Date: Tue, 4 Feb 2020 15:27:25 +0100
From: Julien Rouhaud <rjuju123@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Sergei Kornilov <sk@zsrv.org>,
	Guillaume Lelarge <guillaume@lelarge.info>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Expose lock group leader pid in pg_stat_activity
Message-ID: <20200204142725.GA30965@nol>
References: 
 <CAOBaU_Yaj8Zv-jXQu5VoowSf4kkTLGvFDg6ZZyfireBSVgooNQ@mail.gmail.com>
 <20200116072727.GI3117@paquier.xyz>
 <20200116074912.GA98418@paquier.xyz>
 <CAOBaU_ZPHkriFVYF=OrFyLsrXzcdzPizr0SPh+QdNBsHjT2GmQ@mail.gmail.com>
 <20200118025102.GB230692@paquier.xyz>
 <CAOBaU_Z6WcT8FJfcCMU_Bp+PTzUpTEY9BfBQE4gbGKpE5wpfwQ@mail.gmail.com>
 <20200128130910.anay3mlhaminob6w@development>
 <CAOBaU_YpsBA2puV8NtjV_Oit2jbt=wnNSA_2E41kGUi_BhA8rA@mail.gmail.com>
 <20200128135208.6tnr72gamv2dr262@development>
 <20200130130301.GC130922@paquier.xyz>
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On Thu, Jan 30, 2020 at 10:03:01PM +0900, Michael Paquier wrote:
> On Tue, Jan 28, 2020 at 02:52:08PM +0100, Tomas Vondra wrote:
> > On Tue, Jan 28, 2020 at 02:26:34PM +0100, Julien Rouhaud wrote:
> >> There were already some dependencies between the rows since parallel
> >> queries were added, as you could see eg. a parallel worker while no
> >> query is currently active.  This patch will make those corner cases
> >> more obvious.
>
> I was reviewing the code and one thing that I was wondering is if it
> would be better to make the code more defensive and return NULL when
> the PID of the referenced leader is 0 or InvalidPid.  However that
> would mean that we have a dummy 2PC entry from PGPROC or something not
> yet started, which makes no sense.  So your simpler version is
> actually fine.  What you have here is that in the worst case you could
> finish with an incorrect reference to shared memory if a PGPROC is
> recycled between the moment you look for the leader field and the
> moment the PID value is fetched.  That's very unlikely to happen, and
> I agree that it does not really justify the cost of taking extra locks
> every time we scan pg_stat_activity.

Ok.

>
> > Yeah, sure. I mean explicit dependencies, e.g. a column referencing
> > values from another row, like leader_id does.
>
> +      The leader_pid is NULL for processes not involved in parallel query.
> This is missing two markups, one for "NULL" and a second for
> "leader_pid".

The extra "leader_pid" disappeared when I reworked the doc.  I'm not sure what
you meant here for NULL as I don't see any extra markup used in closeby
documentation, so I hope this version is ok.

> The documentation does not match the surroundings
> either, so I would suggest a reformulation for the beginning:
> PID of the leader process if this process is involved in parallel query.

> And actually this paragraph is not completely true, because leader_pid
> remains set even after one parallel query run has been finished for a
> session when leader_pid = pid as lockGroupLeader is set to NULL only
> once the process is stopped in ProcKill().

Oh good point, that's unfortunately not a super friendly behavior.  I tried to
adapt the documentation to address of all that.  It's maybe slightly too
verbose, but I guess that extra clarity is welcome here.

> >> Should I document the possible inconsistencies?
> >
> > I think it's worth mentioning that as a comment in the code, say before
> > the pg_stat_get_activity function. IMO we don't need to document all
> > possible inconsistencies, a generic explanation is enough.
>
> Agreed that adding some information in the area when we look at the
> PGPROC entries for wait events and such would be nice.

I added some code comments to remind that we don't guarantee any consistency
here.

