim
+//   - Be readable from the specified file path
+//
+// # Considerations
+//
+// - Tokens are cached until expiration to avoid excessive file I/O
+// - Transport security is required (RequireTransportSecurity returns true)
+// - Errors in reading tokens or parsing JWTs will result in RPC UNAVAILALBE or
+// UNAUTHENTICATED errors
+// - These errors are cached and retried with exponential backoff.
+//
+// This implementation is originally intended for use in service mesh
+// environments like Istio where JWT tokens are provisioned and rotated by the
+// infrastructure.
+//
+// # Experimental
+//
+// Notice: All APIs in this package are experimental and may be removed in a
+// later release.
+package jwt

credentials/jwt/jwt_token_file.go

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+/*
+ *
+ * Copyright 2025 gRPC 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 jwt implements gRPC credentials using JWT tokens from files.
+package jwt
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/internal/backoff"
+	"google.golang.org/grpc/status"
+)
+
+// jwtClaims represents the JWT claims structure for extracting expiration time.
+type jwtClaims struct {
+	Exp int64 `json:"exp"`
+}
+
+// jwtTokenFileCallCreds provides JWT token-based PerRPCCredentials that reads
+// tokens from a file.
+// This implementation follows the A97 JWT Call Credentials specification.
+type jwtTokenFileCallCreds struct {
+	tokenFilePath   string
+	backoffStrategy backoff.Strategy // Strategy when error occurs
+
+	// Cached token data
+	mu               sync.RWMutex
+	cachedToken      string
+	cachedExpiration time.Time // Slightly less than actual expiration time
+	cachedError      error     // Error from last failed attempt
+	retryAttempt     int       // Current retry attempt number
+	nextRetryTime    time.Time // When next retry is allowed
+
+	// Pre-emptive refresh mutex
+	refreshMu sync.Mutex
+}
+
+// NewTokenFileCallCredentials creates PerRPCCredentials that reads JWT tokens
+// from the specified file path.
+func NewTokenFileCallCredentials(tokenFilePath string) (credentials.PerRPCCredentials, error) {
+	if tokenFilePath == "" {
+		return nil, fmt.Errorf("tokenFilePath cannot be empty")
+	}
+
+	return &jwtTokenFileCallCreds{
+		tokenFilePath:   tokenFilePath,
+		backoffStrategy: backoff.DefaultExponential,
+	}, nil
+}
+
+// GetRequestMetadata gets the current request metadata, refreshing tokens if
+// required. This implementation follows the PerRPCCredentials interface.  The
+// tokens will get automatically refreshed if they are about to expire or if
+// they haven't been loaded successfully yet.
+// If it's not possible to extract a token from the file, UNAVAILABLE is
+// returned.
+// If the token is extracted but invalid, then UNAUTHENTICATED is returned.
+// If errors are encoutered, a backoff is applied before retrying.
+func (c *jwtTokenFileCallCreds) GetRequestMetadata(ctx context.Context, _ ...string) (map[string]string, error) {
+	ri, _ := credentials.RequestInfoFromContext(ctx)
+	if err := credentials.CheckSecurityLevel(ri.AuthInfo, credentials.PrivacyAndIntegrity); err != nil {
+		return nil, fmt.Errorf("unable to transfer JWT token file PerRPCCredentials: %v", err)
+	}
+
+	// This may be delayed if the token needs to be refreshed from file.
+	token, err := c.getToken()
+	if err != nil {
+		return nil, err
+	}
+
+	return map[string]string{
+		"authorization": "Bearer " + token,
+	}, nil
+}
+
+// RequireTransportSecurity indicates whether the credentials requires
+// transport security.
+func (c *jwtTokenFileCallCreds) RequireTransportSecurity() bool {
+	return true
+}
+
+// getToken returns a valid JWT token, reading from file if necessary.
+// Implements pre-emptive refresh and caches errors with backoff.
+func (c *jwtTokenFileCallCreds) getToken() (string, error) {
+	c.mu.RLock()
+
+	if c.isTokenValidLocked() {
+		token := c.cachedToken
+		shouldRefresh := c.needsPreemptiveRefreshLocked()
+		c.mu.RUnlock()
+
+		if shouldRefresh {
+			c.triggerPreemptiveRefresh()
+		}
+		return token, nil
+	}
+
+	// If still within backoff period, return cached error to avoid repeated
+	// file reads.
+	if c.cachedError != nil && time.Now().Before(c.nextRetryTime) {
+		err := c.cachedError
+		c.mu.RUnlock()
+		return "", err
+	}
+
+	c.mu.RUnlock()
+	// Token is expired or missing or the retry backoff period has expired.
+	// So we should refresh synchronously.
+	// NOTE: refreshTokenSync itself acquires the write lock.
+	return c.refreshTokenSync(false)
+}
+
+// isTokenValidLocked checks if the cached token is still valid.
+// Caller must hold c.mu.RLock().
+func (c *jwtTokenFileCallCreds) isTokenValidLocked() bool {
+	if c.cachedToken == "" {
+		return false
+	}
+	return c.cachedExpiration.After(time.Now())
+}
+
+// needsPreemptiveRefreshLocked checks if a pre-emptive refresh should be
+// triggered.
+// Returns true if the cached token is valid but expires within 1 minute.
