Add replay queue spill-to-disk for large transactions

When the replay queue exceeds spock.exception_replay_queue_size (in MB),
subsequent entries are written to a temporary BufFile on disk instead of
accumulating in memory.  This prevents OOM on the subscriber when
replaying large transactions with exception handling enabled.

Key design decisions:

- In-memory queue entries (ApplyReplayContext) hold libpq-allocated data
  buffers.  apply_replay_queue_reset walks the list to PQfreemem them
  before calling MemoryContextReset.

- Spilled entries are relocated to TopMemoryContext so they survive the
  MemoryContextReset in handle_commit.  The caller frees them explicitly
  after replication_handler returns.

- When the replay queue is exhausted mid-transaction (exception before
  COMMIT was received), the loop seamlessly transitions to stream reading
  via continue instead of break, matching the pre-spill behavior.

- Added from_pq field to ApplyReplayEntry to distinguish libpq-allocated
  buffers (PQfreemem) from palloc'd spill-read buffers (pfree).

Includes regression tests covering all three exception_behaviour modes
(transdiscard, discard, sub_disable) both without conflict and with a
primary key conflict on the last record.

Author: danolivo

Makefile

@@ -56,7 +56,7 @@ REGRESS = preseed infofuncs init_fail init preseed_check basic conflict_secondar
 		  interfaces foreign_key copy sequence triggers parallel functions row_filter \
 		  row_filter_sampling att_list column_filter apply_delay \
 		  extended node_origin_cascade multiple_upstreams tuple_origin autoddl \
-		  sync_event sync_table generated_columns drop
+		  sync_event sync_table generated_columns spill_transaction drop
 
 # The following test cases are disabled while developing.
 #

include/spock.h

@@ -50,7 +50,7 @@ extern bool spock_include_ddl_repset;
 extern bool allow_ddl_from_functions;
 extern int	restart_delay_default;
 extern int	restart_delay_on_exception;
-extern int	spock_replay_queue_size;	/* Deprecated - no longer used */
+extern int	spock_replay_queue_size;
 extern int	spock_feedback_frequency;
 extern bool check_all_uc_indexes;
 extern bool	spock_enable_quiet_mode;

src/spock.c

@@ -142,7 +142,7 @@ bool		spock_include_ddl_repset = false;
 bool		allow_ddl_from_functions = false;
 int			restart_delay_default;
 int			restart_delay_on_exception;
-int			spock_replay_queue_size;	/* Deprecated - no longer used */
+int			spock_replay_queue_size;
 int			spock_feedback_frequency;
 bool		check_all_uc_indexes = false;
 bool		spock_enable_quiet_mode = false;
@@ -1153,12 +1153,13 @@ _PG_init(void)
 							NULL);
 
 	DefineCustomIntVariable("spock.exception_replay_queue_size",
-							"DEPRECATED: apply-worker replay queue size (no longer used)",
-							"This setting is deprecated and has no effect. "
-							"The replay queue now dynamically allocates memory as needed.",
+							"Maximum in-memory size for the apply replay queue",
+							"When the replay queue exceeds this size, subsequent "
+							"entries are spilled to a temporary file on disk. "
+							"Set to 0 to disable spilling (unlimited memory).",
 							&spock_replay_queue_size,
 							4,
-							0,
+							-1,
 							MAX_KILOBYTES / 1024,
 							PGC_SIGHUP,
 							GUC_UNIT_MB,