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diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 8839699079..5e1f6c057b 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -622,6 +622,18 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
      <entry><type>integer</type></entry>
      <entry>Process ID of this backend</entry>
     </row>
+    <row>
+     <entry><structfield>leader_pid</structfield></entry>
+     <entry><type>integer</type></entry>
+     <entry>
+      Process ID of the leader process if this process is or has been involved
+      in parallel query, or null.  When a process wants to cooperate with
+      parallel workers, it becomes a parallel group leader, which means that
+      this field will be valued to its own process ID, and will remain a group
+      leader as long as the process exist.  When a parallel worker starts up,
+      this field will be valued with the parallel group leader process ID.
+     </entry>
+    </row>
     <row>
      <entry><structfield>usesysid</structfield></entry>
      <entry><type>oid</type></entry>
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c9e6060035..f681aafcf9 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -741,6 +741,7 @@ CREATE VIEW pg_stat_activity AS
             S.datid AS datid,
             D.datname AS datname,
             S.pid,
+            S.leader_pid,
             S.usesysid,
             U.rolname AS usename,
             S.application_name,
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 7b2da2b36f..f4bea2edd2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -547,7 +547,7 @@ pg_stat_get_progress_info(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_activity(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_ACTIVITY_COLS	29
+#define PG_STAT_GET_ACTIVITY_COLS	30
 	int			num_backends = pgstat_fetch_stat_numbackends();
 	int			curr_backend;
 	int			pid = PG_ARGISNULL(0) ? -1 : PG_GETARG_INT32(0);
@@ -686,33 +686,36 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 			values[5] = CStringGetTextDatum(clipped_activity);
 			pfree(clipped_activity);
 
+			nulls[29] = true;
 			proc = BackendPidGetProc(beentry->st_procpid);
+
+			/*
+			 * For an auxiliary process, retrieve process info from
+			 * AuxiliaryProcs stored in shared-memory.
+			 */
+			if (!proc && (beentry->st_backendType != B_BACKEND))
+				proc = AuxiliaryPidGetProc(beentry->st_procpid);
+
+			/*
+			 * If we retrieved process info, display wait events and lock group
+			 * leader info if any.  To avoid extra overhead, no extra lock is
+			 * being held, meaning that we don't guarantee consistency over the
+			 * various rows being returned.
+			 */
 			if (proc != NULL)
 			{
 				uint32		raw_wait_event;
+				PGPROC	   *leader;
 
 				raw_wait_event = UINT32_ACCESS_ONCE(proc->wait_event_info);
 				wait_event_type = pgstat_get_wait_event_type(raw_wait_event);
 				wait_event = pgstat_get_wait_event(raw_wait_event);
 
-			}
-			else if (beentry->st_backendType != B_BACKEND)
-			{
-				/*
-				 * For an auxiliary process, retrieve process info from
-				 * AuxiliaryProcs stored in shared-memory.
-				 */
-				proc = AuxiliaryPidGetProc(beentry->st_procpid);
-
-				if (proc != NULL)
+				leader = proc->lockGroupLeader;
+				if (leader)
 				{
-					uint32		raw_wait_event;
-
-					raw_wait_event =
-						UINT32_ACCESS_ONCE(proc->wait_event_info);
-					wait_event_type =
-						pgstat_get_wait_event_type(raw_wait_event);
-					wait_event = pgstat_get_wait_event(raw_wait_event);
+					values[29] = Int32GetDatum(leader->pid);
+					nulls[29] = false;
 				}
 			}
 
