Add some error cross-checks to gen_node_support.pl.

Check that if we generate a call to copy, compare, write, or read
a specific node type, that node type does have the appropriate
support function.  (This doesn't protect against trying to invoke
nonexistent code when considering generic field types such as
"Node *", but it seems like a useful check anyway.)

Check that array_size() refers to a field appearing earlier in
the struct.  Aside from catching obvious errors like a misspelled
field name, this protects against a more subtle mistake: if the
size field appears later in the struct than the array field, then
compare and read functions would misbehave.  There is actually
exactly that situation in PlannerInfo, but it's okay since we
do not need compare or read functionality for that (today anyway).

Discussion: https://postgr.es/m/263413.1669513145@sss.pgh.pa.us

src/backend/nodes/gen_node_support.pl

@@ -123,6 +123,8 @@ my @no_equal;
 my @no_read;
 # node types we don't want read/write support for
 my @no_read_write;
+# node types that have handmade read/write support
+my @special_read_write;
 # node types we don't want any support functions for, just node tags
 my @nodetag_only;
 
@@ -152,9 +154,12 @@ my @extra_tags = qw(
 # since we won't use its internal structure here anyway.
 push @node_types, qw(List);
 # Lists are specially treated in all four support files, too.
-push @no_copy,       qw(List);
-push @no_equal,      qw(List);
-push @no_read_write, qw(List);
+# (Ideally we'd mark List as "special copy/equal" not "no copy/equal".
+# But until there's other use-cases for that, just hot-wire the tests
+# that would need to distinguish.)
+push @no_copy,            qw(List);
+push @no_equal,           qw(List);
+push @special_read_write, qw(List);
 
 # Nodes with custom copy/equal implementations are skipped from
 # .funcs.c but need case statements in .switch.c.
@@ -338,16 +343,7 @@ foreach my $infile (@ARGV)
 						}
 						elsif ($attr eq 'special_read_write')
 						{
-							# This attribute is called
-							# "special_read_write" because there is
-							# special treatment in outNode() and
-							# nodeRead() for these nodes.  For this
-							# script, it's the same as
-							# "no_read_write", but calling the
-							# attribute that externally would probably
-							# be confusing, since read/write support
-							# does in fact exist.
-							push @no_read_write, $in_struct;
+							push @special_read_write, $in_struct;
 						}
 						elsif ($attr =~ /^nodetag_number\((\d+)\)$/)
 						{
@@ -681,6 +677,9 @@ _equal${n}(const $n *a, const $n *b)
 {
 " unless $struct_no_equal;
 
+	# track already-processed fields to support field order checks
+	my %previous_fields;
+
 	# print instructions for each field
 	foreach my $f (@{ $node_type_info{$n}->{fields} })
 	{
@@ -697,6 +696,10 @@ _equal${n}(const $n *a, const $n *b)
 			if ($a =~ /^array_size\(([\w.]+)\)$/)
 			{
 				$array_size_field = $1;
+				# insist that we copy or compare the array size first!
+				die
+				  "array size field $array_size_field for field $n.$f must precede $f\n"
+				  if (!$previous_fields{$array_size_field});
 			}
 			elsif ($a =~ /^copy_as\(([\w.]+)\)$/)
 			{
@@ -786,6 +789,17 @@ _equal${n}(const $n *a, const $n *b)
 		elsif (($t =~ /^(\w+)\*$/ or $t =~ /^struct\s+(\w+)\*$/)
 			and elem $1, @node_types)
 		{
+			die
+			  "node type \"$1\" lacks copy support, which is required for struct \"$n\" field \"$f\"\n"
+			  if (elem $1, @no_copy or elem $1, @nodetag_only)
+			  and $1 ne 'List'
+			  and !$copy_ignore;
+			die
+			  "node type \"$1\" lacks equal support, which is required for struct \"$n\" field \"$f\"\n"
+			  if (elem $1, @no_equal or elem $1, @nodetag_only)
+			  and $1 ne 'List'
+			  and !$equal_ignore;
+
 			print $cff "\tCOPY_NODE_FIELD($f);\n"    unless $copy_ignore;
 			print $eff "\tCOMPARE_NODE_FIELD($f);\n" unless $equal_ignore;
 		}
@@ -809,6 +823,8 @@ _equal${n}(const $n *a, const $n *b)
 			die
 			  "could not handle type \"$t\" in struct \"$n\" field \"$f\"\n";
 		}
+
+		$previous_fields{$f} = 1;
 	}
 
 	print $cff "
@@ -851,6 +867,7 @@ foreach my $n (@node_types)
 	next if elem $n, @abstract_types;
 	next if elem $n, @nodetag_only;
 	next if elem $n, @no_read_write;
+	next if elem $n, @special_read_write;
 
 	my $no_read = (elem $n, @no_read);
 
@@ -891,6 +908,11 @@ _read${n}(void)
 ";
 	}
 
+	# track already-processed fields to support field order checks
+	# (this isn't quite redundant with the previous loop, since
+	# we may be considering structs that lack copy/equal support)
+	my %previous_fields;
+
 	# print instructions for each field
 	foreach my $f (@{ $node_type_info{$n}->{fields} })
 	{
@@ -906,6 +928,10 @@ _read${n}(void)
 			if ($a =~ /^array_size\(([\w.]+)\)$/)
 			{
 				$array_size_field = $1;
+				# insist that we read the array size first!
+				die
+				  "array size field $array_size_field for field $n.$f must precede $f\n"
+				  if (!$previous_fields{$array_size_field} && !$no_read);
 			}
 			elsif ($a =~ /^read_as\(([\w.]+)\)$/)
 			{
@@ -1083,6 +1109,14 @@ _read${n}(void)
 		elsif (($t =~ /^(\w+)\*$/ or $t =~ /^struct\s+(\w+)\*$/)
 			and elem $1, @node_types)
 		{
+			die
+			  "node type \"$1\" lacks write support, which is required for struct \"$n\" field \"$f\"\n"
+			  if (elem $1, @no_read_write or elem $1, @nodetag_only);
+			die
+			  "node type \"$1\" lacks read support, which is required for struct \"$n\" field \"$f\"\n"
+			  if (elem $1, @no_read or elem $1, @nodetag_only)
+			  and !$no_read;
+
 			print $off "\tWRITE_NODE_FIELD($f);\n";
 			print $rff "\tREAD_NODE_FIELD($f);\n" unless $no_read;
 		}
@@ -1145,6 +1179,8 @@ _read${n}(void)
 		{
 			print $rff "\tlocal_node->$f = $read_as_field;\n" unless $no_read;
 		}
+
+		$previous_fields{$f} = 1;
 	}
 
 	print $off "}