{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 while probing the HW entries and `enumDecoders()` catches it, returning a non-OK result. The SW `hevc`/`h264` decoders were already collected before the throw (they precede the HW entries in `kPreferredDecoderImplementations`), 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. Note the HW decoders never enter the list at all: the throw happens before `codecs.push_back`.

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 candidate instead of returning, so the HW->SW fallback actually runs. This matters on a machine that does have a GPU, where enumeration succeeds but `init()` fails (NVDEC session limit, VRAM pressure).
- Return `nullptr` rather than `decoder` when the loop is exhausted. Raised by DevMate: with the `continue` above, a candidate that initialized but failed `decode()` could otherwise be returned by a later iteration's fall-through. A non-null return is also load-bearing in `DecoderFactory::makeDecoder`, which stops at the first maker that returns anything, so handing back a known-bad decoder would prevent other registered makers from being tried. This also removes a pre-existing path to the same stale return.

The GPU path is unchanged.

Reviewed By: kongchen1992

Differential Revision: D114806413

fbshipit-source-id: e0886dcd9523d3b4cb20dd6efcd96f76c0f1b6fa

Author: YLouWashU

vrs/utils/xprs/XprsDecoder.cpp

@@ -373,7 +373,11 @@ 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{};
@@ -399,7 +403,7 @@ 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) {
@@ -424,8 +428,11 @@ unique_ptr<utils::DecoderI> xprsDecoderMaker(
       }
     }
   }
+  // Reaching here means no candidate both initialized and decoded, so never
+  // hand back the last instance: it may have failed decode, and a non-null
+  // return stops DecoderFactory from trying any other registered maker.
   TelemetryLogger::error(context, "No decoder found for " + codecFormatName);
-  return decoder;
+  return nullptr;
 }
 
 } // namespace vrs::vxprs