Fix running out of file descriptors for spill files.

Currently while decoding changes, if the number of changes exceeds a
certain threshold, we spill those to disk.  And this happens for each
(sub)transaction.  Now, while reading all these files, we don't close them
until we read all the files.  While reading these files, if the number of
such files exceeds the maximum number of file descriptors, the operation
errors out.

Use PathNameOpenFile interface to open these files as that internally has
the mechanism to release kernel FDs as needed to get us under the
max_safe_fds limit.

Reported-by: Amit Khandekar
Author: Amit Khandekar
Reviewed-by: Amit Kapila
Backpatch-through: 9.4
Discussion: https://postgr.es/m/CAJ3gD9c-sECEn79zXw4yBnBdOttacoE-6gAyP0oy60nfs_sabQ@mail.gmail.com

Author: Amit Kapila

src/backend/replication/logical/reorderbuffer.c

@@ -131,13 +131,21 @@ typedef struct ReorderBufferTupleCidEnt
 	CommandId	combocid;		/* just for debugging */
 } ReorderBufferTupleCidEnt;
 
+/* Virtual file descriptor with file offset tracking */
+typedef struct TXNEntryFile
+{
+	File		vfd;			/* -1 when the file is closed */
+	off_t		curOffset;		/* offset for next write or read. Reset to 0
+								 * when vfd is opened. */
+} TXNEntryFile;
+
 /* k-way in-order change iteration support structures */
 typedef struct ReorderBufferIterTXNEntry
 {
 	XLogRecPtr	lsn;
 	ReorderBufferChange *change;
 	ReorderBufferTXN *txn;
-	int			fd;
+	TXNEntryFile file;
 	XLogSegNo	segno;
 } ReorderBufferIterTXNEntry;
 
@@ -207,7 +215,8 @@ static void AssertTXNLsnOrder(ReorderBuffer *rb);
  * subtransactions
  * ---------------------------------------
  */
-static ReorderBufferIterTXNState *ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn);
+static void ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn,
+									 ReorderBufferIterTXNState *volatile *iter_state);
 static ReorderBufferChange *ReorderBufferIterTXNNext(ReorderBuffer *rb, ReorderBufferIterTXNState *state);
 static void ReorderBufferIterTXNFinish(ReorderBuffer *rb,
 									   ReorderBufferIterTXNState *state);
@@ -223,7 +232,7 @@ static void ReorderBufferSerializeTXN(ReorderBuffer *rb, ReorderBufferTXN *txn);
 static void ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 										 int fd, ReorderBufferChange *change);
 static Size ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
-										int *fd, XLogSegNo *segno);
+										TXNEntryFile *file, XLogSegNo *segno);
 static void ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 									   char *change);
 static void ReorderBufferRestoreCleanup(ReorderBuffer *rb, ReorderBufferTXN *txn);
@@ -996,15 +1005,23 @@ ReorderBufferIterCompare(Datum a, Datum b, void *arg)
 /*
  * Allocate & initialize an iterator which iterates in lsn order over a
  * transaction and all its subtransactions.
+ *
+ * Note: The iterator state is returned through iter_state parameter rather
+ * than the function's return value.  This is because the state gets cleaned up
+ * in a PG_CATCH block in the caller, so we want to make sure the caller gets
+ * back the state even if this function throws an exception.
  */
-static ReorderBufferIterTXNState *
-ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
+static void
+ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn,
+						 ReorderBufferIterTXNState *volatile *iter_state)
 {
 	Size		nr_txns = 0;
 	ReorderBufferIterTXNState *state;
 	dlist_iter	cur_txn_i;
 	int32		off;
 
+	*iter_state = NULL;
+
 	/*
 	 * Calculate the size of our heap: one element for every transaction that
 	 * contains changes.  (Besides the transactions already in the reorder
@@ -1039,7 +1056,7 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 
 	for (off = 0; off < state->nr_txns; off++)
 	{
-		state->entries[off].fd = -1;
+		state->entries[off].file.vfd = -1;
 		state->entries[off].segno = 0;
 	}
 
