C lib loading: add fallback with sensible error msg

bitsandbytes/cextension.py

@@ -3,11 +3,12 @@
 import os
 from pathlib import Path
 import re
+from typing import Optional
 
 import torch
 
 from bitsandbytes.consts import DYNAMIC_LIBRARY_SUFFIX, PACKAGE_DIR
-from bitsandbytes.cuda_specs import CUDASpecs, get_cuda_specs
+from bitsandbytes.cuda_specs import CUDASpecs, get_cuda_specs, get_cuda_version_tuple
 
 logger = logging.getLogger(__name__)
 
@@ -44,11 +45,19 @@ class BNBNativeLibrary:
     def __init__(self, lib: ct.CDLL):
         self._lib = lib
 
-    def __getattr__(self, item):
-        return getattr(self._lib, item)
+    def __getattr__(self, name):
+        def throw_on_call(*args, **kwargs):
+            if hasattr(self._lib, name):
+                return getattr(self._lib, name)(*args, **kwargs)
+            raise RuntimeError(
+                f"Method '{name}' not available in CPU-only version of bitsandbytes.\n"
+                "Reinstall with GPU support or use CUDA-enabled hardware."
+            )
+
+        return throw_on_call
 
     def __getitem__(self, item):
-        return getattr(self._lib, item)
+        return self.__getattr__(item)
 
 
 class CudaBNBNativeLibrary(BNBNativeLibrary):
@@ -61,42 +70,224 @@ def __init__(self, lib: ct.CDLL):
         lib.cget_managed_ptr.restype = ct.c_void_p
 
