fix: robust spatial-correlation-sampler install with CUDA_HOME detection and fallback

- Auto-detect CUDA_HOME from common paths (/usr/local/cuda, etc.)
- Add CUDA bin dir to PATH if nvcc not found
- Try git source first, fall back to PyPI
- Detailed debug logging for build failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -235,25 +235,62 @@ def build_args_list_for_test(d16_batch_path: str,
 # ===== ZeroGPU: install spatial-correlation-sampler at runtime =====
 
 def ensure_correlation_extension_installed():
-    """Install spatial_correlation_sampler at runtime (needs CUDA)."""
+    """Install spatial_correlation_sampler at runtime inside @spaces.GPU where CUDA is live."""
     try:
         import spatial_correlation_sampler
         return
     except ImportError:
         pass
+
+    import torch
+    cuda_home = os.environ.get("CUDA_HOME", "")
+    nvcc_ok = shutil.which("nvcc") is not None
+    print(f"[DEBUG] torch.cuda.is_available={torch.cuda.is_available()}, "
+          f"CUDA_HOME={cuda_home!r}, nvcc_in_PATH={nvcc_ok}")
+
+    # Ensure CUDA_HOME is set so setuptools can find nvcc
+    if not cuda_home:
+        for candidate in ["/usr/local/cuda", "/usr/lib/cuda", "/opt/cuda"]:
+            if path.isdir(candidate):
+                os.environ["CUDA_HOME"] = candidate
+                print(f"[INFO] Set CUDA_HOME={candidate}")
+                break
+
+    # Also make sure nvcc is in PATH
+    cuda_home = os.environ.get("CUDA_HOME", "")
+    if cuda_home and not nvcc_ok:
+        cuda_bin = path.join(cuda_home, "bin")
+        if path.isdir(cuda_bin):
+            os.environ["PATH"] = cuda_bin + ":" + os.environ.get("PATH", "")
+            print(f"[INFO] Added {cuda_bin} to PATH")
+
     print("[INFO] Installing spatial-correlation-sampler from git source...")
     result = subprocess.run(
         [sys.executable, "-m", "pip", "install", "--no-build-isolation",
-        "git+https://github.com/ClementPinard/Pytorch-Correlation-extension.git"],
-        capture_output=True, text=True
+         "git+https://github.com/ClementPinard/Pytorch-Correlation-extension.git"],
+        capture_output=True, text=True, timeout=120,
+        env={**os.environ},
     )
-    print(f"[DEBUG] pip stdout: {result.stdout[-2000:]}")
-    print(f"[DEBUG] pip stderr: {result.stderr[-2000:]}")
-    print(f"[DEBUG] pip returncode: {result.returncode}")
     if result.returncode != 0:
-        raise RuntimeError(f"Failed to install spatial-correlation-sampler:{result.stderr[-500:]}")
+        print(f"[WARN] pip install from git failed (rc={result.returncode})")
+        print(f"[WARN] stderr tail: {result.stderr[-1500:]}")
+        # Fallback: try PyPI package
+        print("[INFO] Trying pip install spatial-correlation-sampler from PyPI...")
+        result2 = subprocess.run(
+            [sys.executable, "-m", "pip", "install", "spatial-correlation-sampler"],
+            capture_output=True, text=True, timeout=120,
+            env={**os.environ},
+        )
+        if result2.returncode != 0:
+            print(f"[ERROR] PyPI install also failed: {result2.stderr[-500:]}")
+            raise RuntimeError(
+                f"Cannot install spatial-correlation-sampler.\n"
+                f"Git error: {result.stderr[-300:]}\n"
+                f"PyPI error: {result2.stderr[-300:]}"
+            )
+
     importlib.invalidate_caches()
-    import spatial_correlation_sampler
+    import spatial_correlation_sampler  # noqa: F811
     print("[INFO] spatial-correlation-sampler installed successfully.")
 
 # ----------------- GRADIO HANDLER -----------------