Improve Alembic plugin to read assets with ArResolver to support othe…

…r sources except local files

Alembic plugin only accepts assets that are from local paths. In order to fix this, we need to use ArResolver/ArAsset to read/write contents so customized resolver can read assets
from other locations. However, interfaces of Alembic library have limitations that can only accept local urls or `std::istream`. This PR uses the following solution:

Open assets with `ArResolver`. Instead of reading them into the memory, we get the mapped file path with
[ArchGetFileName](https://github.com/PixarAnimationStudios/OpenUSD/blob/59992d21/pxr/base/arch/fileSystem.h#L177) from `ArAsset::GetFileUnsafe`.

Fixes #2961
Closes #3302

(Internal change: 2353336)

pxr/usd/plugin/usdAbc/alembicReader.cpp

@@ -7,18 +7,23 @@
 /// \file alembicReader.cpp
 
 #include "pxr/pxr.h"
+#include "pxr/usd/ar/asset.h"
+#include "pxr/usd/ar/resolvedPath.h"
+#include "pxr/usd/ar/resolver.h"
 #include "pxr/usd/plugin/usdAbc/alembicReader.h"
 #include "pxr/usd/plugin/usdAbc/alembicUtil.h"
 #include "pxr/usd/usdGeom/hermiteCurves.h"
 #include "pxr/usd/usdGeom/tokens.h"
 #include "pxr/usd/usdGeom/xformable.h"
 #include "pxr/usd/sdf/schema.h"
+#include "pxr/base/arch/fileSystem.h"
 #include "pxr/base/work/threadLimits.h"
 #include "pxr/base/trace/trace.h"
 #include "pxr/base/tf/envSetting.h"
 #include "pxr/base/tf/staticData.h"
 #include "pxr/base/tf/staticTokens.h"
 #include "pxr/base/tf/ostreamMethods.h"
+#include "pxr/base/tf/fileUtils.h"
 #include <Alembic/Abc/ArchiveInfo.h>
 #include <Alembic/Abc/IArchive.h>
 #include <Alembic/Abc/IObject.h>
@@ -801,6 +806,8 @@ class _ReaderContext {
                    const ISampleSelector& selector,
                    const UsdAbc_AlembicDataAny& value) const;
 
+    std::string _OpenAndGetMappedFilePath(const std::string& filePath);
+
     // Custom auto-lock that safely ignores a NULL pointer.
     class _Lock {
     public:
@@ -853,6 +860,10 @@ class _ReaderContext {
     _PrimMap _prims;
     Prim* _pseudoRoot;
     UsdAbc_TimeSamples _allTimeSamples;
+
+    // Asset holders to keep a reference alive so that resolver doesn't 
+    // attempt to cleanup asset until the asset is no longer in use.
+    std::vector<std::shared_ptr<ArAsset>> _assetHolders;
 };
 
 static
@@ -872,6 +883,36 @@ _ReaderContext::_ReaderContext() :
     // Do nothing
 }
 
+std::string
+_ReaderContext::_OpenAndGetMappedFilePath(const std::string& filePath)
+{
+    // Return as it is if it's a local path or symlink.
+    if (!TfIsFile(filePath, true))
+    {
+        // Open asset with Ar to support URLs other than local paths.
+        std::shared_ptr<ArAsset> asset =
+            ArGetResolver().OpenAsset(ArResolvedPath(filePath));
+        if (asset)
+        {
+            FILE* fileHandle; size_t fileOffset;
+            std::tie(fileHandle, fileOffset) = asset->GetFileUnsafe();
+            // Check file offset also to ensure that the .abc file
+            // we're looking at isn't embedded in a .usdz or other package
+            if (fileHandle && fileOffset == 0)
+            {
+                // If file handle is presented, use mapped file path instead of original one.
+                const std::string mappedFilePath = ArchGetFileName(fileHandle);
+                _assetHolders.emplace_back(std::move(asset));
+
+                return mappedFilePath;
+            }
+        }
+    }
+
+    // Otherwise, fallback to original asset path.
+    return filePath;
+}
+
 bool
 _ReaderContext::Open(const std::string& filePath, std::string* errorLog,
                      const SdfFileFormat::FileFormatArguments& args)
@@ -883,11 +924,11 @@ _ReaderContext::Open(const std::string& filePath, std::string* errorLog,
         auto abcLayers = args.find("abcLayers");
         if (abcLayers != args.end()) {
             for (auto&& l : TfStringSplit(abcLayers->second, ",")) {
-                layeredABC.emplace_back(std::move(l));
+                layeredABC.emplace_back(_OpenAndGetMappedFilePath(l));
             }
         }
     }
-    layeredABC.emplace_back(filePath);
+    layeredABC.emplace_back(_OpenAndGetMappedFilePath(filePath));
 
 #if PXR_HDF5_SUPPORT_ENABLED && !H5_HAVE_THREADSAFE
     // HDF5 may not be thread-safe.
@@ -1507,6 +1548,7 @@ _ReaderContext::_Clear()
     _allTimeSamples.clear();
     _instanceSources.clear();
     _instances.clear();
+    _assetHolders.clear();
 }
 
 const _ReaderContext::Prim*