UsdUtilsExtractExternalReferences() no longer relies on mutating

list-ops (via ModifyItemEdits) to collect layer deps.

Fixes PixarAnimationStudios#1573

(Internal change: 2180649)

pxr/usd/usdUtils/dependencies.cpp

@@ -338,10 +338,16 @@ _FileAnalyzer::_RemapRefOrPayload(const RefOrPayloadType &refOrPayload)
 void
 _FileAnalyzer::_ProcessPayloads(const SdfPrimSpecHandle &primSpec)
 {
-    SdfPayloadsProxy payloadList = primSpec->GetPayloadList();
-    payloadList.ModifyItemEdits(std::bind(
-        &_FileAnalyzer::_RemapRefOrPayload<SdfPayload, _DepType::Payload>, 
-        this, std::placeholders::_1));
+    if (_remapPathFunc) {
+        primSpec->GetPayloadList().ModifyItemEdits(std::bind(
+            &_FileAnalyzer::_RemapRefOrPayload<SdfPayload, _DepType::Payload>, 
+            this, std::placeholders::_1));
+    } else {
+        for (SdfPayload const& payload:
+             primSpec->GetPayloadList().GetAddedOrExplicitItems()) {
+            _ProcessDependency(payload.GetAssetPath(), _DepType::Payload);
+        }
+    }
 }
 
 void
@@ -529,10 +535,16 @@ _FileAnalyzer::_ProcessMetadata(const SdfPrimSpecHandle &primSpec)
 void
 _FileAnalyzer::_ProcessReferences(const SdfPrimSpecHandle &primSpec)
 {
-    SdfReferencesProxy refList = primSpec->GetReferenceList();
-    refList.ModifyItemEdits(std::bind(
-        &_FileAnalyzer::_RemapRefOrPayload<SdfReference, _DepType::Reference>, 
-        this, std::placeholders::_1));
+    if (_remapPathFunc) {
+        primSpec->GetReferenceList().ModifyItemEdits(std::bind(
+            &_FileAnalyzer::_RemapRefOrPayload<SdfReference,
+            _DepType::Reference>, this, std::placeholders::_1));
+    } else {
+        for (SdfReference const& reference:
+            primSpec->GetReferenceList().GetAddedOrExplicitItems()) {
+            _ProcessDependency(reference.GetAssetPath(), _DepType::Reference);
+        }
+    }
 }
 
 void

pxr/usd/usdUtils/testenv/testUsdUtilsDependencyExtractor.py

@@ -22,7 +22,7 @@
 # KIND, either express or implied. See the Apache License for the specific
 # language governing permissions and limitations under the Apache License.
 from __future__ import print_function
-from pxr import UsdUtils
+from pxr import UsdUtils, Sdf
 import argparse, contextlib, sys, os
 
 @contextlib.contextmanager
@@ -50,6 +50,11 @@ def presult(ostr, fileName, refType, refs):
         print('Error: cannot access file {}'.format(args.infile), file=sys.stderr)
         sys.exit(1)
 
+    # Open layer and turn off edit permission, to validate that the dependency
+    # extractor does not require modifying the layer
+    layer = Sdf.Layer.FindOrOpen(args.infile)
+    layer.SetPermissionToEdit(False)
+
     sublayers, references, payloads = \
         UsdUtils.ExtractExternalReferences(args.infile)
     with stream(args.outfile, 'w') as ofp: