Make local copy of client hostnames in backend status array.

The other strings, application_name and query string, were snapshotted to
local memory in pgstat_read_current_status(), but we forgot to do that for
client hostnames. As a result, the client hostname would appear to change in
the local copy, if the client disconnected.

Backpatch to all supported versions.

Author: Edmund Horner
Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/CAMyN-kA7aOJzBmrYFdXcc7Z0NmW%2B5jBaf_m%3D_-77uRNyKC9r%3DA%40mail.gmail.com

src/backend/postmaster/pgstat.c

@@ -3058,6 +3058,7 @@ pgstat_read_current_status(void)
 	LocalPgBackendStatus *localtable;
 	LocalPgBackendStatus *localentry;
 	char	   *localappname,
+			   *localclienthostname,
 			   *localactivity;
 #ifdef USE_SSL
 	PgBackendSSLStatus *localsslstatus;
@@ -3076,6 +3077,9 @@ pgstat_read_current_status(void)
 	localappname = (char *)
 		MemoryContextAlloc(pgStatLocalContext,
 						   NAMEDATALEN * MaxBackends);
+	localclienthostname = (char *)
+		MemoryContextAlloc(pgStatLocalContext,
+						   NAMEDATALEN * MaxBackends);
 	localactivity = (char *)
 		MemoryContextAlloc(pgStatLocalContext,
 						   pgstat_track_activity_query_size * MaxBackends);
@@ -3116,6 +3120,8 @@ pgstat_read_current_status(void)
 				 */
 				strcpy(localappname, (char *) beentry->st_appname);
 				localentry->backendStatus.st_appname = localappname;
+				strcpy(localclienthostname, (char *) beentry->st_clienthostname);
+				localentry->backendStatus.st_clienthostname = localclienthostname;
 				strcpy(localactivity, (char *) beentry->st_activity);
 				localentry->backendStatus.st_activity = localactivity;
 				localentry->backendStatus.st_ssl = beentry->st_ssl;
@@ -3147,6 +3153,7 @@ pgstat_read_current_status(void)
 
 			localentry++;
 			localappname += NAMEDATALEN;
+			localclienthostname += NAMEDATALEN;
 			localactivity += pgstat_track_activity_query_size;
 #ifdef USE_SSL
 			localsslstatus++;