fix: warnings when compiling neon extensions (#7053)

proceeding https://github.com/neondatabase/neon/pull/7010, close
https://github.com/neondatabase/neon/issues/6188

## Summary of changes

This pull request (should) fix all warnings except
`-Wdeclaration-after-statement` in the neon extension compilation.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pgxn/hnsw/hnsw.c

@@ -149,7 +149,7 @@ hnsw_check_available_memory(Size requested)
 	struct sysinfo si;
 	Size total;
 	if (sysinfo(&si) < 0)
-		elog(ERROR, "Failed to get amount of RAM: %n");
+		elog(ERROR, "Failed to get amount of RAM: %m");
 
 	total = si.totalram*si.mem_unit;
 	if ((Size)NBuffers*BLCKSZ + requested >= total)

pgxn/neon/extension_server.c

@@ -38,7 +38,6 @@ neon_download_extension_file_http(const char *filename, bool is_library)
 
 	CURLcode	res;
 	char	   *compute_ctl_url;
-	char	   *postdata;
 	bool		ret = false;
 
 	if (handle == NULL)

pgxn/neon/neon.c

@@ -95,7 +95,6 @@ get_num_snap_files_lsn_threshold(void)
 	DIR		   *dirdesc;
 	struct dirent *de;
 	char	   *snap_path = "pg_logical/snapshots/";
-	int			cnt = 0;
 	int			lsns_allocated = 1024;
 	int			lsns_num = 0;
 	XLogRecPtr *lsns;
@@ -161,9 +160,6 @@ get_num_snap_files_lsn_threshold(void)
 PGDLLEXPORT void
 LogicalSlotsMonitorMain(Datum main_arg)
 {
-	TimestampTz now,
-				last_checked;
-
 	/* Establish signal handlers. */
 	pqsignal(SIGUSR1, procsignal_sigusr1_handler);
 	pqsignal(SIGHUP, SignalHandlerForConfigReload);

pgxn/neon/pagestore_smgr.c

@@ -1888,7 +1888,6 @@ neon_zeroextend(SMgrRelation reln, ForkNumber forkNum, BlockNumber blocknum,
 				int nblocks, bool skipFsync)
 {
 	const PGAlignedBlock buffer = {0};
-	BlockNumber curblocknum = blocknum;
 	int			remblocks = nblocks;
 	XLogRecPtr	lsn = 0;
 

pgxn/neon/walproposer_pg.c

@@ -1026,7 +1026,7 @@ static void
 StartProposerReplication(WalProposer *wp, StartReplicationCmd *cmd)
 {
 	XLogRecPtr	FlushPtr;
-	TimeLineID	currTLI;
+	 __attribute__((unused)) TimeLineID	currTLI;
 
 #if PG_VERSION_NUM < 150000
 	if (ThisTimeLineID == 0)
@@ -1230,7 +1230,6 @@ WalProposerRecovery(WalProposer *wp, Safekeeper *sk)
 	TimeLineID	timeline;
 	XLogRecPtr	startpos;
 	XLogRecPtr	endpos;
-	uint64		download_range_mb;
 
 	startpos = GetLogRepRestartLSN(wp);
 	if (startpos == InvalidXLogRecPtr)

pgxn/neon_test_utils/neontest.c

@@ -182,8 +182,6 @@ test_consume_memory(PG_FUNCTION_ARGS)
 Datum
 test_release_memory(PG_FUNCTION_ARGS)
 {
-	TimestampTz start;
-
 	if (PG_ARGISNULL(0))
 	{
 		if (consume_cxt)

pgxn/neon_walredo/walredoproc.c

@@ -220,6 +220,9 @@ enter_seccomp_mode(void)
 }
 #endif /* HAVE_LIBSECCOMP */
 
+PGDLLEXPORT void
+WalRedoMain(int argc, char *argv[]);
+
 /*
  * Entry point for the WAL redo process.
  *