Repository: AzureAD/microsoft-authentication-library-for-python

Issue:
[Feature Request] refresh_in 
Refresh_In (token response) support. 
This feature allows the service to control when MSAL should attempt to refresh the access token (ahead of its expiration)

[API Review](https://identitydivision.visualstudio.com/DevEx/_git/AuthLibrariesApiReview?path=%2FRefreshAtExpirationPercentage%2Foverview.md&version=GBdev&_a=contents)

Discussion:

Patch:
diff --git a/msal/application.py b/msal/application.py
index a1f5003..72bbecf 100644
--- a/msal/application.py
+++ b/msal/application.py
@@ -822,6 +822,7 @@ class ClientApplication(object):
             force_refresh=False,  # type: Optional[boolean]
             claims_challenge=None,
             **kwargs):
+        access_token_from_cache = None
         if not (force_refresh or claims_challenge):  # Bypass AT when desired or using claims
             query={
                     "client_id": self.client_id,
@@ -839,17 +840,27 @@ class ClientApplication(object):
             now = time.time()
             for entry in matches:
                 expires_in = int(entry["expires_on"]) - now
-                if expires_in < 5*60:
+                if expires_in < 5*60:  # Then consider it expired
                     continue  # Removal is not necessary, it will be overwritten
                 logger.debug("Cache hit an AT")
-                return {  # Mimic a real response
+                access_token_from_cache = {  # Mimic a real response
                     "access_token": entry["secret"],
                     "token_type": entry.get("token_type", "Bearer"),
                     "expires_in": int(expires_in),  # OAuth2 specs defines it as int
                     }
-        return self._acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family(
+                if "refresh_on" in entry and int(entry["refresh_on"]) < now:  # aging
+                    break  # With a fallback in hand, we break here to go refresh
+                return access_token_from_cache  # It is still good as new
+        try:
+            result = self._acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family(
                 authority, decorate_scope(scopes, self.client_id), account,
                 force_refresh=force_refresh, claims_challenge=claims_challenge, **kwargs)
+            if (result and "error" not in result) or (not access_token_from_cache):
+                return result
+        except:  # The exact HTTP exception is transportation-layer dependent
+            logger.exception("Refresh token failed")  # Potential AAD outage?
+        return access_token_from_cache
+
 
     def _acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family(
             self, authority, scopes, account, **kwargs):
diff --git a/msal/token_cache.py b/msal/token_cache.py
index 34eff37..028635b 100644
--- a/msal/token_cache.py
+++ b/msal/token_cache.py
@@ -170,6 +170,9 @@ class TokenCache(object):
                     }
                 if data.get("key_id"):  # It happens in SSH-cert or POP scenario
                     at["key_id"] = data.get("key_id")
+                if "refresh_in" in response:
+                    refresh_in = response["refresh_in"]  # It is an integer
+                    at["refresh_on"] = str(now + refresh_in)  # Schema wants a string
                 self.modify(self.CredentialType.ACCESS_TOKEN, at, at)
 
             if client_info and not event.get("skip_account_creation"):


Test patch:
diff --git a/tests/test_application.py b/tests/test_application.py
index 8d48a0a..3c3b464 100644
--- a/tests/test_application.py
+++ b/tests/test_application.py
@@ -319,3 +319,83 @@ class TestApplicationForClientCapabilities(unittest.TestCase):
 
