Core: Classify 503 Service Unavailable errors as transient. (#8182)

Also, pin grpcio < 2.0dev.

Closes #5410.

Author: tseaver

google/api_core/retry.py

@@ -98,7 +98,9 @@ def if_exception_type_predicate(exception):
 # Pylint sees this as a constant, but it is also an alias that should be
 # considered a function.
 if_transient_error = if_exception_type(
-    (exceptions.InternalServerError, exceptions.TooManyRequests)
+    exceptions.InternalServerError,
+    exceptions.TooManyRequests,
+    exceptions.ServiceUnavailable,
 )
 """A predicate that checks if an exception is a transient API error.
 
@@ -107,6 +109,7 @@ def if_exception_type_predicate(exception):
 - :class:`google.api_core.exceptions.InternalServerError` - HTTP 500, gRPC
     ``INTERNAL(13)`` and its subclasses.
 - :class:`google.api_core.exceptions.TooManyRequests` - HTTP 429
+- :class:`google.api_core.exceptions.ServiceUnavailable` - HTTP 503
 - :class:`google.api_core.exceptions.ResourceExhausted` - gRPC
     ``RESOURCE_EXHAUSTED(8)``
 """

setup.py

@@ -39,7 +39,7 @@
     'futures >= 3.2.0; python_version < "3.2"',
 ]
 extras = {
-    "grpc": "grpcio >= 1.8.2",
+    "grpc": "grpcio >= 1.8.2, < 2.0dev",
     "grpcgcp": "grpcio-gcp >= 0.2.2",
     "grpcio-gcp": "grpcio-gcp >= 0.2.2",
 }

tests/unit/test_retry.py

@@ -41,6 +41,7 @@ def test_if_exception_type_multiple():
 def test_if_transient_error():
     assert retry.if_transient_error(exceptions.InternalServerError(""))
     assert retry.if_transient_error(exceptions.TooManyRequests(""))
+    assert retry.if_transient_error(exceptions.ServiceUnavailable(""))
     assert not retry.if_transient_error(exceptions.InvalidArgument(""))