Avoid spamming the client with multiple ParameterStatus messages.

Up to now, we sent a ParameterStatus message to the client immediately
upon any change in the active value of any GUC_REPORT variable.  This
was only barely okay when the feature was designed; now that we have
things like function SET clauses, there are very plausible use-cases
where a GUC_REPORT variable might change many times within a query
--- and even end up back at its original value, perhaps.  Fortunately
most of our GUC_REPORT variables are unlikely to be changed often;
but there are proposals in play to enlarge that set, or even make it
user-configurable.

Hence, let's fix things to not generate more than one ParameterStatus
message per variable per query, and to not send any message at all
unless the end-of-query value is different from what we last reported.

Discussion: https://postgr.es/m/5708.1601145259@sss.pgh.pa.us

src/backend/tcop/postgres.c

@@ -4229,6 +4229,9 @@ PostgresMain(int argc, char *argv[],
 				pgstat_report_activity(STATE_IDLE, NULL);
 			}
 
+			/* Report any recently-changed GUC options */
+			ReportChangedGUCOptions();
+
 			ReadyForQuery(whereToSendOutput);
 			send_ready_for_query = false;
 		}

src/backend/utils/misc/guc.c

@@ -4822,6 +4822,8 @@ static bool guc_dirty;			/* true if need to do commit/abort work */
 
 static bool reporting_enabled;	/* true to enable GUC_REPORT */
 
+static bool report_needed;		/* true if any GUC_REPORT reports are needed */
+
 static int	GUCNestLevel = 0;	/* 1 when in main transaction */
 
 
@@ -5452,6 +5454,7 @@ InitializeOneGUCOption(struct config_generic *gconf)
 	gconf->reset_scontext = PGC_INTERNAL;
 	gconf->stack = NULL;
 	gconf->extra = NULL;
+	gconf->last_reported = NULL;
 	gconf->sourcefile = NULL;
 	gconf->sourceline = 0;
 
@@ -5828,7 +5831,10 @@ ResetAllOptions(void)
 		gconf->scontext = gconf->reset_scontext;
 
 		if (gconf->flags & GUC_REPORT)
-			ReportGUCOption(gconf);
+		{
+			gconf->status |= GUC_NEEDS_REPORT;
+			report_needed = true;
+		}
 	}
 }
 
@@ -6215,7 +6221,10 @@ AtEOXact_GUC(bool isCommit, int nestLevel)
 
 			/* Report new value if we changed it */
 			if (changed && (gconf->flags & GUC_REPORT))
-				ReportGUCOption(gconf);
+			{
+				gconf->status |= GUC_NEEDS_REPORT;
+				report_needed = true;
+			}
 		}						/* end of stack-popping loop */
 
 		if (stack != NULL)
@@ -6257,17 +6266,60 @@ BeginReportingGUCOptions(void)
 		if (conf->flags & GUC_REPORT)
 			ReportGUCOption(conf);
 	}
+
+	report_needed = false;
+}
+
+/*
+ * ReportChangedGUCOptions: report recently-changed GUC_REPORT variables
+ *
+ * This is called just before we wait for a new client query.
+ *
+ * By handling things this way, we ensure that a ParameterStatus message
+ * is sent at most once per variable per query, even if the variable
+ * changed multiple times within the query.  That's quite possible when
+ * using features such as function SET clauses.  Function SET clauses
+ * also tend to cause values to change intraquery but eventually revert
+ * to their prevailing values; ReportGUCOption is responsible for avoiding
+ * redundant reports in such cases.
+ */
+void
+ReportChangedGUCOptions(void)
+{
+	/* Quick exit if not (yet) enabled */
+	if (!reporting_enabled)
+		return;
+
+	/* Quick exit if no values have been changed */
+	if (!report_needed)
+		return;
+
+	/* Transmit new values of interesting variables */
+	for (int i = 0; i < num_guc_variables; i++)
+	{
+		struct config_generic *conf = guc_variables[i];
+
+		if ((conf->flags & GUC_REPORT) && (conf->status & GUC_NEEDS_REPORT))
+			ReportGUCOption(conf);
+	}
+
+	report_needed = false;
 }
 
 /*
  * ReportGUCOption: if appropriate, transmit option value to frontend
+ *
+ * We need not transmit the value if it's the same as what we last
+ * transmitted.  However, clear the NEEDS_REPORT flag in any case.
  */
 static void
 ReportGUCOption(struct config_generic *record)
 {
-	if (reporting_enabled && (record->flags & GUC_REPORT))
+	char	   *val = _ShowOption(record, false);
+
+	if (record->last_reported == NULL ||
+		strcmp(val, record->last_reported) != 0)
 	{
-		char	   *val = _ShowOption(record, false);
 		StringInfoData msgbuf;
 
 		pq_beginmessage(&msgbuf, 'S');
@@ -6275,8 +6327,19 @@ ReportGUCOption(struct config_generic *record)
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
 
-		pfree(val);
+		/*
+		 * We need a long-lifespan copy.  If strdup() fails due to OOM, we'll
+		 * set last_reported to NULL and thereby possibly make a duplicate
+		 * report later.
+		 */
+		if (record->last_reported)
+			free(record->last_reported);
+		record->last_reported = strdup(val);
 	}
+
+	pfree(val);
+
+	record->status &= ~GUC_NEEDS_REPORT;
 }
 
 /*
@@ -7695,7 +7758,10 @@ set_config_option(const char *name, const char *value,
 	}
 
 	if (changeVal && (record->flags & GUC_REPORT))
-		ReportGUCOption(record);
+	{
+		record->status |= GUC_NEEDS_REPORT;
+		report_needed = true;
+	}
 
 	return changeVal ? 1 : -1;
 }

src/include/utils/guc.h

@@ -363,6 +363,7 @@ extern void AtStart_GUC(void);
 extern int	NewGUCNestLevel(void);
 extern void AtEOXact_GUC(bool isCommit, int nestLevel);
 extern void BeginReportingGUCOptions(void);
+extern void ReportChangedGUCOptions(void);
 extern void ParseLongOption(const char *string, char **name, char **value);
 extern bool parse_int(const char *value, int *result, int flags,
 					  const char **hintmsg);

src/include/utils/guc_tables.h

@@ -161,6 +161,8 @@ struct config_generic
 	GucContext	reset_scontext; /* context that set the reset value */
 	GucStack   *stack;			/* stacked prior values */
 	void	   *extra;			/* "extra" pointer for current actual value */
+	char	   *last_reported;	/* if variable is GUC_REPORT, value last sent
+								 * to client (NULL if not yet sent) */
 	char	   *sourcefile;		/* file current setting is from (NULL if not
 								 * set in config file) */
 	int			sourceline;		/* line in source file */
@@ -172,7 +174,8 @@ struct config_generic
  * Caution: the GUC_IS_IN_FILE bit is transient state for ProcessConfigFile.
  * Do not assume that its value represents useful information elsewhere.
  */
-#define GUC_PENDING_RESTART 0x0002
+#define GUC_PENDING_RESTART 0x0002	/* changed value cannot be applied yet */
+#define GUC_NEEDS_REPORT	0x0004	/* new value must be reported to client */
 
 
 /* GUC records for specific variable types */