[Feature] Add support for async OAuth token refreshes

config/experimental/auth/auth.go

@@ -0,0 +1,212 @@
+// Package auth is an internal package that provides authentication utilities.
+//
+// IMPORTANT: This package is not meant to be used directly by consumers of the
+// SDK and is subject to change without notice.
+package auth
+
+import (
+	"sync"
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+const (
+	// Default duration for the stale period. The number as been set arbitrarily
+	// and might be changed in the future.
+	defaultStaleDuration = 3 * time.Minute
+
+	// Disable the asynchronous token refresh by default. This is meant to
+	// change in the future once the feature is stable.
+	defaultDisableAsyncRefresh = true
+)
+
+type Option func(*cachedTokenSource)
+
+// WithCachedToken sets the initial token to be used by a cached token source.
+func WithCachedToken(t *oauth2.Token) Option {
+	return func(cts *cachedTokenSource) {
+		cts.cachedToken = t
+	}
+}
+
+// WithAsyncRefresh enables or disables the asynchronous token refresh.
+func WithAsyncRefresh(b bool) Option {
+	return func(cts *cachedTokenSource) {
+		cts.disableAsync = !b
+	}
+}
+
+// NewCachedTokenProvider wraps a [oauth2.TokenSource] to cache the tokens
+// it returns. By default, the cache will refresh tokens asynchronously a few
+// minutes before they expire.
+//
+// The token cache is safe for concurrent use by multiple goroutines and will
+// guarantee that only one token refresh is triggered at a time.
+//
+// The token cache does not take care of retries in case the token source
+// returns and error; it is the responsibility of the provided token source to
+// handle retries appropriately.
+//
+// If the TokenSource is already a cached token source (obtained by calling this
+// function), it is returned as is.
+func NewCachedTokenSource(ts oauth2.TokenSource, opts ...Option) oauth2.TokenSource {
+	// This is meant as a niche optimization to avoid double caching of the
+	// token source in situations where the user calls needs caching guarantees
+	// but does not know if the token source is already cached.
+	if cts, ok := ts.(*cachedTokenSource); ok {
+		return cts
+	}
+
+	cts := &cachedTokenSource{
+		tokenSource:   ts,
+		staleDuration: defaultStaleDuration,
+		disableAsync:  defaultDisableAsyncRefresh,
+		cachedToken:   nil,
+		timeNow:       time.Now,
+	}
+
+	for _, opt := range opts {
+		opt(cts)
+	}
+
+	return cts
+}
+
+type cachedTokenSource struct {
+	// The token source to obtain tokens from.
+	tokenSource oauth2.TokenSource
+
+	// If true, only refresh the token with a blocking call when it is expired.
+	disableAsync bool
+
+	// Duration during which a token is considered stale, see tokenState.
+	staleDuration time.Duration
+
+	mu          sync.Mutex
+	cachedToken *oauth2.Token
+
+	// Indicates that an async refresh is in progress. This is used to prevent
+	// multiple async refreshes from being triggered at the same time.
+	isRefreshing bool
+
+	// Error returned by the last refresh. Async refreshes are disabled if this
+	// value is not nil so that the cache does not continue sending request to
+	// a potentially failing server. The next blocking call will re-enable async
+	// refreshes by setting this value to nil if it succeeds, or return the
+	// error if it fails.
+	refreshErr error
+
+	timeNow func() time.Time // for testing
+}
+
+// Token returns a token from the cache or fetches a new one if the current
+// token is expired.
+func (cts *cachedTokenSource) Token() (*oauth2.Token, error) {
+	if cts.disableAsync {
+		return cts.blockingToken()
+	}
+	return cts.asyncToken()
+}
+
+// tokenState represents the state of the token. Each token can be in one of
+// the following three states:
+//   - fresh: The token is valid.
+//   - stale: The token is valid but will expire soon.
+//   - expired: The token has expired and cannot be used.
+//
+// Token state through time:
+//
+//	issue time     expiry time
+//	    v               v
+//	    | fresh | stale | expired -> time
+//	    |     valid     |
+type tokenState int
+
+const (
+	fresh   tokenState = iota // The token is valid.
+	stale                     // The token is valid but will expire soon.
+	expired                   // The token has expired and cannot be used.
+)
+
+// tokenState returns the state of the token. The function is not thread-safe
+// and should be called with the lock held.
+func (c *cachedTokenSource) tokenState() tokenState {
+	if c.cachedToken == nil {
+		return expired
+	}
+	switch lifeSpan := c.cachedToken.Expiry.Sub(c.timeNow()); {
+	case lifeSpan <= 0:
+		return expired
+	case lifeSpan <= c.staleDuration:
+		return stale
+	default:
+		return fresh
+	}
+}
+
+func (cts *cachedTokenSource) asyncToken() (*oauth2.Token, error) {
+	cts.mu.Lock()
+	ts := cts.tokenState()
+	t := cts.cachedToken
+	cts.mu.Unlock()
+
+	switch ts {
+	case fresh:
+		return t, nil
+	case stale:
+		cts.triggerAsyncRefresh()
+		return t, nil
+	default: // expired
+		return cts.blockingToken()
+	}
+}
+
+func (cts *cachedTokenSource) blockingToken() (*oauth2.Token, error) {
+	cts.mu.Lock()
+
+	// The lock is kept for the entire operation to ensure that only one
+	// blockingToken operation is running at a time.
+	defer cts.mu.Unlock()
+
+	// This is important to recover from potential previous failed attempts
+	// to refresh the token asynchronously, see declaration of refreshErr for
+	// more information.
+	cts.isRefreshing = false
+	cts.refreshErr = nil
+
+	// It's possible that the token got refreshed (either by a blockingToken or
+	// an asyncRefresh call) while this particular call was waiting to acquire
+	// the mutex. This check avoids refreshing the token again in such cases.
+	if ts := cts.tokenState(); ts != expired { // fresh or stale
+		return cts.cachedToken, nil
+	}
+
+	t, err := cts.tokenSource.Token()
+	if err != nil {
+		return nil, err
+	}
+	cts.cachedToken = t
+	return t, nil
+}
+
+func (cts *cachedTokenSource) triggerAsyncRefresh() {
+	cts.mu.Lock()
+	defer cts.mu.Unlock()
+	if !cts.isRefreshing && cts.refreshErr == nil {
+		cts.isRefreshing = true
+
+		go func() {
+			t, err := cts.tokenSource.Token()
+
+			cts.mu.Lock()
+			defer cts.mu.Unlock()
+			cts.isRefreshing = false
+			if err != nil {
+				cts.refreshErr = err
+				return
+			}
+			cts.cachedToken = t
+		}()
+	}
+}