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 2228256907..226c904c04 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5175,9 +5175,9 @@
   proname => 'pg_stat_get_activity', prorows => '100', proisstrict => 'f',
   proretset => 't', provolatile => 's', proparallel => 'r',
   prorettype => 'record', proargtypes => 'int4',
-  proallargtypes => '{int4,oid,int4,oid,text,text,text,text,text,timestamptz,timestamptz,timestamptz,timestamptz,inet,text,int4,xid,xid,text,bool,text,text,int4,bool,text,numeric,text,bool,text,bool}',
-  proargmodes => '{i,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{pid,datid,pid,usesysid,application_name,state,query,wait_event_type,wait_event,xact_start,query_start,backend_start,state_change,client_addr,client_hostname,client_port,backend_xid,backend_xmin,backend_type,ssl,sslversion,sslcipher,sslbits,sslcompression,ssl_client_dn,ssl_client_serial,ssl_issuer_dn,gss_auth,gss_princ,gss_enc}',
+  proallargtypes => '{int4,oid,int4,oid,text,text,text,text,text,timestamptz,timestamptz,timestamptz,timestamptz,inet,text,int4,xid,xid,text,bool,text,text,int4,bool,text,numeric,text,bool,text,bool,int4}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{pid,datid,pid,usesysid,application_name,state,query,wait_event_type,wait_event,xact_start,query_start,backend_start,state_change,client_addr,client_hostname,client_port,backend_xid,backend_xmin,backend_type,ssl,sslversion,sslcipher,sslbits,sslcompression,ssl_client_dn,ssl_client_serial,ssl_issuer_dn,gss_auth,gss_princ,gss_enc,leader_pid}',
   prosrc => 'pg_stat_get_activity' },
 { oid => '3318',
   descr => 'statistics: information about progress of backends running maintenance command',
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 2ab2115fa1..634f8256f7 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1730,6 +1730,7 @@ pg_shmem_allocations| SELECT pg_get_shmem_allocations.name,
 pg_stat_activity| SELECT s.datid,
     d.datname,
     s.pid,
+    s.leader_pid,
     s.usesysid,
     u.rolname AS usename,
     s.application_name,
@@ -1747,7 +1748,7 @@ pg_stat_activity| SELECT s.datid,
     s.backend_xmin,
     s.query,
     s.backend_type
-   FROM ((pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc)
+   FROM ((pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc, leader_pid)
      LEFT JOIN pg_database d ON ((s.datid = d.oid)))
      LEFT JOIN pg_authid u ON ((s.usesysid = u.oid)));
 pg_stat_all_indexes| SELECT c.oid AS relid,
@@ -1851,7 +1852,7 @@ pg_stat_gssapi| SELECT s.pid,
     s.gss_auth AS gss_authenticated,
     s.gss_princ AS principal,
     s.gss_enc AS encrypted
-   FROM pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc)
+   FROM pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc, leader_pid)
   WHERE (s.client_port IS NOT NULL);
 pg_stat_progress_analyze| SELECT s.pid,
     s.datid,
@@ -1984,7 +1985,7 @@ pg_stat_replication| SELECT s.pid,
     w.spill_txns,
     w.spill_count,
     w.spill_bytes
-   FROM ((pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc)
+   FROM ((pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc, leader_pid)
      JOIN pg_stat_get_wal_senders() w(pid, state, sent_lsn, write_lsn, flush_lsn, replay_lsn, write_lag, flush_lag, replay_lag, sync_priority, sync_state, reply_time, spill_txns, spill_count, spill_bytes) ON ((s.pid = w.pid)))
      LEFT JOIN pg_authid u ON ((s.usesysid = u.oid)));
 pg_stat_ssl| SELECT s.pid,
@@ -1996,7 +1997,7 @@ pg_stat_ssl| SELECT s.pid,
     s.ssl_client_dn AS client_dn,
     s.ssl_client_serial AS client_serial,
     s.ssl_issuer_dn AS issuer_dn
-   FROM pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc)
+   FROM pg_stat_get_activity(NULL::integer) s(datid, pid, usesysid, application_name, state, query, wait_event_type, wait_event, xact_start, query_start, backend_start, state_change, client_addr, client_hostname, client_port, backend_xid, backend_xmin, backend_type, ssl, sslversion, sslcipher, sslbits, sslcompression, ssl_client_dn, ssl_client_serial, ssl_issuer_dn, gss_auth, gss_princ, gss_enc, leader_pid)
   WHERE (s.client_port IS NOT NULL);
 pg_stat_subscription| SELECT su.oid AS subid,
     su.subname,

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