Turn PGBE_ACTIVITY_SIZE into a GUC variable, track_activity_query_size.

As the buffer could now be a lot larger than before, and copying it could
thus be a lot more expensive than before, use strcpy instead of memcpy to
copy the query string, as was already suggested in comments. Also, only copy
the PgBackendStatus struct and string if the slot is in use.

Patch by Thomas Lee, with some changes by me.

5 files changed, 73 insertions, 14 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 1fcbb54d13..a54c413823 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -3331,6 +3331,22 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-track-activity-query-size" xreflabel="track_activity_query_size">
+      <term><varname>track_activity_query_size</varname> (<type>integer</type>)</term>
+      <indexterm>
+       <primary><varname>track_activity_query_size</> configuration parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+       Specifies the number of bytes reserved to track the currently
+       executing command for each active session, for the
+       <structname>pg_stat_activity</>.<structfield>current_query</> field.
+       The default value is 1024. This parameter can only be set at server
+       start.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-track-counts" xreflabel="track_counts">
       <term><varname>track_counts</varname> (<type>boolean</type>)</term>
       <indexterm>
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index f178fe31d3..34a2406a9a 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -101,6 +101,7 @@
 bool		pgstat_track_activities = false;
 bool		pgstat_track_counts = false;
 int			pgstat_track_functions = TRACK_FUNC_OFF;
+int			pgstat_track_activity_query_size = 1024;
 
 /*
  * BgWriter global statistics counters (unused in other processes).
@@ -2010,6 +2011,7 @@ pgstat_fetch_global(void)
 
 static PgBackendStatus *BackendStatusArray = NULL;
 static PgBackendStatus *MyBEEntry = NULL;
+static char			   *BackendActivityBuffer = NULL;
 
 
 /*
@@ -2020,20 +2022,25 @@ BackendStatusShmemSize(void)
 {
 	Size		size;
 
-	size = mul_size(sizeof(PgBackendStatus), MaxBackends);
+	size = add_size(mul_size(sizeof(PgBackendStatus), MaxBackends),
+					mul_size(pgstat_track_activity_query_size, MaxBackends));
 	return size;
 }
 
 /*
- * Initialize the shared status array during postmaster startup.
+ * Initialize the shared status array and activity string buffer during
+ * postmaster startup.
  */
 void
 CreateSharedBackendStatus(void)
 {
-	Size		size = BackendStatusShmemSize();
+	Size		size;
 	bool		found;
+	int			i;
+	char	   *buffer;
 
 	/* Create or attach to the shared array */
+	size = mul_size(sizeof(PgBackendStatus), MaxBackends);
 	BackendStatusArray = (PgBackendStatus *)
 		ShmemInitStruct("Backend Status Array", size, &found);
 
@@ -2044,6 +2051,23 @@ CreateSharedBackendStatus(void)
 		 */
 		MemSet(BackendStatusArray, 0, size);
 	}
+
+	/* Create or attach to the shared activity buffer */
+	size = mul_size(pgstat_track_activity_query_size, MaxBackends);
+	BackendActivityBuffer = (char*)
+		ShmemInitStruct("Backend Activity Buffer", size, &found);
+
+	if (!found)
+	{
+		MemSet(BackendActivityBuffer, 0, size);
+
+		/* Initialize st_activity pointers. */
+		buffer = BackendActivityBuffer;
+		for (i = 0; i < MaxBackends; i++) {
+			BackendStatusArray[i].st_activity = buffer;
+			buffer += pgstat_track_activity_query_size;
+		}
+	}
 }
 
 
@@ -2128,7 +2152,7 @@ pgstat_bestart(void)
 	beentry->st_waiting = false;
 	beentry->st_activity[0] = '\0';
 	/* Also make sure the last byte in the string area is always 0 */
-	beentry->st_activity[PGBE_ACTIVITY_SIZE - 1] = '\0';
+	beentry->st_activity[pgstat_track_activity_query_size - 1] = '\0';
 
 	beentry->st_changecount++;
 	Assert((beentry->st_changecount & 1) == 0);
@@ -2188,7 +2212,7 @@ pgstat_report_activity(const char *cmd_str)
 	start_timestamp = GetCurrentStatementStartTimestamp();
 
 	len = strlen(cmd_str);
-	len = pg_mbcliplen(cmd_str, len, PGBE_ACTIVITY_SIZE - 1);
+	len = pg_mbcliplen(cmd_str, len, pgstat_track_activity_query_size - 1);
 