@@ -1048,6 +1065,9 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 									  ReorderBufferIterCompare,
 									  state);
 
+	/* Now that the state fields are initialized, it is safe to return it. */
+	*iter_state = state;
+
 	/*
 	 * Now insert items into the binary heap, in an unordered fashion.  (We
 	 * will run a heap assembly step at the end; this is more efficient.)
@@ -1064,7 +1084,7 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 		{
 			/* serialize remaining changes */
 			ReorderBufferSerializeTXN(rb, txn);
-			ReorderBufferRestoreChanges(rb, txn, &state->entries[off].fd,
+			ReorderBufferRestoreChanges(rb, txn, &state->entries[off].file,
 										&state->entries[off].segno);
 		}
 
@@ -1094,7 +1114,7 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 				/* serialize remaining changes */
 				ReorderBufferSerializeTXN(rb, cur_txn);
 				ReorderBufferRestoreChanges(rb, cur_txn,
-											&state->entries[off].fd,
+											&state->entries[off].file,
 											&state->entries[off].segno);
 			}
 			cur_change = dlist_head_element(ReorderBufferChange, node,
@@ -1110,8 +1130,6 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 
 	/* assemble a valid binary heap */
 	binaryheap_build(state->heap);
-
-	return state;
 }
 
 /*
@@ -1175,7 +1193,7 @@ ReorderBufferIterTXNNext(ReorderBuffer *rb, ReorderBufferIterTXNState *state)
 		dlist_delete(&change->node);
 		dlist_push_tail(&state->old_change, &change->node);
 
-		if (ReorderBufferRestoreChanges(rb, entry->txn, &entry->fd,
+		if (ReorderBufferRestoreChanges(rb, entry->txn, &entry->file,
 										&state->entries[off].segno))
 		{
 			/* successfully restored changes from disk */
@@ -1214,8 +1232,8 @@ ReorderBufferIterTXNFinish(ReorderBuffer *rb,
 
 	for (off = 0; off < state->nr_txns; off++)
 	{
-		if (state->entries[off].fd != -1)
-			CloseTransientFile(state->entries[off].fd);
+		if (state->entries[off].file.vfd != -1)
+			FileClose(state->entries[off].file.vfd);
 	}
 
 	/* free memory we might have "leaked" in the last *Next call */
@@ -1558,7 +1576,7 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 
 		rb->begin(rb, txn);
 
-		iterstate = ReorderBufferIterTXNInit(rb, txn);
+		ReorderBufferIterTXNInit(rb, txn, &iterstate);
 		while ((change = ReorderBufferIterTXNNext(rb, iterstate)) != NULL)
 		{
 			Relation	relation = NULL;
@@ -2765,11 +2783,12 @@ ReorderBufferChangeSize(ReorderBufferChange *change)
  */
 static Size
 ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
-							int *fd, XLogSegNo *segno)
+							TXNEntryFile *file, XLogSegNo *segno)
 {
 	Size		restored = 0;
 	XLogSegNo	last_segno;
 	dlist_mutable_iter cleanup_iter;
+	File	   *fd = &file->vfd;
 
 	Assert(txn->first_lsn != InvalidXLogRecPtr);
 	Assert(txn->final_lsn != InvalidXLogRecPtr);
@@ -2810,7 +2829,11 @@ ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
 			ReorderBufferSerializedPath(path, MyReplicationSlot, txn->xid,
 										*segno);
 
