Reducing the range of OIDs consumed by genbki.pl

The attached patch stems from the conversation at [1];
I'm starting a new thread to avoid confusing the cfbot.

Briefly, the idea is to allow reverting the change made in
commit ab596105b to increase FirstBootstrapObjectId from
12000 to 13000, by teaching genbki.pl to assign OIDs
independently in each catalog rather than from a single
OID counter.  Thus, the OIDs in this range will not be
globally unique anymore, but only unique per-catalog.

The aforesaid commit had to increase FirstBootstrapObjectId
because as of HEAD, genbki.pl needs to consume OIDs up through
12035, overrunning the old limit of 12000.  But moving up that
limit seems a bit risky, cf [2].  It'd be better if we could
avoid doing that.  Since the OIDs in question are spread across
several catalogs, allocating them per-catalog seems to fix the
problem quite effectively.  With the attached patch, the ending
OID counters are

GenbkiNextOid(pg_amop) = 10945
GenbkiNextOid(pg_amproc) = 10697
GenbkiNextOid(pg_cast) = 10230
GenbkiNextOid(pg_opclass) = 10164

so we have quite a lot of daylight before we'll ever approach
12000 again.

Per-catalog OID uniqueness shouldn't be a problem here, because
any code that's assuming global uniqueness is broken anyway;
no such guarantee exists after the OID counter has wrapped
around.

So I propose shoehorning this into v14, to avoid shipping a
release where FirstBootstrapObjectId has been bumped up.

            regards, tom lane

[1] https://www.postgresql.org/message-id/flat/3737988.1618451008%40sss.pgh.pa.us

[2] https://www.postgresql.org/message-id/flat/CAGPqQf3JYTrTB1E1fu_zOGj%2BrG_kwTfa3UcUYPfNZL9o1bcYNw%40mail.gmail.com

diff --git a/doc/src/sgml/bki.sgml b/doc/src/sgml/bki.sgml
index b33e59d5e4..db1b3d5e9a 100644
--- a/doc/src/sgml/bki.sgml
+++ b/doc/src/sgml/bki.sgml
@@ -418,11 +418,11 @@
    <para>
     If <filename>genbki.pl</filename> needs to assign an OID to a catalog
     entry that does not have a manually-assigned OID, it will use a value in
-    the range 10000—12999.  The server's OID counter is set to 13000
+    the range 10000—11999.  The server's OID counter is set to 12000
     at the start of a bootstrap run.  Thus objects created by regular SQL
     commands during the later phases of bootstrap, such as objects created
     while running the <filename>information_schema.sql</filename> script,
-    receive OIDs of 13000 or above.
+    receive OIDs of 12000 or above.
    </para>

    <para>
diff --git a/src/backend/catalog/genbki.pl b/src/backend/catalog/genbki.pl
index 112b3affdf..e5e774267e 100644
--- a/src/backend/catalog/genbki.pl
+++ b/src/backend/catalog/genbki.pl
@@ -167,15 +167,17 @@ foreach my $oid (keys %oidcounts)
 die "found $found duplicate OID(s) in catalog data\n" if $found;


-# Oids not specified in the input files are automatically assigned,
+# OIDs not specified in the input files are automatically assigned,
 # starting at FirstGenbkiObjectId, extending up to FirstBootstrapObjectId.
+# We allow such OIDs to be assigned independently within each catalog.
 my $FirstGenbkiObjectId =
   Catalog::FindDefinedSymbol('access/transam.h', $include_path,
     'FirstGenbkiObjectId');
 my $FirstBootstrapObjectId =
   Catalog::FindDefinedSymbol('access/transam.h', $include_path,
     'FirstBootstrapObjectId');
-my $GenbkiNextOid = $FirstGenbkiObjectId;
+# Hash of next available OID, indexed by catalog name.
+my %GenbkiNextOids;


 # Fetch some special data that we will substitute into the output file.
@@ -563,8 +565,7 @@ EOM
             # Assign oid if oid column exists and no explicit assignment in row
             if ($attname eq "oid" and not defined $bki_values{$attname})
             {
-                $bki_values{$attname} = $GenbkiNextOid;
-                $GenbkiNextOid++;
+                $bki_values{$attname} = assign_next_oid($catname);
             }

             # Replace OID synonyms with OIDs per the appropriate lookup rule.
@@ -669,11 +670,6 @@ foreach my $declaration (@index_decls)
 # last command in the BKI file: build the indexes declared above
 print $bki "build indices\n";

-# check that we didn't overrun available OIDs
-die
-  "genbki OID counter reached $GenbkiNextOid, overrunning FirstBootstrapObjectId\n"
-  if $GenbkiNextOid > $FirstBootstrapObjectId;
-
 # Now generate system_constraints.sql

 foreach my $c (@system_constraints)
@@ -1081,6 +1077,25 @@ sub form_pg_type_symbol
     return $name . $arraystr . 'OID';
 }

+# Assign an unused OID within the specified catalog.
+sub assign_next_oid
+{
+    my $catname = shift;
+
+    # Initialize, if no previous request for this catalog.
+    $GenbkiNextOids{$catname} = $FirstGenbkiObjectId
+      if !defined($GenbkiNextOids{$catname});
+
+    my $result = $GenbkiNextOids{$catname}++;
+
+    # Check that we didn't overrun available OIDs
+    die
+      "genbki OID counter for $catname reached $result, overrunning FirstBootstrapObjectId\n"
+      if $result >= $FirstBootstrapObjectId;
+
+    return $result;
+}
+
 sub usage
 {
     die <<EOM;
diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index 05c6fbffe4..2fe8a59110 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -161,18 +161,20 @@ FullTransactionIdAdvance(FullTransactionId *dest)
  *        development purposes (such as in-progress patches and forks);
  *        they should not appear in released versions.
  *
- *        OIDs 10000-12999 are reserved for assignment by genbki.pl, for use
+ *        OIDs 10000-11999 are reserved for assignment by genbki.pl, for use
  *        when the .dat files in src/include/catalog/ do not specify an OID
- *        for a catalog entry that requires one.
+ *        for a catalog entry that requires one.  Note that genbki.pl assigns
+ *        these OIDs independently in each catalog, so they're not guaranteed
+ *        to be globally unique.
  *
- *        OIDS 13000-16383 are reserved for assignment during initdb
- *        using the OID generator.  (We start the generator at 13000.)
+ *        OIDS 12000-16383 are reserved for assignment during initdb
+ *        using the OID generator.  (We start the generator at 12000.)
  *
  *        OIDs beginning at 16384 are assigned from the OID generator
  *        during normal multiuser operation.  (We force the generator up to
  *        16384 as soon as we are in normal operation.)
  *
- * The choices of 8000, 10000 and 13000 are completely arbitrary, and can be
+ * The choices of 8000, 10000 and 12000 are completely arbitrary, and can be
  * moved if we run low on OIDs in any category.  Changing the macros below,
  * and updating relevant documentation (see bki.sgml and RELEASE_CHANGES),
  * should be sufficient to do this.  Moving the 16384 boundary between
@@ -186,7 +188,7 @@ FullTransactionIdAdvance(FullTransactionId *dest)
  * ----------
  */
 #define FirstGenbkiObjectId        10000
-#define FirstBootstrapObjectId    13000
+#define FirstBootstrapObjectId    12000
 #define FirstNormalObjectId        16384

 /*