 	/*
 	 * Update my status entry, following the protocol of bumping
@@ -2267,6 +2291,7 @@ pgstat_read_current_status(void)
 	volatile PgBackendStatus *beentry;
 	PgBackendStatus *localtable;
 	PgBackendStatus *localentry;
+	char			*localactivity;
 	int			i;
 
 	Assert(!pgStatRunningInCollector);
@@ -2278,6 +2303,9 @@ pgstat_read_current_status(void)
 	localtable = (PgBackendStatus *)
 		MemoryContextAlloc(pgStatLocalContext,
 						   sizeof(PgBackendStatus) * MaxBackends);
+	localactivity = (char *)
+		MemoryContextAlloc(pgStatLocalContext,
+						   pgstat_track_activity_query_size * MaxBackends);
 	localNumBackends = 0;
 
 	beentry = BackendStatusArray;
@@ -2295,11 +2323,17 @@ pgstat_read_current_status(void)
 		{
 			int			save_changecount = beentry->st_changecount;
 
-			/*
-			 * XXX if PGBE_ACTIVITY_SIZE is really large, it might be best to
-			 * use strcpy not memcpy for copying the activity string?
-			 */
-			memcpy(localentry, (char *) beentry, sizeof(PgBackendStatus));
+			localentry->st_procpid = beentry->st_procpid;
+			if (localentry->st_procpid > 0)
+			{
+				memcpy(localentry, (char *) beentry, sizeof(PgBackendStatus));
+				/*
+				 * strcpy is safe even if the string is modified concurrently,
+				 * because there's always a \0 at the end of the buffer.
+				 */
+				strcpy(localactivity, (char *) beentry->st_activity);
+				localentry->st_activity = localactivity;
+			}
 
 			if (save_changecount == beentry->st_changecount &&
 				(save_changecount & 1) == 0)
@@ -2314,6 +2348,7 @@ pgstat_read_current_status(void)
 		if (localentry->st_procpid > 0)
 		{
 			localentry++;
+			localactivity += pgstat_track_activity_query_size;
 			localNumBackends++;
 		}
 	}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index fa964370a9..456560ba3b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1848,6 +1848,15 @@ static struct config_int ConfigureNamesInt[] =
 		-1, -1, INT_MAX, NULL, NULL
 	},
 
+	{
+		{"track_activity_query_size", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the size reserved for pg_stat_activity.current_query, in bytes."),
+			NULL,
+		},
+		&pgstat_track_activity_query_size,
+		1024, 100, 102400, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 033a74b2d4..552d856ae2 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -364,6 +364,7 @@
 #track_activities = on
 #track_counts = on
 #track_functions = none			# none, pl, all
+#track_activity_query_size = 1024
 #update_process_title = on
 
 
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index a9f5501143..8abcc2f264 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -509,9 +509,6 @@ typedef struct PgStat_GlobalStats
  * ----------
  */
 
-/* Max length of st_activity string ... perhaps replace with a GUC var? */
-#define PGBE_ACTIVITY_SIZE	1024
-
 /* ----------
  * PgBackendStatus
  *
@@ -551,7 +548,7 @@ typedef struct PgBackendStatus
 	bool		st_waiting;
 
 	/* current command string; MUST be null-terminated */
-	char		st_activity[PGBE_ACTIVITY_SIZE];
+	char	   *st_activity;
 } PgBackendStatus;
 
 /*
@@ -578,6 +575,7 @@ typedef struct PgStat_FunctionCallUsage
 extern bool pgstat_track_activities;
 extern bool pgstat_track_counts;
 extern int	pgstat_track_functions;
+extern int	pgstat_track_activity_query_size;
 
 /*
  * BgWriter statistics counters are updated directly by bgwriter and bufmgr