Refactor SLRU download interface between Postgres and the neon extension

Move the responsibility of writing the SLRU segment contents to the
neon extension. The neon extension hook now downloads the file and
places it to the correct path, and returns a true/false to the caller
to indicate if the file existed. On successful download, the caller
retries opening the file.

pgxn/neon/communicator.c

@@ -2398,11 +2398,16 @@ communicator_dbsize(Oid dbNode, neon_request_lsns *request_lsns)
 	return db_size;
 }
 
-int
-communicator_read_slru_segment(SlruKind kind, int64 segno, neon_request_lsns *request_lsns,
-							   void *buffer)
+/*
+ * Download the given SLRU segment from the pageserver.
+ *
+ * See comments on read_slru_segment_result for the result.
+ */
+read_slru_segment_result
+communicator_read_slru_segment(SlruKind kind, int64 segno,
+							   neon_request_lsns *request_lsns)
 {
-	int			n_blocks;
+	read_slru_segment_result result = { NULL, 0, NULL };
 	shardno_t	shard_no = 0; /* All SLRUs are at shard 0 */
 	NeonResponse *resp = NULL;
 	NeonGetSlruSegmentRequest request;
@@ -2456,8 +2461,9 @@ communicator_read_slru_segment(SlruKind kind, int64 segno, neon_request_lsns *re
 												request.hdr.reqid, LSN_FORMAT_ARGS(request.hdr.lsn), LSN_FORMAT_ARGS(request.hdr.not_modified_since), kind, (unsigned long long) segno);
 				}
 			}
-			n_blocks = slru_resp->n_blocks;
-			memcpy(buffer, slru_resp->data, n_blocks*BLCKSZ);
+			result.slru_data = slru_resp->data;
+			result.n_blocks = slru_resp->n_blocks;
+			result.buf = slru_resp;
 			break;
 		}
 		case T_NeonErrorResponse:
@@ -2486,10 +2492,10 @@ communicator_read_slru_segment(SlruKind kind, int64 segno, neon_request_lsns *re
 										"Expected GetSlruSegment (0x%02x) or Error (0x%02x) response to GetSlruSegmentRequest, but got 0x%02x",
 										T_NeonGetSlruSegmentResponse, T_NeonErrorResponse, resp->tag);
 	}
-	pfree(resp);
+	/* do not pfree(resp) here. The caller is responsible for it */
 
 	communicator_reconfigure_timeout_if_needed();
-	return n_blocks;
+	return result;
 }
 
 void

pgxn/neon/communicator.h

@@ -39,9 +39,24 @@ extern void communicator_prefetch_register_bufferv(BufferTag tag, neon_request_l
 												   BlockNumber nblocks, const bits8 *mask);
 extern bool communicator_prefetch_receive(BufferTag tag);
 
-extern int communicator_read_slru_segment(SlruKind kind, int64 segno,
-										  neon_request_lsns *request_lsns,
-										  void *buffer);
+/* Return type for communicator_read_slru_segment */
+typedef struct read_slru_segment_result
+{
+	/* the SLRU segment content is returned here */
+	void	   *slru_data;
+
+	/* Size of 'slru_data', in BLCKSZ blocks. 0 if the segment was not found */
+	int			n_blocks;
+
+	/*
+	 * 'slru_data' points to a larger palloc'd structure that's opaque
+	 * to the caller. Once you're done with the result, pfree this.
+	 */
+	void	   *buf;
+} read_slru_segment_result;
+
+extern read_slru_segment_result communicator_read_slru_segment(SlruKind kind, int64 segno,
+															   neon_request_lsns *request_lsns);
 
 extern void communicator_reconfigure_timeout_if_needed(void);
 extern void communicator_prefetch_pump_state(void);

pgxn/neon/pagestore_smgr.c

@@ -49,6 +49,7 @@
 #include "access/xlog_internal.h"
 #include "access/xlogutils.h"
 #include "catalog/pg_class.h"
+#include "common/file_utils.h"
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
 #include "postmaster/interrupt.h"
@@ -56,6 +57,7 @@
 #include "replication/walsender.h"
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
+#include "storage/fd.h"
 #include "storage/fsm_internals.h"
 #include "storage/md.h"
 #include "storage/smgr.h"
@@ -2127,13 +2129,20 @@ neon_end_unlogged_build(SMgrRelation reln)
 