     def test_both_claims_and_capabilities_none(self):
         self.assertEqual(_merge_claims_challenge_and_capabilities(None, None), None)
+
+
+class TestApplicationForRefreshInBehaviors(unittest.TestCase):
+    """The following test cases were based on design doc here
+    https://identitydivision.visualstudio.com/DevEx/_git/AuthLibrariesApiReview?path=%2FRefreshAtExpirationPercentage%2Foverview.md&version=GBdev&_a=preview&anchor=scenarios
+    """
+    def setUp(self):
+        self.authority_url = "https://login.microsoftonline.com/common"
+        self.authority = msal.authority.Authority(
+            self.authority_url, MinimalHttpClient())
+        self.scopes = ["s1", "s2"]
+        self.uid = "my_uid"
+        self.utid = "my_utid"
+        self.account = {"home_account_id": "{}.{}".format(self.uid, self.utid)}
+        self.rt = "this is a rt"
+        self.cache = msal.SerializableTokenCache()
+        self.client_id = "my_app"
+        self.app = ClientApplication(
+            self.client_id, authority=self.authority_url, token_cache=self.cache)
+
+    def populate_cache(self, access_token="at", expires_in=86400, refresh_in=43200):
+        self.cache.add({
+            "client_id": self.client_id,
+            "scope": self.scopes,
+            "token_endpoint": "{}/oauth2/v2.0/token".format(self.authority_url),
+            "response": TokenCacheTestCase.build_response(
+                access_token=access_token,
+                expires_in=expires_in, refresh_in=refresh_in,
+                uid=self.uid, utid=self.utid, refresh_token=self.rt),
+            })
+
+    def test_fresh_token_should_be_returned_from_cache(self):
+        # a.k.a. Return unexpired token that is not above token refresh expiration threshold
+        access_token = "An access token prepopulated into cache"
+        self.populate_cache(access_token=access_token, expires_in=900, refresh_in=450)
+        self.assertEqual(
+            access_token,
+            self.app.acquire_token_silent(['s1'], self.account).get("access_token"))
+
+    def test_aging_token_and_available_aad_should_return_new_token(self):
+        # a.k.a. Attempt to refresh unexpired token when AAD available
+        self.populate_cache(access_token="old AT", expires_in=3599, refresh_in=-1)
+        new_access_token = "new AT"
+        self.app._acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family = (
+            lambda *args, **kwargs: {"access_token": new_access_token})
+        self.assertEqual(
+            new_access_token,
+            self.app.acquire_token_silent(['s1'], self.account).get("access_token"))
+
+    def test_aging_token_and_unavailable_aad_should_return_old_token(self):
+        # a.k.a. Attempt refresh unexpired token when AAD unavailable
+        old_at = "old AT"
+        self.populate_cache(access_token=old_at, expires_in=3599, refresh_in=-1)
+        self.app._acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family = (
+            lambda *args, **kwargs: {"error": "sth went wrong"})
+        self.assertEqual(
+            old_at,
+            self.app.acquire_token_silent(['s1'], self.account).get("access_token"))
+
+    def test_expired_token_and_unavailable_aad_should_return_error(self):
+        # a.k.a. Attempt refresh expired token when AAD unavailable
+        self.populate_cache(access_token="expired at", expires_in=-1, refresh_in=-900)
+        error = "something went wrong"
+        self.app._acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family = (
+            lambda *args, **kwargs: {"error": error})
+        self.assertEqual(
+            error,
+            self.app.acquire_token_silent_with_error(  # This variant preserves error
+                ['s1'], self.account).get("error"))
+
+    def test_expired_token_and_available_aad_should_return_new_token(self):
+        # a.k.a. Attempt refresh expired token when AAD available
+        self.populate_cache(access_token="expired at", expires_in=-1, refresh_in=-900)
+        new_access_token = "new AT"
+        self.app._acquire_token_silent_by_finding_rt_belongs_to_me_or_my_family = (
+            lambda *args, **kwargs: {"access_token": new_access_token})
+        self.assertEqual(
+            new_access_token,
+            self.app.acquire_token_silent(['s1'], self.account).get("access_token"))
+
diff --git a/tests/test_token_cache.py b/tests/test_token_cache.py
index c846883..92ab7c3 100644
--- a/tests/test_token_cache.py
+++ b/tests/test_token_cache.py
@@ -29,30 +29,20 @@ class TokenCacheTestCase(unittest.TestCase):
     def build_response(  # simulate a response from AAD
             uid=None, utid=None,  # If present, they will form client_info
             access_token=None, expires_in=3600, token_type="some type",
-            refresh_token=None,
-            foci=None,
-            id_token=None,  # or something generated by build_id_token()
-            error=None,
+            **kwargs  # Pass-through: refresh_token, foci, id_token, error, refresh_in, ...
             ):
         response = {}
         if uid and utid:  # Mimic the AAD behavior for "client_info=1" request
             response["client_info"] = base64.b64encode(json.dumps({
                 "uid": uid, "utid": utid,
                 }).encode()).decode('utf-8')
-        if error:
-            response["error"] = error
         if access_token:
             response.update({
                 "access_token": access_token,
                 "expires_in": expires_in,
                 "token_type": token_type,
                 })
-        if refresh_token:
-            response["refresh_token"] = refresh_token
-        if id_token:
-            response["id_token"] = id_token
-        if foci:
-            response["foci"] = foci
+        response.update(kwargs)  # Pass-through key-value pairs as top-level fields
         return response
 
     def setUp(self):
@@ -222,6 +212,21 @@ class TokenCacheTestCase(unittest.TestCase):
             {}).get("key_id")
         self.assertEqual(my_key_id, cached_key_id, "AT should be bound to the key")
 
+    def test_refresh_in_should_be_recorded_as_refresh_on(self):  # Sounds weird. Yep.
+        self.cache.add({
+            "client_id": "my_client_id",
+            "scope": ["s2", "s1", "s3"],  # Not in particular order
+            "token_endpoint": "https://login.example.com/contoso/v2/token",
+            "response": self.build_response(
+                uid="uid", utid="utid",  # client_info
+                expires_in=3600, refresh_in=1800, access_token="an access token",
+                ),  #refresh_token="a refresh token"),
+            }, now=1000)
+        refresh_on = self.cache._cache["AccessToken"].get(
+            'uid.utid-login.example.com-accesstoken-my_client_id-contoso-s2 s1 s3',
+            {}).get("refresh_on")
+        self.assertEqual("2800", refresh_on, "Should save refresh_on")
+
     def test_old_rt_data_with_wrong_key_should_still_be_salvaged_into_new_rt(self):
         sample = {
             'client_id': 'my_client_id',
@@ -241,6 +246,7 @@ class TokenCacheTestCase(unittest.TestCase):
                 'uid.utid-login.example.com-refreshtoken-my_client_id--s2 s1 s3')
             )
 
+
 class SerializableTokenCacheTestCase(TokenCacheTestCase):
     # Run all inherited test methods, and have extra check in tearDown()