Alternative to copying part of a recent Xcode `libSystem.tbd` to `src/mach_dyld_atfork.tbd`

[benz0li]

I find copying part of a recent Xcode libSystem.tbd file into the Julia source code rather odd.

It also prevents MacPorts from building current Julia versions (≥ 1.8.0) on 'OS X'/macOS 10.10 - 10.14 (Yosemite - Mojave).

---

@quentinmit was able to get Julia to build on macOS 10.14 (Mojave)

[...] with a simple Makefile patch; I replaced

OSLIBS += $(SRCDIR)/mach_dyld_atfork.tbd

with

CLANG_LDFLAGS += -Wl,-U,__dyld_atfork_prepare -Wl,-U,__dyld_atfork_parent

and added $(CLANG_LDFLAGS) to the rules building libjulia-internal and libjulia-internal-debug. The latter is probably safe on every platform but the former needs a guard for older OS versions, of course.

It's possible that the additional -U flags are unnecessary, but I didn't test that (the previous line sets a different flag that theoretically should allow unknown symbols as well).

– #65550 (julia: link fails for 10.14 and earlier: ld: unsupported tapi file type '!tapi-tbd' in YAML file) – MacPorts

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Cross references:

• macOS: workaround a dyld/libunwind deadlock issue since 12.1 #43701
  • Fixed Profile: @profile hanging on MacOS 12 #43578
• macOS: avoid deadlock inside dyld4 deadlock workaround #49740
  • Fixed macOS dyld4 introduces a deadlock bug into Profile code #49733

[benz0li]

@quentinmit Could you please provide a full diff of your changes [that got Julia to build on macOS 10.14 (Mojave)] so @vtjnash may look into those?

I myself am not familiar enough with Xcode and Clang to help any further. Adding/Modifying a Clang command line argument instead of file src/mach_dyld_atfork.tbd makes more sense to me, though.

[quentinmit]

Unfortunately, I can't for the life of me find the patched source tree I used to build my julia :(

It looks like MacPorts has more or less applied my diff here, though: macports/macports-ports@56bf1a4#diff-ca423e7f15760e3e098d84955b5b68cc727d53538b6d50aa95d6a1fee7f49818

Basically, it's just telling the linker that it's okay if __dyld_atfork_prepare and __dyld_atfork_parent are undefined.

[vtjnash]

I reviewed it at #50017. Currently that is breaking for most platforms, but if you remove the unnecessary Makefile changes and just add the -Wl,-U, we can merge that.

[benz0li]

@vtjnash Thank you for looking into this and for your feedback.

Very much appreciated.