Move env var checks into patch_needed() function

With this change, the `patch_needed()` function can also be used from
other libraries without them needing to reimplement the logic and env
vars to decide whether to check the driver and runtime versions.

ptxcompiler/patch.py

@@ -107,6 +107,37 @@ def get_cubin(self, cc=None):
 def patch_needed():
     logger = get_logger()
 
+    # The patch is needed if the user explicitly forced it with an environment
+    # variable.
+    apply = os.getenv("PTXCOMPILER_APPLY_NUMBA_CODEGEN_PATCH")
+    if apply is not None:
+        logger.debug(f"PTXCOMPILER_APPLY_NUMBA_CODEGEN_PATCH={apply}")
+        try:
+            apply = int(apply)
+        except ValueError:
+            apply = False
+
+    if apply:
+        return True
+
+    # We should avoid checking whether the patch is needed if the user
+    # requested that we don't check (e.g. in a non-fork-safe environment)
+    check = os.getenv("PTXCOMPILER_CHECK_NUMBA_CODEGEN_PATCH_NEEDED")
+    if check is not None:
+        logger.debug(f"PTXCOMPILER_CHECK_NUMBA_CODEGEN_PATCH_NEEDED={check}")
+        try:
+            check = int(check)
+        except ValueError:
+            check = False
+    else:
+        check = True
+
+    if not check:
+        return False
+
+    # Check whether the patch is needed by comparing the driver and runtime
+    # versions - it is needed if the runtime version exceeds the driver
+    # version.
     cp = subprocess.run([sys.executable, '-c', CMD], capture_output=True)
 
     if cp.returncode:
@@ -128,26 +159,9 @@ def patch_needed():
 
 def patch_numba_codegen_if_needed():
     logger = get_logger()
-    check = os.getenv("PTXCOMPILER_CHECK_NUMBA_CODEGEN_PATCH_NEEDED")
-    apply = os.getenv("PTXCOMPILER_APPLY_NUMBA_CODEGEN_PATCH")
-    if check is not None:
-        logger.debug(f"PTXCOMPILER_CHECK_NUMBA_CODEGEN_PATCH_NEEDED={check}")
-        try:
-            check = int(check)
-        except ValueError:
-            check = False
-    else:
-        check = True
-    if apply is not None:
-        logger.debug(f"PTXCOMPILER_APPLY_NUMBA_CODEGEN_PATCH={apply}")
-        try:
-            apply = int(apply)
-        except ValueError:
-            apply = False
-    else:
-        apply = False
-    if apply or (check and patch_needed()):
+
+    if patch_needed():
         logger.debug("Patching Numba codegen for forward compatibility")
         codegen.JITCUDACodegen._library_class = PTXStaticCompileCodeLibrary
     else:
-        logger.debug("Driver version sufficient: not patching Numba codegen")
+        logger.debug("Not patching Numba codegen")