Commits on Jul 9, 2021

1. sql/pgwire: fix statement buffer memory leak when using suspended por… …

   …tals
   
   The connection statement buffer grows indefinitely when the client uses the
   execute portal with limit feature of the Postgres protocol, eventually causing
   the node to crash out of memory. Any long running query that uses the limit
   feature will cause this memory leak such as the `EXPERIMENTAL CHANGEFEED FOR`
   statement. The execute portal with limit feature of the Postgres protocol is
   used by the JDBC Postgres driver to fetch a limited number of rows at a time.
   
   The leak is caused by commands accumulating in the buffer and never getting
   cleared out. The client sends 2 commands every time it wants to receive more
   rows:
   
   - `Execute {"Portal": "C_1", "MaxRows": 1}`
   - `Sync`
   
   The server processes the commands and leaves them in the buffer, every
   iteration causes 2 more commands to leak.
   
   A similar memory leak was fixed by cockroachdb#48859, however the execute with limit
   feature is implemented by a side state machine in limitedCommandResult. The
   cleanup routine added by cockroachdb#48859 is never executed for suspended portals as
   they never return to the main conn_executor loop.
   
   After this change the statement buffer gets trimmed to reclaim memory after
   each client command is processed in the limitedCommandResult side state
   machine. The StmtBuf.Ltrim function was changed to be public visibility to
   enable this. While this is not ideal, it does scope the fix to the
   limitedCommandResult side state machine and could be addressed when the
   limitedCommandResult functionality is refactored into the conn_executor.
   
   Added a unit test which causes the leak, used the PGWire client in the test as
   neither the pg or pgx clients use execute with limit, so cant be used to
   demonstrate the leak. Also tested the fix in a cluster by following the steps
   outlined in cockroachdb#66849.
   
   Resolves: cockroachdb#66849
   
   See also: cockroachdb#48859
   
   Release note (bug fix): fix statement buffer memory leak when using
   suspended portals

   

   joesankey committed Jul 9, 2021
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