ParseXS: allow XS files with no XS again

GH #24016

ParseXS / xsubpp allowed .xs files to contain only C code; i.e. no XS
code. A warning was issued about about a missing MODULE line, but the C
contents of the file was emitted as-is.

My recent refactoring broke this - it was now generating a C code file
with zero lines. This was because ParseXS now splits the parsing and
code-emitting into two separate phases, and this code during parsing:

     warn "Didn't find a 'MODULE ... PACKAGE ... PREFIX' line\n";
     exit 0; # Not a fatal error for the caller process

was now exiting before any lines had been emitted.

The fix is to not suddenly exit, but instead continue with pared-down
parsing and code emitting.

Author: iabyn

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS/Node.pm

@@ -603,10 +603,14 @@ sub parse {
 
     my $C_part = ExtUtils::ParseXS::Node::C_part->new();
     $self->{C_part} = $C_part;
-    $C_part->parse($pxs, $self)
-        or return;
+    my $c_part_result = $C_part->parse($pxs, $self);
     push @{$self->{kids}}, $C_part;
 
+    # A failure when parsing the C part means that there wasn't a MODULE
+    # line. Don't try to parse the missing XS part, but still return
+    # success, passing through the lines from the C part.
+    return 1 unless $c_part_result;
+
     # "Parse" the bit following any C code. Doesn't actually consume any
     # lines: just a placeholder for emitting postamble code.
 
@@ -752,7 +756,7 @@ sub parse {
     }
 
     warn "Didn't find a 'MODULE ... PACKAGE ... PREFIX' line\n";
-    exit 0; # Not a fatal error for the caller process
+    return;
 }
 
 

dist/ExtUtils-ParseXS/t/001-basic.t

@@ -5879,4 +5879,28 @@ EOF
     test_many($preamble, undef, \@test_fns);
 }
 
+{
+    # An XS file without a MODULE line should warn, but
+    # still emit the C code in the C part of the file (the whole file
+    # contents in this case).
+
+    my $preamble = '';
+
+    my @test_fns = (
+        [
+            "No MODULE line",
+            [ Q(<<'EOF') ],
+                |foo
+                |bar
+EOF
+
+            [ 0, 0, qr{#line 1 ".*"\nfoo\nbar\n#line 13 ".*"}, "all C present" ],
+            [ 1, 0, qr{Didn't find a 'MODULE ... PACKAGE ... PREFIX' line},
+                    "got expected MODULE warning"  ],
+        ],
+    );
+
+    test_many($preamble, undef, \@test_fns);
+}
+
 done_testing;