Clarify handling of special-case values in bootstrap catalog data.

I (tgl) originally coded the special case for pg_proc.pronargs as
though it were a kind of default value.  It seems better though to
treat computable columns as an independent concern: this makes the
code clearer, and probably a bit faster too since we needn't do
work inside the per-column loop.

Improve related comments, as well, in the expectation that there
might be more cases like this in future.

John Naylor, some additional comment-hacking by me

Discussion: https://postgr.es/m/CAJVSVGW-D7OobzU=dybVT2JqZAx-4X1yvBJdavBmqQL05Q6CLw@mail.gmail.com

src/backend/catalog/Catalog.pm

@@ -292,6 +292,21 @@ sub AddDefaultValues
 	my ($row, $schema, $catname) = @_;
 	my @missing_fields;
 
+	# Compute special-case column values.
+	# Note: If you add new cases here, you must also teach
+	# strip_default_values() in include/catalog/reformat_dat_file.pl
+	# to delete them.
+	if ($catname eq 'pg_proc')
+	{
+		# pg_proc.pronargs can be derived from proargtypes.
+		if (defined $row->{proargtypes})
+		{
+			my @proargtypes = split /\s+/, $row->{proargtypes};
+			$row->{pronargs} = scalar(@proargtypes);
+		}
+	}
+
+	# Now fill in defaults, and note any columns that remain undefined.
 	foreach my $column (@$schema)
 	{
 		my $attname = $column->{name};
@@ -305,14 +320,6 @@ sub AddDefaultValues
 		{
 			$row->{$attname} = $column->{default};
 		}
-		elsif ($catname eq 'pg_proc'
-			&& $attname eq 'pronargs'
-			&& defined($row->{proargtypes}))
-		{
-			# pg_proc.pronargs can be derived from proargtypes.
-			my @proargtypes = split /\s+/, $row->{proargtypes};
-			$row->{$attname} = scalar(@proargtypes);
-		}
 		else
 		{
 			# Failed to find a value.
@@ -320,6 +327,7 @@ sub AddDefaultValues
 		}
 	}
 
+	# Failure to provide all columns is a hard error.
 	if (@missing_fields)
 	{
 		die sprintf "missing values for field(s) %s in %s.dat line %s\n",

src/include/catalog/pg_proc.dat

@@ -30,6 +30,9 @@
 # "convert srctypename to desttypename" for cast functions
 # "less-equal-greater" for B-tree comparison functions
 
+# Note: pronargs is computed when this file is read, so it does not need
+# to be specified in entries here.  See AddDefaultValues() in Catalog.pm.
+
 # Once upon a time these entries were ordered by OID.  Lately it's often
 # been the custom to insert new entries adjacent to related older entries.
 # Try to do one or the other though, don't just insert entries at random.

src/include/catalog/reformat_dat_file.pl

@@ -177,31 +177,31 @@ foreach my $catname (@catnames)
 	close $dat;
 }
 
-# Leave values out if there is a matching default.
+# Remove column values for which there is a matching default,
+# or if the value can be computed from other columns.
 sub strip_default_values
 {
 	my ($row, $schema, $catname) = @_;
 
+	# Delete values that match defaults.
 	foreach my $column (@$schema)
 	{
 		my $attname = $column->{name};
 		die "strip_default_values: $catname.$attname undefined\n"
 		  if !defined $row->{$attname};
 
-		# Delete values that match defaults.
 		if (defined $column->{default}
 			and ($row->{$attname} eq $column->{default}))
 		{
 			delete $row->{$attname};
 		}
+	}
 
-		# Also delete pg_proc.pronargs, since that can be recomputed.
-		if (   $catname eq 'pg_proc'
-			&& $attname eq 'pronargs'
-			&& defined($row->{proargtypes}))
-		{
-			delete $row->{$attname};
-		}
+	# Delete computed values.  See AddDefaultValues() in Catalog.pm.
+	# Note: This must be done after deleting values matching defaults.
+	if ($catname eq 'pg_proc')
+	{
+		delete $row->{pronargs} if defined $row->{proargtypes};
 	}
 }