Get SDK version dynamically

cibuildwheel/macos.py

@@ -1,5 +1,6 @@
 import os
 import platform
+import re
 import shlex
 import shutil
 import subprocess
@@ -63,6 +64,15 @@ def get_xcode_version() -> Tuple[int, int]:
     return (int(version_parts[0]), int(version_parts[1]))
 
 
+def get_macos_sdks() -> List[str]:
+    output = subprocess.check_output(
+        ['xcodebuild', '-showsdks'],
+        universal_newlines=True,
+    )
+
+    return [m.group(1) for m in re.finditer(r'-sdk (macosx\S+)', output)]
+
+
 class PythonConfiguration(NamedTuple):
     version: str
     identifier: str
@@ -258,16 +268,22 @@ def setup_python(python_configuration: PythonConfiguration,
                 env.setdefault('ARCHFLAGS', '-arch arm64 -arch x86_64')
 
     if python_configuration.identifier.endswith('arm64') or python_configuration.identifier.endswith('universal2'):
-        if get_macos_version() < (10, 16):
+        if get_macos_version() < (10, 16) and 'SDKROOT' not in env:
             # xcode 12.2 or higher can build arm64 on macos 10.15 or below, but
-            # needs the correct SDK selected
+            # needs the correct SDK selected.
+            sdks = get_macos_sdks()
 
-            # however, different versions of Xcode contain different SDK
+            # Different versions of Xcode contain different SDK versions...
-            # versions...
+            # we're happy with anything newer than macOS 11.0
-            if get_xcode_version() == (12, 2):
+            arm64_compatible_sdks = [s for s in sdks if not s.startswith('macosx10.')]
-                env.setdefault('SDKROOT', 'macosx11.0')
+
+            if not arm64_compatible_sdks:
+                log.warning(unwrap('''
+                    SDK for building arm64-compatible wheels not found. You need Xcode 12.2 or later
+                    to build universal2 or arm64 wheels.
+                '''))
             else:
-                env.setdefault('SDKROOT', 'macosx11.1')
+                env.setdefault('SDKROOT', arm64_compatible_sdks[0])
 
     log.step('Installing build tools...')
     call(['pip', 'install', '--upgrade', 'setuptools', 'wheel', 'delocate', *dependency_constraint_flags], env=env)