Status: closed in favor of the earlier and broader #5612. The tests remain available for that PR. The evidence below is historical.

compiler,runtime: call fcntl through a C wrapper on darwin

What this does

The third parameter of fcntl is variadic, and on darwin/arm64 a variadic
argument goes on the stack and not in a register. A call to libc fcntl through
a plain three-argument function pointer thus makes the callee read that argument
from an unrelated stack slot. open() already has a C wrapper in
src/runtime/os_darwin.c for the same reason, and this adds the matching one
for fcntl.

The wrapper takes the argument as a uintptr_t so that the pointer commands
reached through syscall.fcntlPtr use it too. Both spellings go through
libc_fcntl_trampoline, and on a little-endian target the int commands read the
low half of the same stack slot.

Why it matters

The failure is quiet. fcntl(fd, F_SETFD, FD_CLOEXEC) sets the flag or does
not, which depends on the stack contents, so the result is the same for one
binary and different between binaries. syscall.CloseOnExec is the main caller,
so when it fails, every descriptor of the program goes into every process that
it starts, and a child that holds a copy of the write end of a pipe keeps that
pipe from a report of EOF.

Evidence

Measured on macOS 26.6 arm64 before this change.

• fcntl(fd, F_DUPFD, 100) returns EINVAL.
• Three F_SETFL calls with 0x4, 0x0 and 0x8 all leave F_GETFL reading 0x48.

Two tests are added to src/os/fcntl_test.go. The os package is already in
TEST_PACKAGES_FAST, so both run in the linux and the macOS CI jobs with no
makefile change.

Both pass on macOS 26.6 arm64 with this change. Linux is unaffected, because it
does not use the darwin trampoline path, and the tests are a regression guard
there.

A downstream product also ships binaries built with a fork that carries this
change, in the published release dispat v1.4.0.
https://github.com/yohimik/dispat/releases/tag/services%2Fdispat%2Fv1.4.0

Scope

• compiler/syscall.go only changes createDarwinFuncPCABI0Call, so no other
  target sees a difference.
• No API change.

Related

This is a prerequisite for correct process creation on darwin. os.Pipe on
darwin has no pipe2, so it must mark both ends close-on-exec with fcntl.

Related pull requests

Each open PR in this series has a separate change. A dependency is not a copied commit.

• sync: hand the RWMutex over instead of a re-test of the reader count #5630 fixes RWMutex handover.
• runtime: deliver os/signal notifications under the threads scheduler #5631 is closed in favor of runtime: deliver signals under the threads scheduler when blocked on I/O #5530, owned by @0pcom. The fork still uses the tested runtime: deliver os/signal notifications under the threads scheduler #5631 implementation until a replacement is checked.
• compiler,runtime: call fcntl through a C wrapper on darwin #5632 is closed in favor of compiler: implement go:cgo_import_dynamic and lower Darwin cgo import trampolines (#5604) #5612, owned by @neomantra. compiler: implement go:cgo_import_dynamic and lower Darwin cgo import trampolines (#5604) #5612 covers both stdlib and x/sys Darwin trampolines.
• runtime: implement weak.runtime_makeStrongFromWeak #5633 provides the weak-pointer runtime hook.
• os: start processes with posix_spawn #5634 provides process spawning and process state. Darwin also needs compiler: implement go:cgo_import_dynamic and lower Darwin cgo import trampolines (#5604) #5612 and builder: declare Darwin socket and process symbols #5636. Concurrent use needs sync: hand the RWMutex over instead of a re-test of the reader count #5630.
• loader: use the real crypto/tls on hosted linux and darwin #5635 selects software TLS on hosted Linux and Darwin. It needs runtime: implement weak.runtime_makeStrongFromWeak #5633. The overlapping crypto/tls: use Go's software TLS on the host target #5645 is under discussion with @0pcom.
• builder: declare Darwin socket and process symbols #5636 provides the Darwin socket and process link symbols. It does not include process or TLS implementation changes.

In tinygo-org/net

• net/http: follow redirects in the client again net#72 restores HTTP redirects. It shares http/client.go with the timeout work in http: port Go net/http timeout handling net#67, so the two changes need integration review.
• net: report a name that does not exist as a not-found lookup error net#73 preserves DNS not-found errors.
• net: extend the host netdev to darwin net#74 adds Darwin host netdev support. It shares the host netdev with net: non-blocking host netdev I/O with an epoll poller net#77. Linux epoll and Darwin support must remain separate.
• net/http: give the HTTPS client trust roots on darwin net#75 supplies Darwin trust roots to the HTTP client.
• net: non-blocking host netdev I/O with an epoll poller net#77 owns Linux cancellation and deadline work, net: report the kernel-assigned port after listening on port 0 net#80 owns port-zero reporting, and net/http: add ListenAndServeTLS / ServeTLS (software TLS) net#79 owns HTTP server TLS. These are separate contributors' changes, not completed parts of this series.
• net: API-compat fills for grpc and wazero (SyscallConn, Dialer.Control, IPConn.Read) net#78 adds client API fields and related compatibility methods.

Full Darwin networking also needs the merged net changes and a later src/net pin update. No upstream merge or current full-suite pass is implied by this list.