Add local-address escape "%L" to log_line_prefix.

This escape shows the numeric server IP address that the client
has connected to.  Unix-socket connections will show "[local]".
Non-client processes (e.g. background processes) will show "[none]".

We expect that this option will be of interest to only a fairly
small number of users.  Therefore the implementation is optimized
for the case where it's not used (that is, we don't do the string
conversion until we have to), and we've not added the field to
csvlog or jsonlog formats.

Author: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Cary Huang <cary.huang@highgo.ca>
Reviewed-by: David Steele <david@pgmasters.net>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAKAnmmK-U+UicE-qbNU23K--Q5XTLdM6bj+gbkZBZkjyjrd3Ow@mail.gmail.com

doc/src/sgml/config.sgml

@@ -7689,6 +7689,12 @@ local0.*    /var/log/postgresql
              <entry>Remote host name or IP address</entry>
              <entry>yes</entry>
             </row>
+            <row>
+             <entry><literal>%L</literal></entry>
+             <entry>Local address (the IP address on the server that the
+             client connected to)</entry>
+             <entry>yes</entry>
+            </row>
             <row>
              <entry><literal>%b</literal></entry>
              <entry>Backend type</entry>

src/backend/utils/error/elog.c

@@ -67,6 +67,7 @@
 #endif
 
 #include "access/xact.h"
+#include "common/ip.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "mb/pg_wchar.h"
@@ -3084,6 +3085,38 @@ log_status_format(StringInfo buf, const char *format, ErrorData *edata)
 					appendStringInfoSpaces(buf,
 										   padding > 0 ? padding : -padding);
 				break;
+			case 'L':
+				{
+					const char *local_host;
+
+					if (MyProcPort)
+					{
+						if (MyProcPort->local_host[0] == '\0')
+						{
+							/*
+							 * First time through: cache the lookup, since it
+							 * might not have trivial cost.
+							 */
+							(void) pg_getnameinfo_all(&MyProcPort->laddr.addr,
+													  MyProcPort->laddr.salen,
+													  MyProcPort->local_host,
+													  sizeof(MyProcPort->local_host),
+													  NULL, 0,
+													  NI_NUMERICHOST | NI_NUMERICSERV);
+						}
+						local_host = MyProcPort->local_host;
+					}
+					else
+					{
+						/* Background process, or connection not yet made */
+						local_host = "[none]";
+					}
+					if (padding != 0)
+						appendStringInfo(buf, "%*s", padding, local_host);
+					else
+						appendStringInfoString(buf, local_host);
+				}
+				break;
 			case 'r':
 				if (MyProcPort && MyProcPort->remote_host)
 				{

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

@@ -604,6 +604,7 @@
 					#   %d = database name
 					#   %r = remote host and port
 					#   %h = remote host
+					#   %L = local address
 					#   %b = backend type
 					#   %p = process ID
 					#   %P = process ID of parallel group leader

src/include/libpq/libpq-be.h

@@ -139,6 +139,9 @@ typedef struct Port
 	int			remote_hostname_errcode;	/* see above */
 	char	   *remote_port;	/* text rep of remote port */
 
+	/* local_host is filled only if needed (see log_status_format) */
+	char		local_host[64]; /* ip addr of local socket for client conn */
+
 	/*
 	 * Information that needs to be saved from the startup packet and passed
 	 * into backend execution.  "char *" fields are NULL if not set.