[RFC PATCH 2/9] Add pass to drop empty XML elements

[Giuliano Procida <gprocida@google.com>]

Certain elements in ABI XML are effectvely containers and can be
dropped if empty and their attributes don't carry ABI information.

- elf-variable-symbols: pure container
- elf-function-symbols: pure container
- namespace-decl: has a name
- abi-instr: compilation unit (path etc.)
- abi-corpus: binary object (architecture)
- abi-corpus-group: binary objects (architecture)

It could be argued that abi-corpus (or abi-corpus-group) should be
kept around to hold the architecture of an object or set of objects.
However, if a binary object has no symbols (say, if it is empty), it
hardly matters what the architecture is.

Note that:

- abidiff rejects XML files with an XML declaration at the top
- abidiff rejects completely empty files

Resolving the first would make the second moot. In the meantime, we
avoid dropping top-level elements.

	* scripts/abitidy.pl (drop_if_empty): New variable containing
	the tags of elements that can be dropped if empty.
	(drop_empty): New Function that removes empty elements, except
	top-level ones.

Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 scripts/abitidy.pl | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/scripts/abitidy.pl b/scripts/abitidy.pl
index 66d636d7..1f74e267 100755
--- a/scripts/abitidy.pl
+++ b/scripts/abitidy.pl
@@ -105,20 +105,58 @@ sub indent($indent, $node) {
   }
 }
 
+# Remove an XML element and any preceeding comment.
+sub remove_node($node) {
+  my $prev = $node->previousSibling();
+  if ($prev && $prev->nodeType == XML_COMMENT_NODE) {
+    $prev->unbindNode();
+  }
+  $node->unbindNode();
+}
+
+# These container elements can be dropped if empty.
+my %drop_if_empty = map { $_ => undef } qw(
+  elf-variable-symbols
+  elf-function-symbols
+  namespace-decl
+  abi-instr
+  abi-corpus
+  abi-corpus-group
+);
+
+# This is a XML DOM traversal as we want post-order traversal so we
+# delete nodes that become empty during the process.
+sub drop_empty;
+sub drop_empty($node) {
+  my $node_name = $node->getName();
+  for my $child ($node->childNodes()) {
+    drop_empty($child);
+  }
+  if (!$node->hasChildNodes() && $node->nodeType == XML_ELEMENT_NODE && exists $drop_if_empty{$node->getName()}) {
+    # Until abidiff accepts empty ABIs, avoid dropping top-level elements.
+    if ($node->parentNode->nodeType == XML_ELEMENT_NODE) {
+      remove_node($node);
+    }
+  }
+}
+
 # Parse arguments.
 my $input_opt;
 my $output_opt;
 my $all_opt;
+my $drop_opt;
 GetOptions('i|input=s' => \$input_opt,
            'o|output=s' => \$output_opt,
            'a|all' => sub {
-             1
+             $drop_opt = 1
            },
+           'd|drop-empty!' => \$drop_opt,
   ) and !@ARGV or die("usage: $0",
                       map { (' ', $_) } (
                         '[-i|--input file]',
                         '[-o|--output file]',
                         '[-a|--all]',
+                        '[-d|--[no-]drop-empty]',
                       ), "\n");
 
 exit 0 unless defined $input_opt;
@@ -131,6 +169,9 @@ close $input;
 # This simplifies DOM analysis and manipulation.
 strip_text($dom);
 
+# Drop empty elements.
+drop_empty($dom) if $drop_opt;
+
 exit 0 unless defined $output_opt;
 
 # Reformat for human consumption.
-- 
2.31.0.291.g576ba9dcdaf-goog