auth: Use a custom function to determine access token expiry

sdk/auth/cached_authorizer.go

@@ -27,10 +27,10 @@ type CachedAuthorizer struct {
 // Token returns the current token if it's still valid, else will acquire a new token
 func (c *CachedAuthorizer) Token(ctx context.Context, req *http.Request) (*oauth2.Token, error) {
 	c.mutex.RLock()
-	valid := c.token != nil && c.token.Valid()
+	dueForRenewal := tokenDueForRenewal(c.token)
 	c.mutex.RUnlock()
 
-	if !valid {
+	if dueForRenewal {
 		c.mutex.Lock()
 		defer c.mutex.Unlock()
 		var err error
@@ -46,16 +46,15 @@ func (c *CachedAuthorizer) Token(ctx context.Context, req *http.Request) (*oauth
 // AuxiliaryTokens returns additional tokens for auxiliary tenant IDs, for use in multi-tenant scenarios
 func (c *CachedAuthorizer) AuxiliaryTokens(ctx context.Context, req *http.Request) ([]*oauth2.Token, error) {
 	c.mutex.RLock()
-	var valid bool
+	var dueForRenewal bool
 	for _, token := range c.auxTokens {
-		valid = token != nil && token.Valid()
-		if !valid {
+		if dueForRenewal = tokenDueForRenewal(token); dueForRenewal {
 			break
 		}
 	}
 	c.mutex.RUnlock()
 
-	if !valid {
+	if !dueForRenewal {
 		c.mutex.Lock()
 		defer c.mutex.Unlock()
 		var err error

sdk/auth/client_credentials.go

@@ -343,6 +343,7 @@ func clientCredentialsToken(ctx context.Context, endpoint string, params *url.Va
 		AccessToken: tokenRes.AccessToken,
 		TokenType:   tokenRes.TokenType,
 	}
+
 	var secs time.Duration
 	if exp, ok := tokenRes.ExpiresIn.(string); ok && exp != "" {
 		if v, err := strconv.Atoi(exp); err == nil {

sdk/auth/token.go

@@ -0,0 +1,44 @@
+package auth
+
+import (
+	"golang.org/x/oauth2"
+	"time"
+
+	"github.com/hashicorp/go-azure-sdk/sdk/claims"
+)
+
+const tokenExpiryDelta = 10 * time.Minute
+
+// tokenExpiresSoon returns true if the token expires within 10 minutes, or if more than 50% of its validity period has elapsed (if this can be determined), whichever is later
+func tokenDueForRenewal(token *oauth2.Token) bool {
+	if token == nil {
+		return true
+	}
+
+	// Some tokens may never expire
+	if token.Expiry.IsZero() {
+		return false
+	}
+
+	expiry := token.Expiry.Round(0)
+	delta := tokenExpiryDelta
+	now := time.Now()
+	expiresWithinTenMinutes := expiry.Add(-delta).Before(now)
+
+	// Try to parse the token claims to retrieve the issuedAt time
+	if claims, err := claims.ParseClaims(token); err != nil {
+		if claims.IssuedAt > 0 {
+			issued := time.Unix(claims.IssuedAt, 0)
+			validity := expiry.Sub(issued)
+
+			// If the validity period is less than double the expiry delta, then instead
+			// determine whether >50% of the validity period has elapsed
+			if validity < delta*2 {
+				halfValidityHasElapsed := issued.Add(validity / 2).Before(now)
+				return halfValidityHasElapsed
+			}
+		}
+	}
+
+	return expiresWithinTenMinutes
+}

sdk/claims/claims.go

@@ -15,6 +15,7 @@ import (
 // Claims is used to unmarshall the claims from a JWT issued by the Microsoft Identity Platform.
 type Claims struct {
 	Audience          string   `json:"aud"`
+	IssuedAt          int64    `json:"iat"`
 	Issuer            string   `json:"iss"`
 	IdentityProvider  string   `json:"idp"`
 	ObjectId          string   `json:"oid"`