Add %P to log_line_prefix for parallel group leader

This is useful for monitoring purposes with log parsing.  Similarly to
pg_stat_activity, the leader's PID is shown only for active parallel
workers, minimizing the log footprint for the leaders as the equivalent
shared memory field is set as long as a backend is alive.

Author: Justin Pryzby
Reviewed-by: Álvaro Herrera, Michael Paquier, Julien Rouhaud, Tom Lane
Discussion: https://postgr.es/m/20200315111831.GA21492@telsasoft.com

doc/src/sgml/config.sgml

@@ -6694,6 +6694,12 @@ local0.*    /var/log/postgresql
              <entry>Process ID</entry>
              <entry>no</entry>
             </row>
+            <row>
+             <entry><literal>%P</literal></entry>
+             <entry>Process ID of the parallel group leader, if this process
+              is a parallel query worker</entry>
+             <entry>no</entry>
+            </row>
             <row>
              <entry><literal>%t</literal></entry>
              <entry>Time stamp without milliseconds</entry>
@@ -7026,7 +7032,7 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
         character count of the error position therein,
         location of the error in the PostgreSQL source code
         (if <varname>log_error_verbosity</varname> is set to <literal>verbose</literal>),
-        application name, and backend type.
+        application name, backend type, and process ID of parallel group leader.
         Here is a sample table definition for storing CSV-format log output:
 
 <programlisting>
@@ -7056,6 +7062,7 @@ CREATE TABLE postgres_log
   location text,
   application_name text,
   backend_type text,
+  leader_pid integer,
   PRIMARY KEY (session_id, session_line_num)
 );
 </programlisting>

src/backend/utils/error/elog.c

@@ -2448,6 +2448,29 @@ log_line_prefix(StringInfo buf, ErrorData *edata)
 				else
 					appendStringInfo(buf, "%d", MyProcPid);
 				break;
+
+			case 'P':
+				if (MyProc)
+				{
+					PGPROC	   *leader = MyProc->lockGroupLeader;
+
+					/*
+					 * Show the leader only for active parallel workers. This
+					 * leaves out the leader of a parallel group.
+					 */
+					if (leader == NULL || leader->pid == MyProcPid)
+						appendStringInfoSpaces(buf,
+											   padding > 0 ? padding : -padding);
+					else if (padding != 0)
+						appendStringInfo(buf, "%*d", padding, leader->pid);
+					else
+						appendStringInfo(buf, "%d", leader->pid);
+				}
+				else if (padding != 0)
+					appendStringInfoSpaces(buf,
+										   padding > 0 ? padding : -padding);
+				break;
+
 			case 'l':
 				if (padding != 0)
 					appendStringInfo(buf, "%*ld", padding, log_line_number);
@@ -2836,6 +2859,21 @@ write_csvlog(ErrorData *edata)
 	else
 		appendCSVLiteral(&buf, GetBackendTypeDesc(MyBackendType));
 
+	appendStringInfoChar(&buf, ',');
+
+	/* leader PID */
+	if (MyProc)
+	{
+		PGPROC	   *leader = MyProc->lockGroupLeader;
+
+		/*
+		 * Show the leader only for active parallel workers.  This leaves out
+		 * the leader of a parallel group.
+		 */
+		if (leader && leader->pid != MyProcPid)
+			appendStringInfo(&buf, "%d", leader->pid);
+	}
+
 	appendStringInfoChar(&buf, '\n');
 
 	/* If in the syslogger process, try to write messages direct to file */

src/backend/utils/misc/postgresql.conf.sample

@@ -537,6 +537,7 @@
 					#   %h = remote host
 					#   %b = backend type
 					#   %p = process ID
+					#   %P = process ID of parallel group leader
 					#   %t = timestamp without milliseconds
 					#   %m = timestamp with milliseconds
 					#   %n = timestamp with milliseconds (as a Unix epoch)