Refactor the syslogger pipe protocol to use a bitmask for its options

The previous protocol expected a set of matching characters to check if
a message sent was the last one or not, that changed depending on the
destination wanted:
- 't' and 'f' tracked the last message of a log sent to stderr.
- 'T' and 'F' tracked the last message of a log sent to csvlog.

This could be extended with more characters when introducing new
destinations, but using a bitmask is much more elegant.  This commit
changes the protocol so as a bitmask is used in the header of a log
chunk message sent to the syslogger, with the following options
available for now:
- log_destination as stderr.
- log_destination as csvlog.
- if a message is the last chunk of a message.

Sehrope found this issue in a patch set to introduce JSON as an option
for log_destination, but his patch made the size of the protocol header
larger.  This commit keeps the same size as the original, and adapts the
protocol as wanted.

Thanks also to Andrew Dunstan and Greg Stark for the discussion.

Author: Michael Paquier, Sehrope Sarkuni
Discussion: https://postgr.es/m/CAH7T-aqswBM6JWe4pDehi1uOiufqe06DJWaU5=X7dDLyqUExHg@mail.gmail.com

1 files changed, 14 insertions, 5 deletions

diff --git a/src/backend/postmaster/syslogger.c b/src/backend/postmaster/syslogger.c
index cad43bdef23..bca38835726 100644
--- a/src/backend/postmaster/syslogger.c
+++ b/src/backend/postmaster/syslogger.c
@@ -38,6 +38,7 @@
 #include "nodes/pg_list.h"
 #include "pgstat.h"
 #include "pgtime.h"
+#include "port/pg_bitutils.h"
 #include "postmaster/fork_process.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/postmaster.h"
@@ -885,14 +886,15 @@ process_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 	{
 		PipeProtoHeader p;
 		int			chunklen;
+		bits8		dest_flags;
 
 		/* Do we have a valid header? */
 		memcpy(&p, cursor, offsetof(PipeProtoHeader, data));
+		dest_flags = p.flags & (PIPE_PROTO_DEST_STDERR | PIPE_PROTO_DEST_CSVLOG);
 		if (p.nuls[0] == '\0' && p.nuls[1] == '\0' &&
 			p.len > 0 && p.len <= PIPE_MAX_PAYLOAD &&
 			p.pid != 0 &&
-			(p.is_last == 't' || p.is_last == 'f' ||
-			 p.is_last == 'T' || p.is_last == 'F'))
+			pg_popcount((char *) &dest_flags, 1) == 1)
 		{
 			List	   *buffer_list;
 			ListCell   *cell;
@@ -906,8 +908,15 @@ process_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 			if (count < chunklen)
 				break;
 
-			dest = (p.is_last == 'T' || p.is_last == 'F') ?
-				LOG_DESTINATION_CSVLOG : LOG_DESTINATION_STDERR;
+			if ((p.flags & PIPE_PROTO_DEST_STDERR) != 0)
+				dest = LOG_DESTINATION_STDERR;
+			else if ((p.flags & PIPE_PROTO_DEST_CSVLOG) != 0)
+				dest = LOG_DESTINATION_CSVLOG;
+			else
+			{
+				/* this should never happen as of the header validation */
+				Assert(false);
+			}
 
 			/* Locate any existing buffer for this source pid */
 			buffer_list = buffer_lists[p.pid % NBUFFER_LISTS];
@@ -924,7 +933,7 @@ process_pipe_input(char *logbuffer, int *bytes_in_logbuffer)
 					free_slot = buf;
 			}
 
-			if (p.is_last == 'f' || p.is_last == 'F')
+			if ((p.flags & PIPE_PROTO_IS_LAST) == 0)
 			{
 				/*
 				 * Save a complete non-final chunk in a per-pid buffer