Fix syslogger to not lose log coherency under high load.

The original coding of the syslogger had an arbitrary limit of 20 large
messages concurrently in progress, after which it would just punt and dump
message fragments to the output file separately.  Our ambitions are a bit
higher than that now, so allow the data structure to expand as necessary.

Reported and patched by Andrew Dunstan; some editing by Tom

src/backend/postmaster/syslogger.c

@@ -34,6 +34,7 @@
 #include "lib/stringinfo.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
+#include "nodes/pg_list.h"
 #include "pgtime.h"
 #include "postmaster/fork_process.h"
 #include "postmaster/postmaster.h"
@@ -93,11 +94,14 @@ static char *last_file_name = NULL;
 static char *last_csv_file_name = NULL;
 
 /*
- * Buffers for saving partial messages from different backends. We don't expect
- * that there will be very many outstanding at one time, so 20 seems plenty of
- * leeway. If this array gets full we won't lose messages, but we will lose
- * the protocol protection against them being partially written or interleaved.
+ * Buffers for saving partial messages from different backends.
  *
+ * Keep NBUFFER_LISTS lists of these, with the entry for a given source pid
+ * being in the list numbered (pid % NBUFFER_LISTS), so as to cut down on
+ * the number of entries we have to examine for any one incoming message.
+ * There must never be more than one entry for the same source pid.
+ *
+ * An inactive buffer is not removed from its list, just held for re-use.
  * An inactive buffer has pid == 0 and undefined contents of data.
  */
 typedef struct
@@ -106,8 +110,8 @@ typedef struct
 	StringInfoData data;		/* accumulated data, as a StringInfo */
 } save_buffer;
 
-#define CHUNK_SLOTS 20
-static save_buffer saved_chunks[CHUNK_SLOTS];
+#define NBUFFER_LISTS 256
+static List *buffer_lists[NBUFFER_LISTS];
 
 /* These must be exported for EXEC_BACKEND case ... annoying */
 #ifndef WIN32
@@ -592,7 +596,7 @@ SysLogger_Start(void)
 				 * Now we are done with the write end of the pipe.
 				 * CloseHandle() must not be called because the preceding
 				 * close() closes the underlying handle.
-				*/
+				 */
 				syslogPipe[1] = 0;
 #endif
 				redirection_done = true;
@@ -734,6 +738,12 @@ process_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 			(p.is_last == 't' || p.is_last == 'f' ||
 			 p.is_last == 'T' || p.is_last == 'F'))
 		{
+			List	   *buffer_list;
+			ListCell   *cell;
+			save_buffer *existing_slot = NULL,
+					   *free_slot = NULL;
+			StringInfo	str;
+
 			chunklen = PIPE_HEADER_SIZE + p.len;
 
 			/* Fall out of loop if we don't have the whole chunk yet */
@@ -743,53 +753,54 @@ process_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 			dest = (p.is_last == 'T' || p.is_last == 'F') ?
 				LOG_DESTINATION_CSVLOG : LOG_DESTINATION_STDERR;
 
-			if (p.is_last == 'f' || p.is_last == 'F')
+			/* Locate any existing buffer for this source pid */
+			buffer_list = buffer_lists[p.pid % NBUFFER_LISTS];
+			foreach(cell, buffer_list)
 			{
-				/*
-				 * Save a complete non-final chunk in the per-pid buffer if
-				 * possible - if not just write it out.
-				 */
-				int			free_slot = -1,
-							existing_slot = -1;
-				int			i;
-				StringInfo	str;
+				save_buffer *buf = (save_buffer *) lfirst(cell);
 
