MachO: fix dynamic lookup of undefined symbols at runtime

Ensures `MH_NOUNDEFS` is not set when dynamic lookup is enabled for
undefined symbols via `linker_allow_shlib_undefined`.

Author: mlafeldt

src/link/MachO.zig

@@ -2937,7 +2937,13 @@ fn writeLoadCommands(self: *MachO) !struct { usize, usize, u64 } {
 
 fn writeHeader(self: *MachO, ncmds: usize, sizeofcmds: usize) !void {
     var header: macho.mach_header_64 = .{};
-    header.flags = macho.MH_NOUNDEFS | macho.MH_DYLDLINK;
+    header.flags = macho.MH_DYLDLINK;
+
+    // Only set MH_NOUNDEFS if we're not allowing undefined symbols via dynamic lookup.
+    // When dynamic_lookup is enabled, undefined symbols are resolved at runtime by dyld.
+    if (self.undefined_treatment != .dynamic_lookup) {
+        header.flags |= macho.MH_NOUNDEFS;
+    }
 
     // TODO: if (self.options.namespace == .two_level) {
     header.flags |= macho.MH_TWOLEVEL;

test/link/macho.zig

@@ -68,6 +68,7 @@ pub fn testAll(b: *Build, build_opts: BuildOptions) *Step {
     macho_step.dependOn(testTlsLargeTbss(b, .{ .target = default_target }));
     macho_step.dependOn(testTlsZig(b, .{ .target = default_target }));
     macho_step.dependOn(testUndefinedFlag(b, .{ .target = default_target }));
+    macho_step.dependOn(testUndefinedDynamicLookup(b, .{ .target = default_target }));
     macho_step.dependOn(testDiscardLocalSymbols(b, .{ .target = default_target }));
     macho_step.dependOn(testUnresolvedError(b, .{ .target = default_target }));
     macho_step.dependOn(testUnresolvedError2(b, .{ .target = default_target }));
@@ -2632,6 +2633,29 @@ fn testUndefinedFlag(b: *Build, opts: Options) *Step {
     return test_step;
 }
 
+fn testUndefinedDynamicLookup(b: *Build, opts: Options) *Step {
+    const test_step = addTestStep(b, "undefined-dynamic-lookup", opts);
+
+    // Create a dylib with an undefined external symbol reference
+    const dylib = addSharedLibrary(b, opts, .{ .name = "a" });
+    addCSourceBytes(dylib,
+        \\extern int undefined_symbol(void);
+        \\int call_undefined(void) {
+        \\    return undefined_symbol();
+        \\}
+    , &.{});
+    dylib.linker_allow_shlib_undefined = true;
+
+    // Verify the Mach-O header does NOT contain NOUNDEFS flag
+    const check = dylib.checkObject();
+    check.checkInHeaders();
+    check.checkExact("header");
+    check.checkNotPresent("NOUNDEFS");
+    test_step.dependOn(&check.step);
+
+    return test_step;
+}
+
 fn testUnresolvedError(b: *Build, opts: Options) *Step {
     const test_step = addTestStep(b, "unresolved-error", opts);