adding api to configure target service accounts for client ALTS

src/python/grpcio/grpc/__init__.py

@@ -1833,15 +1833,17 @@ def local_server_credentials(local_connect_type=LocalConnectionType.LOCAL_TCP):
         _cygrpc.server_credentials_local(local_connect_type.value))
 
     
-def alts_channel_credentials():
+def alts_channel_credentials(service_accounts=[]):
     """Creates a ChannelCredentials for use with an ALTS-enabled Channel.
 
     This is an EXPERIMENTAL API.
 
+    Args:
+      service_accounts: list of strings, target service accounts
     Returns:
       A ChannelCredentials for use with an ALTS-enabled Channel
     """
-    return ChannelCredentials(_cygrpc.channel_credentials_alts())
+    return ChannelCredentials(_cygrpc.channel_credentials_alts(service_accounts))
 
 
 def alts_server_credentials():

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx.pxi

@@ -354,8 +354,10 @@ def server_credentials_local(grpc_local_connect_type local_connect_type):
 
 cdef class ALTSChannelCredentials(ChannelCredentials):
 
-  def __cinit__(self):
+  def __cinit__(self, service_accounts):
     self.c_options = grpc_alts_credentials_client_options_create()
+    for account in service_accounts:
+      grpc_alts_credentials_client_options_add_target_service_account(self.c_options, account)
  
   def __dealloc__(self):
     if self.c_options != NULL:
@@ -365,8 +367,8 @@ cdef class ALTSChannelCredentials(ChannelCredentials):
     return grpc_alts_credentials_create(self.c_options)
     
 
-def channel_credentials_alts():
+def channel_credentials_alts(service_accounts):
-  return ALTSChannelCredentials()
+  return ALTSChannelCredentials(service_accounts)
 
 
 def server_credentials_alts():

src/python/grpcio/grpc/_cython/_cygrpc/grpc.pxi

@@ -618,6 +618,7 @@ cdef extern from "grpc/grpc_security.h":
   grpc_alts_credentials_options* grpc_alts_credentials_client_options_create()
   grpc_alts_credentials_options* grpc_alts_credentials_server_options_create()
   void grpc_alts_credentials_options_destroy(grpc_alts_credentials_options *options)
+  void grpc_alts_credentials_client_options_add_target_service_account(grpc_alts_credentials_options *options, const char *service_account)
 
 
 

src/python/grpcio_tests/tests/interop/client.py

@@ -107,7 +107,7 @@ def get_secure_channel_parameters(args):
                 args.server_host_override,
             ),)
     else args.use_alts:
-        channel_credentials = grpc.alts_channel_credentials()
+        channel_credentials = grpc.alts_channel_credentials(['svc_account1@gmail.com'])
 
     return channel_credentials, channel_opts