{CUDA Decoding} Restore SW decoder fallback in xprsDecoderMaker

Summary:
A wheel built with `ENABLE_NVCODEC=ON` segfaults on any machine without an NVIDIA driver. The GitHub CI "Test wheel" job crashes with exit 139 after:

    Cannot load libcuda.so.1
    [XPRS][WARNING]: Loading CUDA functions failed
    [DecoderFactory][WARNING]: Could not create a decoder for 'H.265'!

With no driver, `getNvCodecContext()` throws and `enumDecoders()` catches it, returning a non-OK result -- but the SW `hevc`/`h264` decoders were already collected before the throw, so the list is usable. `xprsDecoderMaker` bailed on the non-OK result and discarded that list, so no decoder was created and the null decoder was dereferenced downstream.

Three changes:
- Ignore the `enumDecoders()` result and fail only when the decoder list is genuinely empty. This is what fixes the crash.
- On decoder `init()` failure, continue to the next decoder instead of returning, so HW->SW fallback works.
- Skip HW decoders for monochrome streams, which NVDEC cannot decode.

These changes were reviewed as part of D103253728 and D110227872 but never reached master: the landed commit for D103253728 (`a4556645655d`) contains only its four `arvr/projects/compression/xprs/` files, and D110227872 was never landed. This file was last modified on 2026-04-29, so ShipIt exported a copy that had never received them.

The GPU path is unchanged.

Reviewed By: kongchen1992

Differential Revision: D114806413

Author: YLouWashU

vrs/utils/xprs/XprsDecoder.cpp

@@ -93,6 +93,18 @@ Ocean::Worker* getWorkers(uint32_t pixels) {
   return pixels >= 640 * 480 ? Ocean::WorkerPool::get().scopedWorker()() : nullptr;
 }
 
+bool isMonochromePixelFormat(PixelFormat fmt) {
+  switch (fmt) {
+    case PixelFormat::GREY8:
+    case PixelFormat::GREY10:
+    case PixelFormat::GREY12:
+    case PixelFormat::GREY16:
+      return true;
+    default:
+      return false;
+  }
+}
+
 } // namespace
 
 namespace vrs::vxprs {
@@ -373,7 +385,11 @@ unique_ptr<utils::DecoderI> xprsDecoderMaker(
   const string& codecFormatName = outputImageSpec.getCodecName();
   unique_ptr<VXDecoder> decoder;
   xprs::CodecList decoders;
-  if (xprs::enumDecoders(decoders) != XprsResult::OK) {
+  // enumDecoders can report a failure while still having collected usable SW
+  // decoders, e.g. when CUDA init throws on a machine with no NVIDIA driver.
+  (void)xprs::enumDecoders(decoders);
+  if (decoders.empty()) {
+    XR_LOGE("xprs::enumDecoders returned no decoders");
     return nullptr;
   }
   xprs::VideoCodecFormat codecFormat{};
@@ -388,6 +404,11 @@ unique_ptr<utils::DecoderI> xprsDecoderMaker(
     if (!options.useHwCodecs && dec.hwAccel) {
       continue;
     }
+    // NVDEC cannot decode monochrome H.265, used by the grayscale SLAM and
+    // eye-tracking cameras.
+    if (dec.hwAccel && isMonochromePixelFormat(outputImageSpec.getPixelFormat())) {
+      continue;
+    }
     unique_ptr<xprs::IVideoDecoder> xprsDecoder{createDecoder(dec)};
     if (!xprsDecoder) {
       XR_LOGE("Creating xprs decoder '{}' for {} failed!", dec.implementationName, codecFormatName);
@@ -399,7 +420,7 @@ unique_ptr<utils::DecoderI> xprsDecoderMaker(
             dec.implementationName,
             codecFormatName,
             xprs::getErrorMessage(res));
-        return nullptr;
+        continue;
       }
       decoder = make_unique<VXDecoder>(std::move(xprsDecoder));
       if (decoder->decode(encodedFrame, outDecodedFrame, outputImageSpec) == 0) {