-			*fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
+			*fd = PathNameOpenFile(path, O_RDONLY | PG_BINARY);
+
+			/* No harm in resetting the offset even in case of failure */
+			file->curOffset = 0;
+
 			if (*fd < 0 && errno == ENOENT)
 			{
 				*fd = -1;
@@ -2830,14 +2853,14 @@ ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
 		 * end of this file.
 		 */
 		ReorderBufferSerializeReserve(rb, sizeof(ReorderBufferDiskChange));
-		pgstat_report_wait_start(WAIT_EVENT_REORDER_BUFFER_READ);
-		readBytes = read(*fd, rb->outbuf, sizeof(ReorderBufferDiskChange));
-		pgstat_report_wait_end();
+		readBytes = FileRead(file->vfd, rb->outbuf,
+							 sizeof(ReorderBufferDiskChange),
+							 file->curOffset, WAIT_EVENT_REORDER_BUFFER_READ);
 
 		/* eof */
 		if (readBytes == 0)
 		{
-			CloseTransientFile(*fd);
+			FileClose(*fd);
 			*fd = -1;
 			(*segno)++;
 			continue;
@@ -2853,16 +2876,19 @@ ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
 							readBytes,
 							(uint32) sizeof(ReorderBufferDiskChange))));
 
+		file->curOffset += readBytes;
+
 		ondisk = (ReorderBufferDiskChange *) rb->outbuf;
 
 		ReorderBufferSerializeReserve(rb,
 									  sizeof(ReorderBufferDiskChange) + ondisk->size);
 		ondisk = (ReorderBufferDiskChange *) rb->outbuf;
 
-		pgstat_report_wait_start(WAIT_EVENT_REORDER_BUFFER_READ);
-		readBytes = read(*fd, rb->outbuf + sizeof(ReorderBufferDiskChange),
-						 ondisk->size - sizeof(ReorderBufferDiskChange));
-		pgstat_report_wait_end();
+		readBytes = FileRead(file->vfd,
+							 rb->outbuf + sizeof(ReorderBufferDiskChange),
+							 ondisk->size - sizeof(ReorderBufferDiskChange),
+							 file->curOffset,
+							 WAIT_EVENT_REORDER_BUFFER_READ);
 
 		if (readBytes < 0)
 			ereport(ERROR,
@@ -2875,6 +2901,8 @@ ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
 							readBytes,
 							(uint32) (ondisk->size - sizeof(ReorderBufferDiskChange)))));
 
+		file->curOffset += readBytes;
+
 		/*
 		 * ok, read a full change from disk, now restore it into proper
 		 * in-memory format

src/test/recovery/t/006_logical_decoding.pl

@@ -7,7 +7,7 @@
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 10;
+use Test::More tests => 11;
 use Config;
 
 # Initialize master node
@@ -135,5 +135,43 @@
 is($node_master->slot('otherdb_slot')->{'slot_name'},
 	undef, 'logical slot was actually dropped with DB');
 
+# Test to ensure that we don't run out of file descriptors even if there
+# are more spill files than maxAllocatedDescs.
+
+# Set max_files_per_process to a small value to make it more likely to run out
+# of max open file descriptors.
+$node_master->safe_psql('postgres',
+	'ALTER SYSTEM SET max_files_per_process = 26;');
+$node_master->restart;
+
+$node_master->safe_psql(
+	'postgres', q{
+do $$
+BEGIN
+    FOR i IN 1..10 LOOP
+        BEGIN
+            INSERT INTO decoding_test(x) SELECT generate_series(1,5000);
+        EXCEPTION
+            when division_by_zero then perform 'dummy';
+        END;
+    END LOOP;
+END $$;
+});
+
+$result = $node_master->safe_psql('postgres',
+	qq[
+set logical_decoding_work_mem to 64; -- generate plenty of .spill files
+SELECT data from pg_logical_slot_get_changes('test_slot', NULL, NULL)
+    WHERE data LIKE '%INSERT%' ORDER BY lsn LIMIT 1;
+]);
+
+$expected = q{table public.decoding_test: INSERT: x[integer]:1 y[text]:null};
+is($result, $expected, 'got expected output from spilling subxacts session');
+
+# Reset back max_files_per_process
+$node_master->safe_psql('postgres',
+	'ALTER SYSTEM SET max_files_per_process = DEFAULT;');
+$node_master->restart;
+
 # done with the node
 $node_master->stop;