{CUDA Decoding} Destroy existing NVDEC decoder before recreating on sequence re-signal (#261)

Summary:
Pull Request resolved: #261

`NvDecoder::HandleVideoSequence` is the NVDEC parser's sequence callback and, as its own header doc notes, can fire again mid-stream on a new sequence header or format change. The body unconditionally called `cuvidCreateDecoder(&_decoder, ...)`, so on any re-invocation it overwrote `_decoder` with a fresh handle without destroying the previous one — leaking the old `CUvideodecoder` and leaving the parser driving a stale/duplicated decoder. This is the "returning stale decoder" issue raised in review on D103253728.

Fix: guard the create call. When `_decoder` is already non-null, `cuvidDestroyDecoder` it (and null it, so a subsequent create failure can't double-free in the destructor) before creating the replacement. Destroy-before-recreate is chosen over `cuvidReconfigureDecoder` for correctness and simplicity: Aria streams have constant format so re-signal is rare, making the recreate cost negligible; reconfigure (an in-place perf optimization bounded by the original ulMaxWidth/Height) can be a later change if a re-signaling workload ever needs it.

Follow-up to D103253728.

Reviewed By: georges-berenger

Differential Revision: D113848493

fbshipit-source-id: 7b3bf7f32f379e2cd9dcc44f482c541a63506f4f

Author: YLouWashU

xprs/nvDecoder.cpp

@@ -210,6 +210,18 @@ int NvDecoder::HandleVideoSequence(CUVIDEOFORMAT* vdeo_format) {
     }
   }
 
+  // HandleVideoSequence can fire again mid-stream on a new sequence header or
+  // format change (see header). Never overwrite a live decoder handle: that
+  // leaks the previous CUvideodecoder and leaves the parser driving a stale one.
+  // Destroy the existing decoder before creating its replacement.
+  if (_decoder != nullptr) {
+    CUDA_API_CALL(
+        _nvcodec_context._cuvid_functions->cuvidDestroyDecoder(_decoder),
+        _nvcodec_context._cuda_functions,
+        DO_NOT_THROW);
+    _decoder = nullptr;
+  }
+
   CUDA_API_CALL(
       _nvcodec_context._cuvid_functions->cuvidCreateDecoder(&_decoder, &video_decode_create_info),
       _nvcodec_context._cuda_functions,