Do not generate diffs for slots with identical content

When the old and new revisions have identical content in a given slot,
do not include the relevant SlotDiffRenderer in the return value of
DifferenceEngine::getSlotDiffRenderers(), so that the relevant table
prefixes and modules will be omitted.

Also:

* In TextSlotDiffRenderer::getTextDiff(), return an empty string when
  the inputs are equal, instead of invoking the diff engine. Previously,
  the inline format would produce a <tr> element in this case, causing
  the diff-empty message to be omitted.
* Clarify the doc comment up the stack, indicating SlotDiffRenderer may
  return zero <tr> tags, it doesn't have to be one or more.
* Fix the documented return type of getSlotContents() so that @var is
  not needed in the caller.
* Don't convert the return value of SlotDiffRenderer::getDiff() to
  boolean. It's always a string so compare it to the empty string.

Note that the cache key for inline empty diffs will change, because
TextSlotDiffRenderer::getExtraCacheKeys() is not called anymore for
empty diffs.

Bug: T338670
Change-Id: I0774ba0b159ac43ec214403cf2d06740f6d067cd

1 files changed, 5 insertions, 1 deletions

diff --git a/includes/diff/TextSlotDiffRenderer.php b/includes/diff/TextSlotDiffRenderer.php
index bc526956e5b6..5f0bfed882fa 100644
--- a/includes/diff/TextSlotDiffRenderer.php
+++ b/includes/diff/TextSlotDiffRenderer.php
@@ -208,7 +208,7 @@ class TextSlotDiffRenderer extends SlotDiffRenderer {
 	 * Diff the text representations of two content objects (or just two pieces of text in general).
 	 * @param string $oldText
 	 * @param string $newText
-	 * @return string HTML, one or more <tr> tags.
+	 * @return string HTML. One or more <tr> tags, or an empty string if the inputs are identical.
 	 */
 	public function getTextDiff( string $oldText, string $newText ) {
 		$diff = function () use ( $oldText, $newText ) {
@@ -269,6 +269,10 @@ class TextSlotDiffRenderer extends SlotDiffRenderer {
 		$oldText = str_replace( "\r\n", "\n", $oldText );
 		$newText = str_replace( "\r\n", "\n", $newText );
 
+		if ( $oldText === $newText ) {
+			return '';
+		}
+
 		// Better external diff engine, the 2 may some day be dropped
 		// This one does the escaping and segmenting itself
 		if ( $this->engine === self::ENGINE_WIKIDIFF2 ) {