From cfc70e656d7226d7f6050a99b33eeb9d09841584 Mon Sep 17 00:00:00 2001 From: Greg Nancarrow Date: Tue, 1 Dec 2020 14:17:38 +1100 Subject: [PATCH v20 1/2] Add "in_hot_standby" reportable GUC and make "transaction_read_only" GUC reportable. Add "in_hot_standby" as a GUC_REPORT variable, to indicate to clients when hot-standby is currently active, by having the end-of-query function check (when needed) to see if the active hot-standby value changed. (Implementation: Tom Lane) Also, enhance "transaction_read_only" to be a GUC_REPORT variable, for client connections to read-only/read-write servers. Making these GUC variables reportable avoids having to execute a query post-connection in order to determine whether a host is in hot-standby mode or is read-write (and reduces time to make the connection). Discussion: https://www.postgresql.org/message-id/flat/CAF3+xM+8-ztOkaV9gHiJ3wfgENTq97QcjXQt+rbFQ6F7oNzt9A@mail.gmail.com --- doc/src/sgml/high-availability.sgml | 11 +++++---- src/backend/utils/misc/check_guc | 2 +- src/backend/utils/misc/guc.c | 48 ++++++++++++++++++++++++++++++++++++- 3 files changed, 54 insertions(+), 7 deletions(-) diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml index 19d7bd2..1f5aef9 100644 --- a/doc/src/sgml/high-availability.sgml +++ b/doc/src/sgml/high-availability.sgml @@ -1848,8 +1848,9 @@ if (!triggered) - During hot standby, the parameter transaction_read_only is always - true and may not be changed. But as long as no attempt is made to modify + During hot standby, the parameters in_hot_standby and + transaction_read_only are always true and may not be + changed. But as long as no attempt is made to modify the database, connections during hot standby will act much like any other database connection. If failover or switchover occurs, the database will switch to normal processing mode. Sessions will remain connected while the @@ -1859,9 +1860,9 @@ if (!triggered) - Users will be able to tell whether their session is read-only by - issuing SHOW transaction_read_only. In addition, a set of - functions () allow users to + Users will be able to tell whether hot standby is currently active for their + session by issuing SHOW in_hot_standby. In addition, a set + of functions () allow users to access information about the standby server. These allow you to write programs that are aware of the current state of the database. These can be used to monitor the progress of recovery, or to allow you to diff --git a/src/backend/utils/misc/check_guc b/src/backend/utils/misc/check_guc index 416a087..5e7a54a 100755 --- a/src/backend/utils/misc/check_guc +++ b/src/backend/utils/misc/check_guc @@ -21,7 +21,7 @@ is_superuser lc_collate lc_ctype lc_messages lc_monetary lc_numeric lc_time \ pre_auth_delay role seed server_encoding server_version server_version_num \ session_authorization trace_lock_oidmin trace_lock_table trace_locks trace_lwlocks \ trace_notify trace_userlocks transaction_isolation transaction_read_only \ -zero_damaged_pages" +zero_damaged_pages in_hot_standby" ### What options are listed in postgresql.conf.sample, but don't appear ### in guc.c? diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c index 245a347..311474e 100644 --- a/src/backend/utils/misc/guc.c +++ b/src/backend/utils/misc/guc.c @@ -209,6 +209,7 @@ static bool check_cluster_name(char **newval, void **extra, GucSource source); static const char *show_unix_socket_permissions(void); static const char *show_log_file_mode(void); static const char *show_data_directory_mode(void); +static const char *show_in_hot_standby(void); static bool check_backtrace_functions(char **newval, void **extra, GucSource source); static void assign_backtrace_functions(const char *newval, void *extra); static bool check_recovery_target_timeline(char **newval, void **extra, GucSource source); @@ -607,6 +608,8 @@ static int max_identifier_length; static int block_size; static int segment_size; static int wal_block_size; +static bool in_hot_standby; +static bool last_reported_in_hot_standby; static bool data_checksums; static bool integer_datetimes; static bool assert_enabled; @@ -1618,7 +1621,7 @@ static struct config_bool ConfigureNamesBool[] = {"transaction_read_only", PGC_USERSET, CLIENT_CONN_STATEMENT, gettext_noop("Sets the current transaction's read-only status."), NULL, - GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE + GUC_REPORT | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE }, &XactReadOnly, false, @@ -1845,6 +1848,17 @@ static struct config_bool ConfigureNamesBool[] = }, { + {"in_hot_standby", PGC_INTERNAL, PRESET_OPTIONS, + gettext_noop("Shows whether hot standby is currently active."), + NULL, + GUC_REPORT | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE + }, + &in_hot_standby, + false, + NULL, NULL, show_in_hot_standby + }, + + { {"allow_system_table_mods", PGC_SUSET, DEVELOPER_OPTIONS, gettext_noop("Allows modifications of the structure of system tables."), NULL, @@ -6267,6 +6281,9 @@ BeginReportingGUCOptions(void) ReportGUCOption(conf); } + /* Hack for in_hot_standby: remember the value we just sent */ + last_reported_in_hot_standby = in_hot_standby; + report_needed = false; } @@ -6290,6 +6307,23 @@ ReportChangedGUCOptions(void) if (!reporting_enabled) return; + /* + * Since in_hot_standby isn't actually changed by normal GUC actions, we + * need a hack to check whether a new value needs to be reported to the + * client. For speed, we rely on the assumption that it can never + * transition from false to true. + */ + if (last_reported_in_hot_standby && !RecoveryInProgress()) + { + struct config_generic *record; + + record = find_option("in_hot_standby", false, ERROR); + Assert(record != NULL); + record->status |= GUC_NEEDS_REPORT; + report_needed = true; + last_reported_in_hot_standby = false; + } + /* Quick exit if no values have been changed */ if (!report_needed) return; @@ -11783,6 +11817,18 @@ show_data_directory_mode(void) return buf; } +static const char * +show_in_hot_standby(void) +{ + /* + * Unlike most show hooks, this has a side-effect of updating the dummy + * GUC variable to contain the value last shown. See confederate code in + * BeginReportingGUCOptions and ReportChangedGUCOptions. + */ + in_hot_standby = RecoveryInProgress(); + return in_hot_standby ? "on" : "off"; +} + /* * We split the input string, where commas separate function names * and certain whitespace chars are ignored, into a \0-separated (and -- 1.8.3.1