Drop from _sysconfig

Author: charliermarsh

cpython-unix/build-cpython.sh

@@ -627,10 +627,14 @@ def replace_in_all(search, replace):
     replace_in_file(MAKEFILE, search, replace)
     replace_in_file(SYSCONFIGDATA, search, replace)
 
-# Replace the XCode path with a generic value.
+# Remove the Xcode path from the compiler flags.
+#
+# CPython itself will drop this from `sysconfig.get_config_var("CFLAGS")` and
+# similar calls, but _not_ if `CFLAGS` is set in the environment (regardless of
+# the `CFLAGS` value). It will almost always be wrong, so we drop it unconditionally.
 xcode_path = os.getenv("APPLE_SDK_PATH")
 if xcode_path:
-    replace_in_all(xcode_path, "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk")
+    replace_in_all("-isysroot %s" % xcode_path, "")
 
 # -fdebug-default-version is Clang only. Strip so compiling works on GCC.
 replace_in_all("-fdebug-default-version=4", "")

cpython-unix/build.py

@@ -165,11 +165,29 @@ def add_target_env(env, build_platform, target_triple, build_env):
         # non-system (e.g. Homebrew) executables from being used.
         env["PATH"] = "/usr/bin:/bin"
 
-        sdk_path = os.environ["APPLE_SDK_PATH"]
+        if "APPLE_SDK_PATH" in os.environ:
+            sdk_path = os.environ["APPLE_SDK_PATH"]
+        else:
+            # macOS SDK has historically been in /usr courtesy of an
+            # installer provided by Xcode. But with Catalina, the files
+            # are now typically in
+            # /Applications/Xcode.app/Contents/Developer/Platforms/.
+            # The proper way to resolve this path is with xcrun, which
+            # will give us the headers that Xcode is configured to use.
+            res = subprocess.run(
+                ["xcrun", "--sdk", sdk_platform, "--show-sdk-path"],
+                check=True,
+                capture_output=True,
+                encoding="utf-8",
+            )
+
+            sdk_path = res.stdout.strip()
 
         if not os.path.exists(sdk_path):
             raise Exception("macOS SDK path %s does not exist" % sdk_path)
 
+        env["APPLE_SDK_PATH"] = sdk_path
+
         # Grab the version from the SDK so we can put it in PYTHON.json.
         sdk_settings_path = pathlib.Path(sdk_path) / "SDKSettings.json"
         with sdk_settings_path.open("rb") as fh:
@@ -801,11 +819,6 @@ def build_cpython(
         if "lto" in parsed_build_options:
             env["CPYTHON_LTO"] = "1"
 
-        sdk_path = os.environ["APPLE_SDK_PATH"]
-        if not os.path.exists(sdk_path):
-            raise Exception("macOS SDK path %s does not exist" % sdk_path)
-        env["APPLE_SDK_PATH"] = sdk_path
-
         add_target_env(env, host_platform, target_triple, build_env)
 
         build_env.run("build-cpython.sh", environment=env)