 
+def get_available_cuda_binary_versions() -> list[str]:
+    """Get formatted CUDA versions from existing library files using cuda_specs logic"""
+    lib_pattern = f"libbitsandbytes_cuda*{DYNAMIC_LIBRARY_SUFFIX}"
+    versions = []
+    for lib in Path(__file__).parent.glob(lib_pattern):
+        match = re.search(r"cuda(\d{3})", lib.name)
+        if match:
+            ver_code = int(match.group(1))
+            major = ver_code // 10
+            minor = ver_code % 10
+            versions.append(f"{major}.{minor}")
+    return sorted(versions)
+
+
+def parse_cuda_version(version_str: str) -> str:
+    """Convert raw version string (e.g. '118' from env var) to formatted version (e.g. '11.8')"""
+    if version_str.isdigit() and len(version_str) == 3:
+        return f"{version_str[:2]}.{version_str[2]}"
+    return version_str  # fallback as safety net
+
+
+class ErrorHandlerMockBNBNativeLibrary(BNBNativeLibrary):
+    """
+    Mock library handler that defers errors until native methods are called.
+
+    This class serves as a fallback when the native bitsandbytes library fails to load.
+    It captures the original error and generates detailed troubleshooting guidance.
+
+    Key behaviors:
+    - Allows attribute access and method assignment without immediate errors
+    - Throws a RuntimeError with diagnostic information only when a native method is called, as otherwise it would error out on import, breaking backward compatibility
+    - Handles both missing CUDA dependencies and version mismatch scenarios
+
+    Error scenarios covered:
+    1. Missing shared library dependencies (e.g., libcudart.so not in LD_LIBRARY_PATH or through PyTorch CUDA installation)
+    2. CUDA version mismatch between PyTorch and available pre-compiled binaries
+    3. Completely missing pre-compiled binaries when CUDA is detected
+    4. Custom BNB_CUDA_VERSION override but mismatch
+    5. CPU-only installation attempts when GPU functionality is requested
+
+    """
+
+    def __init__(self, error_msg: str):
+        self.error_msg = error_msg
+        self.user_cuda_version = get_cuda_version_tuple()
+        self.available_versions = get_available_cuda_binary_versions()
+        self.override_value = os.environ.get("BNB_CUDA_VERSION")
+        self.requested_version = (
+            parse_cuda_version(self.override_value)
+            if self.override_value
+            else f"{self.user_cuda_version[0]}.{self.user_cuda_version[1]}"
+            if self.user_cuda_version
+            else "unknown"
+        )
+
+        # Pre-generate the error message based on error type
+        if "cannot open shared object file" in error_msg:
+            self.formatted_error = self._format_dependency_error()
+        else:  # lib loading errors
+            self.formatted_error = self._format_lib_error_message(
+                available_versions=self.available_versions,
+                user_cuda_version=f"{self.user_cuda_version[0]}.{self.user_cuda_version[1]}"
+                if self.user_cuda_version
+                else "unknown",
+                original_error=f"Original error: {self.error_msg}\n" if self.error_msg else "",
+                requested_version=self.requested_version,
+            )
+
+    def _format_lib_error_message(
+        self,
+        available_versions: list[str],
+        user_cuda_version: str,
+        original_error: str = "",
+        requested_version: Optional[str] = None,
+    ) -> str:
+        """Format detailed error message for library loading failures"""
+        analysis = ""
+        no_cpu_lib_found = "libbitsandbytes_cpu.so: cannot open" in original_error
+        no_cuda_lib_found = "CUDA binary not found" in original_error
+
+        if no_cpu_lib_found:
+            analysis = "\n🚨 Failed to load CPU-only bitsandbytes library 🚨\n\n"
+
+        elif no_cuda_lib_found:
+            version_list_str = "\n  - " + "\n  - ".join(available_versions) if available_versions else "NONE"
+            analysis = (
+                (
+                    f"\n🚨 CUDA VERSION MISMATCH 🚨\n"
+                    f"Requested CUDA version:          {requested_version}\n"
+                    f"Detected PyTorch CUDA version:   {user_cuda_version}\n"
+                    f"Available pre-compiled versions: {version_list_str}\n\n"
+                    "This means:\n"
+                    "The version you're trying to use is NOT distributed with this package\n\n"
+                )
+                if available_versions
+                else "\n🚨 Forgot to compile the bitsandbytes library? 🚨\n"
+                "1. You're not using the package but checked-out the source code\n"
+                "2. You MUST compile from source\n\n"
+            )
+
+        base_msg = "Attempted to use bitsandbytes native library functionality but it's not available.\n\n"
+
+        troubleshooting = (
+            (
+                "This typically happens when:\n"
+                "1. bitsandbytes doesn't ship with a pre-compiled binary for your CUDA version\n"
+                "2. The library wasn't compiled properly during installation from source\n\n"
+            )
+            if no_cuda_lib_found
+            else "This typically happens when you checked the code out from source and your torch installation doesn't detect CUDA on your machine.\n\n"
+        )
+
+        note = (
+            (
+                "To make bitsandbytes work, the compiled library version MUST exactly match the linked CUDA version.\n"
+                "If your CUDA version doesn't have a pre-compiled binary, you MUST compile from source.\n\n"
+            )
+            if no_cuda_lib_found
+            else ""
+        )
+
+        compile_instructions = (
+            (
+                "You have two options:\n"
+                "1. COMPILE FROM SOURCE (required if no binary exists):\n"
+                "   https://huggingface.co/docs/bitsandbytes/main/en/installation#cuda-compile\n"
+                "2. Use BNB_CUDA_VERSION to specify a DIFFERENT CUDA version from the detected one, which is installed on your machine and matching an available pre-compiled version listed above\n\n"
+            )
+            if no_cuda_lib_found
+            else "COMPILE FROM SOURCE for CPU-only:\n  `cmake -DCOMPUTE_BACKEND=cpu -S . && make`\n\n"
+        )
+
+        diagnostics = (
+            "🔍 Run this command for detailed diagnostics:\n"
+            "python -m bitsandbytes\n\n"
+            "If you've tried everything and still have issues:\n"
+            "1. Include ALL version info (operating system, bitsandbytes, pytorch, cuda, python)\n"
+            "2. Describe what you've tried in detail\n"
+            "3. Open an issue with this information:\n"
+            "   https://github.com/bitsandbytes-foundation/bitsandbytes/issues\n\n"
+        )
+
+        return f"{analysis}{base_msg}{troubleshooting}{note}{compile_instructions}{original_error}\n{diagnostics}"
+
+    def _format_dependency_error(self) -> str:
+        """Format error message for missing shared libraries"""
+        # Extract missing library name from error
+        error_parts = self.error_msg.split(":")
+        missing_lib = error_parts[0].strip() if len(error_parts) > 0 else "unknown library"
+        cuda_major_version = (
+            self.requested_version.split(".")[0] if "." in self.requested_version else self.requested_version
+        )
+
+        return (
+            f"\n🚨 CUDA SETUP ERROR: Missing dependency: {missing_lib} 🚨\n\n"
+            f"CUDA {cuda_major_version}.x runtime libraries were not found in the LD_LIBRARY_PATH.\n\n"
+            f"To fix this, make sure that:\n"
+            f"1. You have installed CUDA {cuda_major_version}.x toolkit on your system\n"
+            f"2. The CUDA runtime libraries are in your LD_LIBRARY_PATH\n\n"
+            f"You can add them with (and persist the change by adding the line to your .bashrc):\n"
+            f"   export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/cuda-{cuda_major_version}.x/lib64\n\n"
+            f"Original error: {self.error_msg}\n\n"
+            f"🔍 Run this command for detailed diagnostics:\n"
+            f"python -m bitsandbytes\n\n"
+            f"If you've tried everything and still have issues:\n"
+            f"1. Include ALL version info (operating system, bitsandbytes, pytorch, cuda, python)\n"
+            f"2. Describe what you've tried in detail\n"
+            f"3. Open an issue with this information:\n"
+            f"   https://github.com/bitsandbytes-foundation/bitsandbytes/issues\n\n"
+        )
+
+    def __getattr__(self, name):
+        """Return a dummy function that throws when called, rather than on attribute access"""
+
+        def throw_on_call(*args, **kwargs):
+            raise RuntimeError(f"{self.formatted_error}Native code method attempted to call: lib.{name}()")
+
+        return throw_on_call
+
+    def __getitem__(self, name):
+        return self.__getattr__(name)
+
+
 def get_native_library() -> BNBNativeLibrary:
-    binary_path = PACKAGE_DIR / f"libbitsandbytes_cpu{DYNAMIC_LIBRARY_SUFFIX}"
+    """
+    Load CUDA library XOR CPU, as the latter contains a subset of symbols of the former.
+    """
     cuda_specs = get_cuda_specs()
+    binary_path = PACKAGE_DIR / f"libbitsandbytes_cpu{DYNAMIC_LIBRARY_SUFFIX}"
+
     if cuda_specs:
         cuda_binary_path = get_cuda_bnb_library_path(cuda_specs)
-        if cuda_binary_path.exists():
-            binary_path = cuda_binary_path
-        else:
-            logger.warning("Could not find the bitsandbytes CUDA binary at %r", cuda_binary_path)
+
+        if not cuda_binary_path.exists():
+            raise RuntimeError(f"Configured CUDA binary not found at {cuda_binary_path}")
+
+        binary_path = cuda_binary_path
+
     logger.debug(f"Loading bitsandbytes native library from: {binary_path}")
+
+    # Try to load the library - any errors will propagate up
     dll = ct.cdll.LoadLibrary(str(binary_path))
 
