Release gRPC stream call messages that are no longer needed

[Myllyenko]

Release gRPC stream call messages that are no longer needed

Problem

Two gRPC stream call classes in core keep possibly large messages in memory long after they're needed:

• ReadWriteStreamCall queues outgoing messages in messagesQueue while the transport isn't ready to send, for example when the network is slow. Once the call is half-closed, cancelled, or closed by the server, those messages can never be sent: gRPC's ClientCall.isReady() returns false after halfClose(), so flush() never empties the queue. But nothing cleared the queue either, and messages passed to sendNext() after closing were added to it too.
  • The messages stayed in memory until the call object was garbage-collected. On a stalled connection, a half-closed call can live until the keepalive timeout or the deadline.
  • For the topic writer, each queued item is a WriteRequest possibly dozens megabytes-long. After a reconnect the writer re-sends the same message data on a new stream, so the dead queue kept the payloads in memory even after the server acknowledged them.
• ReadStreamCall stores its request in a final field for the whole lifetime of the stream, but only uses it once, in start(). For server-streaming calls with large requests (for example, queries with big parameters), the request stayed in memory until the last response arrived.

Fix

• ReadWriteStreamCall: close(), cancel() and onClose() now mark the call as stopped and clear the queue, and sendNext() drops messages once the call is stopped. Nothing changes in what gets delivered: these messages were never sent before either.
• ReadStreamCall: the request field is set to null right after sendMessage().

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.97%. Comparing base (3d9dc0c) to head (e426ce5).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/java/tech/ydb/core/impl/call/ReadStreamCall.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #730      +/-   ##
============================================
+ Coverage     74.59%   74.97%   +0.38%     
- Complexity     3640     3671      +31     
============================================
  Files           392      393       +1     
  Lines         16554    16618      +64     
  Branches       1748     1757       +9     
============================================
+ Hits          12349    12460     +111     
+ Misses         3592     3532      -60     
- Partials        613      626      +13     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.