+// We only trigger pre-emptive refresh for valid tokens - if the token is
+// invalid or expired, the next RPC will handle synchronous refresh instead.
+// Caller must hold c.mu.RLock().
+func (c *jwtTokenFileCallCreds) needsPreemptiveRefreshLocked() bool {
+	return c.isTokenValidLocked() && time.Until(c.cachedExpiration) < time.Minute
+}
+
+// triggerPreemptiveRefresh starts a background refresh if needed.
+// Multiple concurrent calls are safe - only one refresh will run at a time.
+// The refresh runs in a separate goroutine and does not block the caller.
+func (c *jwtTokenFileCallCreds) triggerPreemptiveRefresh() {
+	go func() {
+		c.refreshMu.Lock()
+		defer c.refreshMu.Unlock()
+
+		// Re-check if refresh is still needed under mutex.
+		c.mu.RLock()
+		stillNeeded := c.needsPreemptiveRefreshLocked()
+		c.mu.RUnlock()
+
+		if !stillNeeded {
+			return // Another goroutine already refreshed or token expired.
+		}
+
+		// Force refresh to read new token even if current one is still valid.
+		_, _ = c.refreshTokenSync(true)
+	}()
+}
+
+// refreshTokenSync reads a new token from the file and updates the cache.  If
+// forceRefresh is true, bypasses the validity check of the currently
+// cached token and always reads from file.
+// This is used for pre-emptive refresh to ensure new tokens are loaded even
+// when the cached token is still valid. If forceRefresh is false, skips
+// file read when cached token is still valid, optimizing concurrent synchronous
+// refresh calls where one RPC may have already updated the cache while another
+// was waiting on the lock.
+func (c *jwtTokenFileCallCreds) refreshTokenSync(forceRefresh bool) (string, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	// Double-check under write lock but skip if preemptive refresh is
+	// requested.
+	if !forceRefresh && c.isTokenValidLocked() {
+		return c.cachedToken, nil
+	}
+
+	tokenBytes, err := os.ReadFile(c.tokenFilePath)
+	if err != nil {
+		err = status.Errorf(codes.Unavailable, "failed to read token file %q: %v", c.tokenFilePath, err)
+		c.setErrorWithBackoffLocked(err)
+		return "", err
+	}
+
+	token := strings.TrimSpace(string(tokenBytes))
+	if token == "" {
+		err := status.Errorf(codes.Unavailable, "token file %q is empty", c.tokenFilePath)
+		c.setErrorWithBackoffLocked(err)
+		return "", err
+	}
+
+	// Parse JWT to extract expiration.
+	exp, err := c.extractExpiration(token)
+	if err != nil {
+		err = status.Errorf(codes.Unauthenticated, "failed to parse JWT from token file %q: %v", c.tokenFilePath, err)
+		c.setErrorWithBackoffLocked(err)
+		return "", err
+	}
+
+	// Success - clear any cached error and backoff state, update token cache.
+	c.clearErrorAndBackoffLocked()
+	c.cachedToken = token
+	// Per RFC A97: consider token invalid if it expires within the next 30
+	// seconds to accommodate for clock skew and server processing time.
+	c.cachedExpiration = exp.Add(-30 * time.Second)
+
+	return token, nil
+}
+
+// extractExpiration parses the JWT token to extract the expiration time.
+func (c *jwtTokenFileCallCreds) extractExpiration(token string) (time.Time, error) {
+	parts := strings.Split(token, ".")
+	if len(parts) != 3 {
+		return time.Time{}, fmt.Errorf("invalid JWT format: expected 3 parts, got %d", len(parts))
+	}
+
+	payload := parts[1]
+	// Add padding if necessary for base64 decoding.
+	if m := len(payload) % 4; m != 0 {
+		payload += strings.Repeat("=", 4-m)
+	}
+
+	payloadBytes, err := base64.URLEncoding.DecodeString(payload)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("failed to decode JWT payload: %v", err)
+	}
+
+	var claims jwtClaims
+	if err := json.Unmarshal(payloadBytes, &claims); err != nil {
+		return time.Time{}, fmt.Errorf("failed to unmarshal JWT claims: %v", err)
+	}
+
+	if claims.Exp == 0 {
+		return time.Time{}, fmt.Errorf("JWT token has no expiration claim")
+	}
+
+	expTime := time.Unix(claims.Exp, 0)
+
+	// Check if token is already expired.
+	if expTime.Before(time.Now()) {
+		return time.Time{}, fmt.Errorf("JWT token is expired")
+	}
+
+	return expTime, nil
+}
+
+// setErrorWithBackoffLocked caches an error and calculates the next retry time
+// using exponential backoff.
+// Caller must hold c.mu write lock.
+func (c *jwtTokenFileCallCreds) setErrorWithBackoffLocked(err error) {
+	c.cachedError = err
+	c.retryAttempt++
+	backoffDelay := c.backoffStrategy.Backoff(c.retryAttempt - 1)
+	c.nextRetryTime = time.Now().Add(backoffDelay)
+}
+
+// clearErrorAndBackoffLocked clears the cached error and resets backoff state.
+// Caller must hold c.mu write lock.
+func (c *jwtTokenFileCallCreds) clearErrorAndBackoffLocked() {
+	c.cachedError = nil
+	c.retryAttempt = 0
+	c.nextRetryTime = time.Time{}
+}