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Avoid spamming the client with multiple ParameterStatus messages.

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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
This commit is contained in:
Tom Lane
2020-11-25 11:40:44 -05:00
parent f73999262e
commit 2432b1a040
4 changed files with 80 additions and 7 deletions
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78
src/backend/utils/misc/guc.c
View File

@ -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;
}