commit 3a974053b3022538d562ba6068e07495b092a344
Author: Edmund Horner <ejrh00@gmail.com>
Date:   Tue Mar 27 15:23:57 2018 +1300

    Allocate enough shared string memory for stats of auxiliary processes
    
    This fixes a bug whereby the st_appname, st_clienthostname, and
    st_activity_raw fields for auxiliary processes point beyond the end of
    their respective shared memory segments.  This change also allocates
    local memory for the st_clienthostname fields, when copying the stats
    data in pgstat_read_current_status().
    
    Author: Edmund Horner

diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 96ba216..084573e 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -2650,7 +2650,7 @@ CreateSharedBackendStatus(void)
 	}
 
 	/* Create or attach to the shared appname buffer */
-	size = mul_size(NAMEDATALEN, MaxBackends);
+	size = mul_size(NAMEDATALEN, NumBackendStatSlots);
 	BackendAppnameBuffer = (char *)
 		ShmemInitStruct("Backend Application Name Buffer", size, &found);
 
@@ -2668,7 +2668,7 @@ CreateSharedBackendStatus(void)
 	}
 
 	/* Create or attach to the shared client hostname buffer */
-	size = mul_size(NAMEDATALEN, MaxBackends);
+	size = mul_size(NAMEDATALEN, NumBackendStatSlots);
 	BackendClientHostnameBuffer = (char *)
 		ShmemInitStruct("Backend Client Host Name Buffer", size, &found);
 
@@ -3224,6 +3224,7 @@ pgstat_read_current_status(void)
 	LocalPgBackendStatus *localtable;
 	LocalPgBackendStatus *localentry;
 	char	   *localappname,
+			   *localclienthostname,
 			   *localactivity;
 #ifdef USE_SSL
 	PgBackendSSLStatus *localsslstatus;
@@ -3242,6 +3243,9 @@ pgstat_read_current_status(void)
 	localappname = (char *)
 		MemoryContextAlloc(pgStatLocalContext,
 						   NAMEDATALEN * NumBackendStatSlots);
+	localclienthostname = (char *)
+		MemoryContextAlloc(pgStatLocalContext,
+						   NAMEDATALEN * NumBackendStatSlots);
 	localactivity = (char *)
 		MemoryContextAlloc(pgStatLocalContext,
 						   pgstat_track_activity_query_size * NumBackendStatSlots);
@@ -3282,6 +3286,8 @@ 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_raw);
 				localentry->backendStatus.st_activity_raw = localactivity;
 				localentry->backendStatus.st_ssl = beentry->st_ssl;
@@ -3313,6 +3319,7 @@ pgstat_read_current_status(void)
 
 			localentry++;
 			localappname += NAMEDATALEN;
+			localclienthostname += NAMEDATALEN;
 			localactivity += pgstat_track_activity_query_size;
 #ifdef USE_SSL
 			localsslstatus++;