 #define STRPREFIX(str, prefix) (strncmp(str, prefix, strlen(prefix)) == 0)
 
-static int
-neon_read_slru_segment(SMgrRelation reln, const char* path, int segno, void* buffer)
+/*
+ * Attempt to download the given SLRU segment file from the pageserver.
+ *
+ * Returns:
+ *   true if the file was successfully downloaded
+ *   false if the file was not found in pageserver
+ *   ereports if some other error happened
+ */
+static bool
+neon_read_slru_segment(SMgrRelation reln, const char* path, int segno)
 {
 	XLogRecPtr	request_lsn,
 				not_modified_since;
 	SlruKind	kind;
-	int			n_blocks;
 	neon_request_lsns request_lsns;
 
 	/*
@@ -2164,6 +2173,7 @@ neon_read_slru_segment(SMgrRelation reln, const char* path, int segno, void* buf
 	 */
 	not_modified_since = nm_adjust_lsn(GetRedoStartLsn());
 
+	/* Only these SLRUs are stored in the pageserver */
 	if (STRPREFIX(path, "pg_xact"))
 		kind = SLRU_CLOG;
 	else if (STRPREFIX(path, "pg_multixact/members"))
@@ -2171,15 +2181,58 @@ neon_read_slru_segment(SMgrRelation reln, const char* path, int segno, void* buf
 	else if (STRPREFIX(path, "pg_multixact/offsets"))
 		kind = SLRU_MULTIXACT_OFFSETS;
 	else
-		return -1;
+		return false;
 
 	request_lsns.request_lsn = request_lsn;
 	request_lsns.not_modified_since = not_modified_since;
 	request_lsns.effective_request_lsn = request_lsn;
 
-	n_blocks = communicator_read_slru_segment(kind, segno, &request_lsns, buffer);
+	{
+		read_slru_segment_result result;
+		int			fd;
+		struct iovec iov[1];
 
-	return n_blocks;
+		/* Call the pageserver */
+		result = communicator_read_slru_segment(kind, segno, &request_lsns);
+		if (result.n_blocks == 0)
+		{
+			/* "File not found" from pageserver */
+			if (result.buf)
+				pfree(result.buf);
+			return false;
+		}
+
+		/* Success! Write the contents to the file */
+		fd = OpenTransientFile(path, O_WRONLY | O_EXCL | O_CREAT | PG_BINARY);
+		if (fd < 0)
+		{
+			ereport(ERROR,
+					errcode_for_file_access(),
+					errmsg("could not create SLRU file \"%s\" to write downloaded contents: %m",
+						   path));
+		}
+
+		errno = 0;
+		iov[0].iov_base = result.slru_data;
+		iov[0].iov_len = result.n_blocks * BLCKSZ;
+		pgstat_report_wait_start(WAIT_EVENT_SLRU_WRITE);
+		if (pg_pwritev_with_retry(fd, iov, 1, 0) != result.n_blocks * BLCKSZ)
+		{
+			pgstat_report_wait_end();
+			/* if write didn't set errno, assume problem is no disk space */
+			if (errno == 0)
+				errno = ENOSPC;
+			ereport(ERROR,
+					errcode_for_file_access(),
+					errmsg("could not write downloaded contents to SLRU file \"%s\": %m",
+						   path));
+		}
+		pgstat_report_wait_end();
+		pfree(result.buf);
+		CloseTransientFile(fd);
+
+		return true;
+	}
 }
 
 static void

vendor/revisions.json

@@ -1,18 +1,18 @@
 {
   "v17": [
     "17.5",
-    "1e01fcea2a6b38180021aa83e0051d95286d9096"
+    "4bb475f24ebbe860fd5432235bd2ab09413b9816"
   ],
   "v16": [
     "16.9",
-    "a42351fcd41ea01edede1daed65f651e838988fc"
+    "e871de05e4bef58a580e28be393a43e12852d5fc"
   ],
   "v15": [
     "15.13",
-    "2aaab3bb4a13557aae05bb2ae0ef0a132d0c4f85"
+    "6d0324add02106b6843a3d3701f4923e847bcbb0"
   ],
   "v14": [
     "14.18",
-    "2155cb165d05f617eb2c8ad7e43367189b627703"
+    "a9419a4ec4cd9fb80eaf45dffe4a2b82a0f25ec1"
   ]
 }