Fix metadata memory leak

The gRPC Core has two styles for passing metadata: as an integer count
along with a grpc_metadata* pointer, which is used for passing metadata
into the core, and as a grpc_metadata_array, which is used for passing
metadata out of the core. The Cython layer of gRPC Python was using a
single data structure wrapping grpc_metadata_array for both purposes,
but this was complex because the core manages the slices contained in
grpc_metadata_array objects (at least those of which it is aware), so
the Cython layer had to keep track of whether or not the core was aware
of the slices it was using (and it was also defective, leaking slices).

This is solved by realigning with the Cython layer’s intended design of
mirroring as closely as possible in Python the gRPC Core API: we use
one structure for passing metadata into the core (what is now called
cygrpc.Metadata) and second, different structure for receiving metadata
out of the core (what was called cygrpc.Metadata but is now
cygrpc.MetadataArray, reflecting that it wraps the core’s
grpc_metadata_array).

All bug fixes should contain added tests preventing regression but this
doesn't because I don't know at this time how to write a does-not-leak
test for Python that fits well into our existing body of tests. Phooey.

Thanks to Dominik Janků (djanku@email.cz) for investigation and an
earlier draft of a solution.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/python/grpcio/grpc/_cython/_cygrpc/server.pyx.pxi b/src/python/grpcio/grpc/_cython/_cygrpc/server.pyx.pxi
index dd276fd57b..b8db27469f 100644
--- a/src/python/grpcio/grpc/_cython/_cygrpc/server.pyx.pxi
+++ b/src/python/grpcio/grpc/_cython/_cygrpc/server.pyx.pxi
@@ -44,7 +44,7 @@ cdef class Server:
     cdef OperationTag operation_tag = OperationTag(tag)
     operation_tag.operation_call = Call()
     operation_tag.request_call_details = CallDetails()
-    operation_tag.request_metadata = Metadata([])
+    operation_tag.request_metadata = MetadataArray()
     operation_tag.references.extend([self, call_queue, server_queue])
     operation_tag.is_new_request = True
     operation_tag.batch_operations = Operations([])