Re: Libpq support to connect to standby server as priority - Mailing list pgsql-hackers

I wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> Agreed.  If this is just a few hundred bytes of server-side local memory
>> per backend, it seems definitely worth it.

> Yeah, given the current set of GUC_REPORT variables, it's hard to see
> the storage for their last-reported values amounting to much.  The need
> for an extra pointer field in each GUC variable record might eat more
> space than the actually-live values :-(

Here's a v2 that does it like that.

            regards, tom lane

diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 7c5f7c775b..34ed0e7558 100644
--- a/src/backend/tcop/postgres.c
+++ b/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;
         }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index bb34630e8e..245a3472bc 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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;
 }
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index 073c8f3e06..6a20a3bcec 100644
--- a/src/include/utils/guc.h
+++ b/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);
diff --git a/src/include/utils/guc_tables.h b/src/include/utils/guc_tables.h
index 04431d0eb2..7f36e1146f 100644
--- a/src/include/utils/guc_tables.h
+++ b/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 */