refactor: Split "communicator" routines to a separate source file (#11459)

pagestore_smgr.c had grown pretty large. Split into two parts, such
that the smgr routines that PostgreSQL code calls stays in
pagestore_smgr.c, and all the prefetching logic and other lower-level
routines related to communicating with the pageserver are moved to a
new source file, "communicator.c".

There are plans to replace communicator parts with a new
implementation. See https://github.com/neondatabase/neon/pull/10799.
This commit doesn't implement any of the new things yet, but it is
good preparation for it. I'm imagining that the new implementation
will approximately replace the current "communicator.c" code, exposing
roughly the same functions to pagestore_smgr.c.

This commit doesn't change any functionality or behavior, or make any
other changes to the existing code: It just moves existing code
around.

pgxn/neon/Makefile

@@ -4,6 +4,7 @@
 MODULE_big = neon
 OBJS = \
 	$(WIN32RES) \
+	communicator.o \
 	extension_server.o \
 	file_cache.o \
 	hll.o \

pgxn/neon/communicator.h

@@ -0,0 +1,48 @@
+/*-------------------------------------------------------------------------
+ *
+ * communicator.h
+ *	  internal interface for communicating with remote pageservers
+ *
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef COMMUNICATOR_h
+#define COMMUNICATOR_h
+
+#include "neon_pgversioncompat.h"
+
+#include "storage/buf_internals.h"
+
+#include "pagestore_client.h"
+
+/* initialization at postmaster startup */
+extern void pg_init_communicator(void);
+
+/* initialization at backend startup */
+extern void communicator_init(void);
+
+extern bool communicator_exists(NRelFileInfo rinfo, ForkNumber forkNum,
+								neon_request_lsns *request_lsns);
+extern BlockNumber communicator_nblocks(NRelFileInfo rinfo, ForkNumber forknum,
+										neon_request_lsns *request_lsns);
+extern int64 communicator_dbsize(Oid dbNode, neon_request_lsns *request_lsns);
+extern void communicator_read_at_lsnv(NRelFileInfo rinfo, ForkNumber forkNum,
+									  BlockNumber base_blockno, neon_request_lsns *request_lsns,
+									  void **buffers, BlockNumber nblocks, const bits8 *mask);
+extern int communicator_prefetch_lookupv(NRelFileInfo rinfo, ForkNumber forknum, BlockNumber blocknum,
+										 neon_request_lsns *lsns,
+										 BlockNumber nblocks, void **buffers, bits8 *mask);
+extern void communicator_prefetch_register_bufferv(BufferTag tag, neon_request_lsns *frlsns,
+												   BlockNumber nblocks, const bits8 *mask);
+extern int communicator_read_slru_segment(SlruKind kind, int64 segno,
+										  neon_request_lsns *request_lsns,
+										  void *buffer);
+
+extern void communicator_reconfigure_timeout_if_needed(void);
+extern void communicator_prefetch_pump_state(bool IsHandlingInterrupts);
+
+
+#endif

pgxn/neon/neon.c

@@ -28,6 +28,7 @@
 #include "utils/guc.h"
 #include "utils/guc_tables.h"
 
+#include "communicator.h"
 #include "extension_server.h"
 #include "file_cache.h"
 #include "neon.h"
@@ -439,7 +440,7 @@ _PG_init(void)
 	pg_init_walproposer();
 	init_lwlsncache();
 
-	pagestore_smgr_init();
+	pg_init_communicator();
 	Custom_XLogReaderRoutines = NeonOnDemandXLogReaderRoutines;
 
 	InitUnstableExtensionsSupport();

pgxn/neon/neon.h

@@ -59,7 +59,6 @@ extern uint32		WAIT_EVENT_NEON_WAL_DL;
 
 extern void pg_init_libpagestore(void);
 extern void pg_init_walproposer(void);
-extern void pagestore_smgr_init(void);
 
 extern uint64 BackpressureThrottlingTime(void);
 extern void SetNeonCurrentClusterSize(uint64 size);

pgxn/neon/pagestore_client.h

@@ -233,6 +233,7 @@ extern const f_smgr *smgr_neon(ProcNumber backend, NRelFileInfo rinfo);
 extern void smgr_init_neon(void);
 extern void readahead_buffer_resize(int newsize, void *extra);
 
+
 /*
  * LSN values associated with each request to the pageserver
  */
@@ -269,6 +270,10 @@ extern PGDLLEXPORT void neon_read_at_lsn(NRelFileInfo rnode, ForkNumber forkNum,
 										 neon_request_lsns request_lsns, void *buffer);
 extern int64 neon_dbsize(Oid dbNode);
 
+extern void neon_get_request_lsns(NRelFileInfo rinfo, ForkNumber forknum,
+								  BlockNumber blkno, neon_request_lsns *output,
+								  BlockNumber nblocks);
+
 /* utils for neon relsize cache */
 extern void relsize_hash_init(void);
 extern bool get_cached_relsize(NRelFileInfo rinfo, ForkNumber forknum, BlockNumber *size);