pcp: Fix simplified specializes regression with variants

In certain cases where a specializes arc was introduced inside
a variant, the process of adding implied specializes arcs up to
the root of the prim index would be cut short causing valid
opinions to be omitted. This worked correctly prior to the
introduction of simplied specializes.

For example, consider the case where the variant node at
/A/B{x=y} introduces a specializes arc to /A/C. That arc would
be immediately propagated to the root of the graph and have
other composition tasks enqueued -- particularly the implied
class task. Pcp would try to add the corresponding implied arc
to the parent of the variant node at /A/B. The computed implied
specializes path would be /A/C, so an inert placeholder node would
be added at the path and would then be immediately propagated to
the root. However, since there was already a propagated specializes
arc at /A/C, no node would be added and no tasks enqueued, which
would cause the implied class process to stop.

This change detects this situation and ensures an implied class
task is always enqueued to ensure the process continues. While
this fixes the reported regression, it introduces a separate
regression in an internal test. This second regression involves
a much more complicated composition structure that we think is
less likely, so I'm landing this fix first to address the more
common case for the 25.08 release. This change will be reverted
afterwards while work on a more complete fix is underway.

See #3750

(Internal change: 2374321)

Author: sunyab

pxr/usd/pcp/CMakeLists.txt

@@ -592,6 +592,11 @@ pxr_install_test_dir(
     DEST testPcpMuseum_SpecializesAndVariants3
 )
 
+pxr_install_test_dir(
+    SRC testenv/testPcpMuseum_SpecializesAndVariants4.testenv
+    DEST testPcpMuseum_SpecializesAndVariants4
+)
+
 pxr_install_test_dir(
     SRC testenv/testPcpMuseum_SubrootInheritsAndVariants.testenv
     DEST testPcpMuseum_SubrootInheritsAndVariants
@@ -1431,6 +1436,13 @@ pxr_register_test(testPcpMuseum_SpecializesAndVariants3
     DIFF_COMPARE compositionResults_SpecializesAndVariants3.txt 
 )
 
+pxr_register_test(testPcpMuseum_SpecializesAndVariants4
+    PYTHON
+    COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testPcpCompositionResults --usd SpecializesAndVariants4/root.usda" 
+    STDOUT_REDIRECT compositionResults_SpecializesAndVariants4.txt
+    DIFF_COMPARE compositionResults_SpecializesAndVariants4.txt 
+)
+
 pxr_register_test(testPcpMuseum_SubrootInheritsAndVariants
     PYTHON
     COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testPcpCompositionResults --usd SubrootInheritsAndVariants/root.usda" 

pxr/usd/pcp/primIndex.cpp

@@ -3267,11 +3267,29 @@ _AddClassBasedArc(
             // EvalImpliedSpecializes task.
             // 
             // If we're _not_ in a recursive prim indexing call we can
-            // immediately do the propagation to avoid going the task
+            // immediately do the propagation to avoid paying the task
             // overhead. See also _FindSpecializesToPropagateToRoot.
             if (!indexer->previousFrame && placeholder &&
                 !_IsRelocatesPlaceholderImpliedArc(placeholder)) {
-                return _PropagateNodeToRoot(placeholder, indexer);
+
+                PcpNodeRef propagatedNode = 
+                    _PropagateNodeToRoot(placeholder, indexer);
+
+                // If a new node was created to propagate the placeholder
+                // to the root, tasks will have been enqueued to continue
+                // implying the class to the root of the prim index.
+                //
+                // If a pre-existing node was found instead (i.e., the 
+                // returned node's origin isn't the placeholder), we need
+                // to manually enqueue tasks on the placeholder to continue
+                // that process.
+                if (propagatedNode && 
+                    propagatedNode.GetOriginNode() != placeholder) {
+                    indexer->AddTasksForNode(
+                        placeholder, Task::EvalImpliedClasses);
+                }
+
+                return propagatedNode;
             }
 
             return placeholder;

pxr/usd/pcp/testenv/testPcpMuseum_SpecializesAndVariants4.testenv/SpecializesAndVariants4/root.usda

@@ -0,0 +1,47 @@
+#usda 1.0
+(
+    """
+    Museum case showing combinations of specializes and variants.
+
+    In this example, when composing /A/render, a specializes arc to
+    /_class_/defaultImplementation is introduced within a variant.
+    That specializes arc is propagated to the root of the graph and
+    must also add implied specializes arcs to its parent nodes;
+    in particular, there should be an implied specializes arc added
+    for /A/defaultImplementation.
+    """
+)
+
+class "_class_"
+{
+    class "defaultImplementation"
+    {
+        uniform string test = "value from /_class_/defaultImplementation"
+    }
+
+    def "render" (
+        variants = {
+            string implementation = "default"
+        }
+        prepend variantSets = "implementation"
+    )
+    {
+        variantSet "implementation" = {
+            "default" (
+                prepend specializes = </_class_/defaultImplementation>
+            ) {
+
+            }
+        }
+    }
+}
+
+def "A"  (
+    prepend specializes = </_class_>
+)
+{
+    over "defaultImplementation"
+    {
+        uniform string test = "value from /A/defaultImplementation"
+    }
+}

pxr/usd/pcp/testenv/testPcpMuseum_SpecializesAndVariants4.testenv/baseline/compositionResults_SpecializesAndVariants4.txt

@@ -0,0 +1,91 @@
+Loading @SpecializesAndVariants4/root.usda@
+
+------------------------------------------------------------------------
+Layer Stack:
+     root.usda
+
+------------------------------------------------------------------------
+Results for composing </_class_>
+
+Prim Stack:
+    root.usda            /_class_
+
+Child names:
+     ['defaultImplementation', 'render']
+
+------------------------------------------------------------------------
+Results for composing </_class_/defaultImplementation>
+
+Prim Stack:
+    root.usda            /_class_/defaultImplementation
+
+Property names:
+     ['test']
+
+Property stacks:
+/_class_/defaultImplementation.test:
+    root.usda            /_class_/defaultImplementation.test
+
+------------------------------------------------------------------------
+Results for composing </_class_/render>
+
+Prim Stack:
+    root.usda            /_class_/render
+    root.usda            /_class_/render{implementation=default}
+    root.usda            /_class_/defaultImplementation
+
+Variant Selections:
+    {implementation = default}
+
+Property names:
+     ['test']
+
+Property stacks:
+/_class_/render.test:
+    root.usda            /_class_/defaultImplementation.test
+
+------------------------------------------------------------------------
+Results for composing </A>
+
+Prim Stack:
+    root.usda            /A
+    root.usda            /_class_
+
+Child names:
+     ['defaultImplementation', 'render']
+
+------------------------------------------------------------------------
+Results for composing </A/defaultImplementation>
+
+Prim Stack:
+    root.usda            /A/defaultImplementation
+    root.usda            /_class_/defaultImplementation
+
+Property names:
+     ['test']
+
+Property stacks:
+/A/defaultImplementation.test:
+    root.usda            /A/defaultImplementation.test
+    root.usda            /_class_/defaultImplementation.test
+
+------------------------------------------------------------------------
+Results for composing </A/render>
+
+Prim Stack:
+    root.usda            /_class_/render
+    root.usda            /_class_/render{implementation=default}
+    root.usda            /A/defaultImplementation
+    root.usda            /_class_/defaultImplementation
+
+Variant Selections:
+    {implementation = default}
+
+Property names:
+     ['test']
+
+Property stacks:
+/A/render.test:
+    root.usda            /A/defaultImplementation.test
+    root.usda            /_class_/defaultImplementation.test
+