-				for (i = 0; i < CHUNK_SLOTS; i++)
+				if (buf->pid == p.pid)
 				{
-					if (saved_chunks[i].pid == p.pid)
-					{
-						existing_slot = i;
-						break;
-					}
-					if (free_slot < 0 && saved_chunks[i].pid == 0)
-						free_slot = i;
+					existing_slot = buf;
+					break;
 				}
-				if (existing_slot >= 0)
+				if (buf->pid == 0 && free_slot == NULL)
+					free_slot = buf;
+			}
+
+			if (p.is_last == 'f' || p.is_last == 'F')
+			{
+				/*
+				 * Save a complete non-final chunk in a per-pid buffer
+				 */
+				if (existing_slot != NULL)
 				{
-					str = &(saved_chunks[existing_slot].data);
+					/* Add chunk to data from preceding chunks */
+					str = &(existing_slot->data);
 					appendBinaryStringInfo(str,
 										   cursor + PIPE_HEADER_SIZE,
 										   p.len);
 				}
-				else if (free_slot >= 0)
+				else
 				{
-					saved_chunks[free_slot].pid = p.pid;
-					str = &(saved_chunks[free_slot].data);
+					/* First chunk of message, save in a new buffer */
+					if (free_slot == NULL)
+					{
+						/*
+						 * Need a free slot, but there isn't one in the list,
+						 * so create a new one and extend the list with it.
+						 */
+						free_slot = palloc(sizeof(save_buffer));
+						buffer_list = lappend(buffer_list, free_slot);
+						buffer_lists[p.pid % NBUFFER_LISTS] = buffer_list;
+					}
+					free_slot->pid = p.pid;
+					str = &(free_slot->data);
 					initStringInfo(str);
 					appendBinaryStringInfo(str,
 										   cursor + PIPE_HEADER_SIZE,
 										   p.len);
 				}
-				else
-				{
-					/*
-					 * If there is no free slot we'll just have to take our
-					 * chances and write out a partial message and hope that
-					 * it's not followed by something from another pid.
-					 */
-					write_syslogger_file(cursor + PIPE_HEADER_SIZE, p.len,
-										 dest);
-				}
 			}
 			else
 			{
@@ -797,26 +808,15 @@ process_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 				 * Final chunk --- add it to anything saved for that pid, and
 				 * either way write the whole thing out.
 				 */
-				int			existing_slot = -1;
-				int			i;
-				StringInfo	str;
-
-				for (i = 0; i < CHUNK_SLOTS; i++)
-				{
-					if (saved_chunks[i].pid == p.pid)
-					{
-						existing_slot = i;
-						break;
-					}
-				}
-				if (existing_slot >= 0)
+				if (existing_slot != NULL)
 				{
-					str = &(saved_chunks[existing_slot].data);
+					str = &(existing_slot->data);
 					appendBinaryStringInfo(str,
 										   cursor + PIPE_HEADER_SIZE,
 										   p.len);
 					write_syslogger_file(str->data, str->len, dest);
-					saved_chunks[existing_slot].pid = 0;
+					/* Mark the buffer unused, and reclaim string storage */
+					existing_slot->pid = 0;
 					pfree(str->data);
 				}
 				else
@@ -872,17 +872,27 @@ static void
 flush_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 {
 	int			i;
-	StringInfo	str;
 
 	/* Dump any incomplete protocol messages */
-	for (i = 0; i < CHUNK_SLOTS; i++)
+	for (i = 0; i < NBUFFER_LISTS; i++)
 	{
-		if (saved_chunks[i].pid != 0)
+		List	   *list = buffer_lists[i];
+		ListCell   *cell;
+
+		foreach(cell, list)
 		{
-			str = &(saved_chunks[i].data);
-			write_syslogger_file(str->data, str->len, LOG_DESTINATION_STDERR);
-			saved_chunks[i].pid = 0;
-			pfree(str->data);
+			save_buffer *buf = (save_buffer *) lfirst(cell);
+
+			if (buf->pid != 0)
+			{
+				StringInfo	str = &(buf->data);
+
+				write_syslogger_file(str->data, str->len,
+									 LOG_DESTINATION_STDERR);
+				/* Mark the buffer unused, and reclaim string storage */
+				buf->pid = 0;
+				pfree(str->data);
+			}
 		}
 	}