SMGR GUC variable and chaining

The overall goal of this commit is to introduce a user interface to
the previous SMGR patch.

The idea is to allow a simple configuration for multiple "modificator"
SMGRs similar to the fsync_checker in the original proposal.

* Extensions should be able to declare a named lists of SMGR implementations,
also specifying if the given SMGR is an "end" implementation for
actual storage, or if it is a modifier implementation for some other
purpose.
* Users should be able to specify a list of SMGRs: possibly multiple modifiers,
and one storage implementation at the end to configure how the
storage manager is constructed.

This commit introduces a new GUC variable, `smgr_chain`, which allows
users to configure multiple SMGR implementations: it is a comma separated list,
where the last entry most be a storage implementation, the others must be
modifiers. The default value of this variable is "md".

The internal storage manager API is also refactored to include an easy
way for SMGR implementations to support proper chaining. Modifier SMGR
implementations also only have to implement the functions they actually
change, and can leave everything else as empty (NULL). And with this
change we can make the functions of the md smgr static.

The fsync example extension is also modified to match the new API.

Author: dutow

contrib/fsync_checker/fsync_checker_smgr.c

@@ -27,15 +27,15 @@ typedef struct
 void		_PG_init(void);
 
 static void fsync_checker_extend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
-								 const void *buffer, bool skipFsync);
-static void fsync_checker_immedsync(SMgrRelation reln, ForkNumber forknum);
+								 const void *buffer, bool skipFsync, SmgrChainIndex chain_index);
+static void fsync_checker_immedsync(SMgrRelation reln, ForkNumber forknum, SmgrChainIndex chain_index);
 static void fsync_checker_writev(SMgrRelation reln, ForkNumber forknum,
 								 BlockNumber blocknum, const void **buffers,
-								 BlockNumber nblocks, bool skipFsync);
+								 BlockNumber nblocks, bool skipFsync, SmgrChainIndex chain_index);
 static void fsync_checker_writeback(SMgrRelation reln, ForkNumber forknum,
-									BlockNumber blocknum, BlockNumber nblocks);
+									BlockNumber blocknum, BlockNumber nblocks, SmgrChainIndex chain_index);
 static void fsync_checker_zeroextend(SMgrRelation reln, ForkNumber forknum,
-									 BlockNumber blocknum, int nblocks, bool skipFsync);
+									 BlockNumber blocknum, int nblocks, bool skipFsync, SmgrChainIndex chain_index);
 
 static void fsync_checker_checkpoint_create(const CheckPoint *checkPoint);
 static void fsync_checker_shmem_request(void);
@@ -47,24 +47,25 @@ static void remove_reln(SMgrRelation reln, ForkNumber forknum);
 static SMgrId fsync_checker_smgr_id;
 static const struct f_smgr fsync_checker_smgr = {
 	.name = "fsync_checker",
-	.smgr_init = mdinit,
+	.chain_position = SMGR_CHAIN_MODIFIER,
+	.smgr_init = NULL,
 	.smgr_shutdown = NULL,
-	.smgr_open = mdopen,
-	.smgr_close = mdclose,
-	.smgr_create = mdcreate,
-	.smgr_exists = mdexists,
-	.smgr_unlink = mdunlink,
+	.smgr_open = NULL,
+	.smgr_close = NULL,
+	.smgr_create = NULL,
+	.smgr_exists = NULL,
+	.smgr_unlink = NULL,
 	.smgr_extend = fsync_checker_extend,
 	.smgr_zeroextend = fsync_checker_zeroextend,
-	.smgr_prefetch = mdprefetch,
-	.smgr_maxcombine = mdmaxcombine,
-	.smgr_readv = mdreadv,
+	.smgr_prefetch = NULL,
+	.smgr_maxcombine = NULL,
+	.smgr_readv = NULL,
 	.smgr_writev = fsync_checker_writev,
 	.smgr_writeback = fsync_checker_writeback,
-	.smgr_nblocks = mdnblocks,
-	.smgr_truncate = mdtruncate,
+	.smgr_nblocks = NULL,
+	.smgr_truncate = NULL,
 	.smgr_immedsync = fsync_checker_immedsync,
-	.smgr_registersync = mdregistersync,
+	.smgr_registersync = NULL,
 };
 
 static HTAB *volatile_relns;
@@ -91,8 +92,6 @@ _PG_init(void)
 	 * could use MdSmgrRelation as the parent.
 	 */
 	fsync_checker_smgr_id = smgr_register(&fsync_checker_smgr, 0);
-
-	storage_manager_id = fsync_checker_smgr_id;
 }
 
 static void
@@ -200,50 +199,50 @@ remove_reln(SMgrRelation reln, ForkNumber forknum)
 
 static void
 fsync_checker_extend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
-					 const void *buffer, bool skipFsync)
+					 const void *buffer, bool skipFsync, SmgrChainIndex chain_index)
 {
 	if (!SmgrIsTemp(reln) && !skipFsync)
 		add_reln(reln, forknum);
 
-	mdextend(reln, forknum, blocknum, buffer, skipFsync);
+	smgr_extend_next(reln, forknum, blocknum, buffer, skipFsync, chain_index + 1);
 }
 
 static void
-fsync_checker_immedsync(SMgrRelation reln, ForkNumber forknum)
+fsync_checker_immedsync(SMgrRelation reln, ForkNumber forknum, SmgrChainIndex chain_index)
 {
 	if (!SmgrIsTemp(reln))
 		remove_reln(reln, forknum);
 
-	mdimmedsync(reln, forknum);
+	smgr_immedsync_next(reln, forknum, chain_index + 1);
 }
 
 static void
 fsync_checker_writev(SMgrRelation reln, ForkNumber forknum,
 					 BlockNumber blocknum, const void **buffers,
-					 BlockNumber nblocks, bool skipFsync)
+					 BlockNumber nblocks, bool skipFsync, SmgrChainIndex chain_index)
 {
 	if (!SmgrIsTemp(reln) && !skipFsync)
 		add_reln(reln, forknum);
 
-	mdwritev(reln, forknum, blocknum, buffers, nblocks, skipFsync);
+	smgr_writev_next(reln, forknum, blocknum, buffers, nblocks, skipFsync, chain_index + 1);
 }
 
 static void
 fsync_checker_writeback(SMgrRelation reln, ForkNumber forknum,
-						BlockNumber blocknum, BlockNumber nblocks)
+						BlockNumber blocknum, BlockNumber nblocks, SmgrChainIndex chain_index)
 {
 	if (!SmgrIsTemp(reln))
 		remove_reln(reln, forknum);
 
-	mdwriteback(reln, forknum, blocknum, nblocks);
+	smgr_writeback_next(reln, forknum, blocknum, nblocks, chain_index + 1);
 }
 
 static void
 fsync_checker_zeroextend(SMgrRelation reln, ForkNumber forknum,
-						 BlockNumber blocknum, int nblocks, bool skipFsync)
+						 BlockNumber blocknum, int nblocks, bool skipFsync, SmgrChainIndex chain_index)
 {
 	if (!SmgrIsTemp(reln) && !skipFsync)
 		add_reln(reln, forknum);
 
-	mdzeroextend(reln, forknum, blocknum, nblocks, skipFsync);
+	smgr_zeroextend_next(reln, forknum, blocknum, nblocks, skipFsync, chain_index + 1);
 }

src/backend/postmaster/postmaster.c

@@ -925,6 +925,8 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	process_shared_preload_libraries();
 
+	process_smgr_chain();
+
 	/*
 	 * Initialize SSL library, if specified.
 	 */