     if hasattr(dll, "get_context"):  # only a CUDA-built library exposes this
         return CudaBNBNativeLibrary(dll)
 
     logger.warning(
         "The installed version of bitsandbytes was compiled without GPU support. "
-        "8-bit optimizers and GPU quantization are unavailable.",
+        "8-bit optimizers and GPU quantization are unavailable."
     )
     return BNBNativeLibrary(dll)
 
 
 try:
     lib = get_native_library()
 except Exception as e:
-    lib = None
-    logger.error(f"Could not load bitsandbytes native library: {e}", exc_info=True)
-    if torch.cuda.is_available():
-        logger.warning(
-            """
-CUDA Setup failed despite CUDA being available. Please run the following command to get more information:
-
-python -m bitsandbytes
+    error_msg = str(e)
+    logger.error(f"bitsandbytes library load error: {error_msg}\n", exc_info=True)
 
-Inspect the output of the command and see if you can locate CUDA libraries. You might need to add them
-to your LD_LIBRARY_PATH. If you suspect a bug, please take the information from python -m bitsandbytes
-and open an issue at: https://github.com/bitsandbytes-foundation/bitsandbytes/issues
-""",
-        )
+    # create a mock with error messaging as fallback
+    lib = ErrorHandlerMockBNBNativeLibrary(error_msg)

bitsandbytes/diagnostics/main.py

@@ -13,21 +13,6 @@
 
 
 def sanity_check():
-    from bitsandbytes.cextension import lib
-
-    if lib is None:
-        print_dedented(
-            """
-            Couldn't load the bitsandbytes library, likely due to missing binaries.
-            Please ensure bitsandbytes is properly installed.
-
-            For source installations, compile the binaries with `cmake -DCOMPUTE_BACKEND=cuda -S .`.
-            See the documentation for more details if needed.
-
-            Trying a simple check anyway, but this will likely fail...
-            """,
-        )
-
     from bitsandbytes.optim import Adam
 
     p = torch.nn.Parameter(torch.rand(10, 10).cuda())
@@ -67,12 +52,15 @@ def main():
         print("SUCCESS!")
         print("Installation was successful!")
         return
-    except ImportError:
-        print(
-            f"WARNING: {__package__} is currently running as CPU-only!\n"
-            "Therefore, 8-bit optimizers and GPU quantization are unavailable.\n\n"
-            f"If you think that this is so erroneously,\nplease report an issue!",
-        )
+    except RuntimeError as e:
+        if "not available in CPU-only" in str(e):
+            print(
+                f"WARNING: {__package__} is currently running as CPU-only!\n"
+                "Therefore, 8-bit optimizers and GPU quantization are unavailable.\n\n"
+                f"If you think that this is so erroneously,\nplease report an issue!",
+            )
+        else:
+            raise e
     except Exception:
         traceback.print_exc()
     print_dedented(