Prevent concurrent SimpleLruTruncate() for any given SLRU.

The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com

2 files changed, 19 insertions, 1 deletions

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 0709b35c043..1466606c348 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -10215,7 +10215,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
    and general database objects (identified by class OID and object OID,
    in the same way as in <structname>pg_description</structname> or
    <structname>pg_depend</structname>).  Also, the right to extend a
-   relation is represented as a separate lockable object.
+   relation is represented as a separate lockable object, as is the right to
+   update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>.
    Also, <quote>advisory</quote> locks can be taken on numbers that have
    user-defined meanings.
   </para>
@@ -10243,6 +10244,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        Type of the lockable object:
        <literal>relation</literal>,
        <literal>extend</literal>,
+       <literal>frozenid</literal>,
        <literal>page</literal>,
        <literal>tuple</literal>,
        <literal>transactionid</literal>,
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 408ecbcae05..5243f5f3461 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1739,6 +1739,12 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry>Waiting to extend a relation.</entry>
      </row>
      <row>
+      <entry><literal>frozenid</literal></entry>
+      <entry>Waiting to
+       update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>
+       and <structname>pg_database</structname>.<structfield>datminmxid</structfield>.</entry>
+     </row>
+     <row>
       <entry><literal>object</literal></entry>
       <entry>Waiting to acquire a lock on a non-relation database object.</entry>
      </row>
@@ -1907,6 +1913,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry>Waiting to read or update <command>NOTIFY</command> messages.</entry>
      </row>
      <row>
+      <entry><literal>NotifyQueueTail</literal></entry>
+      <entry>Waiting to update limit on <command>NOTIFY</command> message
+       storage.</entry>
+     </row>
+     <row>
       <entry><literal>NotifySLRU</literal></entry>
       <entry>Waiting to access the <command>NOTIFY</command> message SLRU
        cache.</entry>
@@ -2083,6 +2094,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry>Waiting for WAL buffers to be written to disk.</entry>
      </row>
      <row>
+      <entry><literal>WrapLimitsVacuum</literal></entry>
+      <entry>Waiting to update limits on transaction id and multixact
+       consumption.</entry>
+     </row>
+     <row>
       <entry><literal>XactBuffer</literal></entry>
       <entry>Waiting for I/O on a transaction status SLRU buffer.</entry>
      </row>