macho: allow undefined symbols in dylibs

[kubkon]

We now respect both -fallow-shlib-undefined and -Wl,"-undefined=dynamic_lookup" flags. This is the first step towards solving issues #8180. We currently do not expose any other ld64 equivalent flag for -undefined flag - we basically throw an error should the user specify a different flag. Support for those is conditional on closing #8180. As a result of this change, it is now possible to generate a valid native Node.js addon with Zig for macOS.

You can also check whether the option is on by requesting --verbose-link when building a shared object like so:

$ zig build-lib -dynamic example.zig -I$(brew --prefix node)/include/node -fallow-shlib-undefined -femit-bin=example.node --verbose-link
zig ld -dynamic -dylib -install_name @rpath/example.node zig-cache/o/f33f9be186fda290c12c65772584b1cb/example.o /Users/kubkon/.cache/zig/o/7d88bd701accf5f7b37248e431dd6be8/libcompiler_rt.a -o example.node -lSystem -lc -undefined dynamic_lookup

Fixes #3000

cc @jorangreef @ifreund

[andrewrk]

Quick question- you mentioned that this supports -Wl,"-undefined=dynamic_lookup", but it looks like this actually implements -Wl,-undefined,dynamic_lookup. Is this intentional?

[kubkon]

Uhm, wait, did I make a typo somewhere? It should implement the former, -Wl,"-undefined=dynamic_lookup" unless I messed something up.

[kubkon]

Hmm, I think that's what I did. Note that there is another if statement after we match -undefined:

    5                 } else if (mem.eql(u8, arg, "-undefined")) {
     4                     i += 1;
     3                     if (i >= linker_args.items.len) {
     2                         fatal("expected linker arg after '{s}'", .{arg});
     1                     }
  1711                     if (mem.eql(u8, "dynamic_lookup", linker_args.items[i])) {
     1                         linker_allow_shlib_undefined = true;
     2                     } else {
     3                         fatal("unsupported -undefined option '{s}'", .{linker_args.items[i]});
     4                     }
     5                 } else {

[andrewrk]

Never mind- I forgot that we had this code up there:

                            // Handle nested-joined args like `-Wl,-rpath=foo`.
                            // Must be prefixed with 1 or 2 dashes.
                            if (linker_arg.len >= 3 and linker_arg[0] == '-' and linker_arg[2] != '-') {
                                if (mem.indexOfScalar(u8, linker_arg, '=')) |equals_pos| {
                                    try linker_args.append(linker_arg[0..equals_pos]);
                                    try linker_args.append(linker_arg[equals_pos + 1 ..]);
                                    continue;
                                }
                            }

all good 👍

[jorangreef]

Thanks so much @kubkon and @andrewrk ! Going to go link a cup of coffee now...