unix: disable preadv() and pwritev() on macOS

Compiling against the 11.0 SDK will introduce these symbols. Why,
I'm not sure, as the CPython source code appears to put them behind
a __builtin_available() check. There might be something wonky going
on. Anyway, this symbol didn't exist until the 11.0 SDK. So it should
be safe to disable it.

Author: indygreg

cpython-unix/build-cpython.sh

@@ -525,6 +525,20 @@ if [ "${PYBUILD_PLATFORM}" = "macos" ]; then
     # as Homebrew or MacPorts. So nerf the check to prevent this.
     CONFIGURE_FLAGS="${CONFIGURE_FLAGS} ac_cv_lib_intl_textdomain=no"
 
+    # When building against an 11.0+ SDK, preadv() and pwritev() are
+    # detected and used, despite only being available in the 11.0+ SDK. This
+    # prevents object files from re-linking when built with older SDKs.
+    # So we disable them. But not in aarch64-apple-darwin, as that target
+    # requires the 11.0 SDK.
+    #
+    # This solution is less than ideal. Modern versions of Python support
+    # weak linking and it should be possible to coerce these functions into
+    # being weakly linked.
+    if [ "${TARGET_TRIPLE}" != "aarch64-apple-darwin" ]; then
+        CONFIGURE_FLAGS="${CONFIGURE_FLAGS} ac_cv_func_preadv=no"
+        CONFIGURE_FLAGS="${CONFIGURE_FLAGS} ac_cv_func_pwritev=no"
+    fi
+
     if [ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" ]; then
         # Python's configure doesn't support cross-compiling on macOS. So we need
         # to explicitly set MACHDEP to avoid busted checks. The code for setting

src/main.rs

@@ -351,6 +351,14 @@ static PLATFORM_TAG_BY_TRIPLE: Lazy<HashMap<&'static str, &'static str>> = Lazy:
     .collect()
 });
 
+/// Symbols that we don't want to appear in mach-o binaries.
+const MACHO_BANNED_SYMBOLS_NON_AARCH64: &[&str] = &[
+    // _readv and _pwritev are introduced when building with the macOS 11 SDK.
+    // If present, they can cause errors re-linking object files. So we ban their
+    // existence.
+    "_preadv", "_pwritev",
+];
+
 fn allowed_dylibs_for_triple(triple: &str) -> Vec<MachOAllowedDylib> {
     match triple {
         "aarch64-apple-darwin" => DARWIN_ALLOWED_DYLIBS.clone(),
@@ -496,6 +504,22 @@ fn validate_macho(
         }
     }
 
+    if let Some(symbols) = &macho.symbols {
+        for symbol in symbols {
+            let (name, _) = symbol?;
+
+            if target_triple != "aarch64-apple-darwin"
+                && MACHO_BANNED_SYMBOLS_NON_AARCH64.contains(&name)
+            {
+                errors.push(format!(
+                    "{} references unallowed symbol {}",
+                    path.display(),
+                    name
+                ));
+            }
+        }
+    }
+
     Ok((errors, seen_dylibs))
 }