testing/ostest: split the fork test into task_fork, vfork and fork

[casaroli]

Depends-On: apache/nuttx#19562

Summary

NuttX implements fork() and vfork() as the same function, and apache/nuttx#19562 splits them into three separate primitives: task_fork() (shares memory, private stack copy, both running), vfork() (shares memory, parent suspended until _exit()/exec()) and POSIX fork() (child gets its own copy). This gives each one a test of its own.

Companion PR: apache/nuttx#19562, declared above with Depends-On: so CI builds the combined change rather than each half against a master that does not yet contain the other. This PR must merge after it, because the symbols each test keys on do not exist until it lands. The apps change that unblocks that PR's CI is #3673, which merges first and is independent of this one.

ostest's "vfork" test was never testing vfork(). It has the child write a global and the parent observe the write — the defining property of sharing, not of vfork(), whose defining property is that the parent is suspended and whose contract forbids the child to write anything at all. It is renamed to task_fork.c, unchanged, because that is the primitive it has always described. It is also the clearest single piece of evidence for the proposal: the test upstream has run for years is a task_fork() test wearing vfork()'s name.

vfork.c is rewritten to test what vfork() promises. The child does only what POSIX permits — it calls _exit(42) and nothing else, not even exit(), which would run atexit handlers and flush stdio in the parent's address space. Since the child may not write memory and the parent cannot run while the child lives, the observable is the child's exit status: had the parent not been suspended, it would have reached waitpid() while the child was still alive. Where child status is not retained — ostest_main() sets SA_NOCLDWAIT for the whole run, deliberately — ECHILD is accepted as equally good evidence, since it says the child was already gone when the parent asked.

fork.c is new and tests POSIX fork(): the child's writes to .data, .bss and the heap are invisible to the parent and vice versa, a pointer to a stack local taken before the fork names the same object in both, and the child does everything a vfork() child may not — calls malloc() and printf(), and returns from the function that called fork().

All three run at the top of user_main(). They exercise the lowest-level machinery in the suite — address environments, stack setup, the architecture's register context — so a fault in one takes the process down instead of reporting a failure. Learning that in seconds rather than after everything else has passed matters when a port is being brought up.

Each test gates on the one primitive it tests, and nothing stands in for anything. There is no compatibility layer and no mapping between symbols.

task_fork_test() keys on CONFIG_TASK_FORK rather than on the capability symbol: ARCH_HAVE_TASK_FORK says the architecture can clone a task while TASK_FORK says the build asked for it, and task_fork() is declared only under the latter, so gating on the capability alone would fail to compile a TASK_FORK=n build. vfork_test() and fork_test() have no such split and key on ARCH_HAVE_VFORK and ARCH_HAVE_FORK directly.

The other in-tree callers are audited for which primitive they actually meant:

• interpreters/python's _posixsubprocess and netutils/libwebsockets' LWS_HAVE_WORKING_VFORK want the fork-then-exec path — ARCH_HAVE_VFORK.
• python's os.fork() and libwebsockets' LWS_HAVE_FORK mean real fork() and stay on ARCH_HAVE_FORK, so they become absent rather than silently wrong.
• testing/fs/fdsantest's vfork case follows ARCH_HAVE_VFORK.

Impact

Between #3673 merging and apache/nuttx#19562 merging, ostest has no fork test. That is the deliberate cost of carrying no compatibility layer: CONFIG_TASK_FORK and CONFIG_ARCH_HAVE_VFORK do not exist on a pre-split NuttX, so task_fork_test() and vfork_test() are not built, and fork_test() is not built either because after the split ARCH_HAVE_FORK is off until a per-architecture PR turns it on. Coverage returns the moment the NuttX side lands.

fork_test() costs nothing on size-constrained configurations, because it is not built on them. It keys on ARCH_HAVE_FORK, which no architecture sets until up_addrenv_fork() lands for it, so lm3s6965-ek:qemu-protected — the configuration with the tightest user flash region — never compiles it.

fork_test() itself is exercised by the per-architecture PRs that follow, which are what turn ARCH_HAVE_FORK back on.

Testing

Host: macOS 15 (Darwin 25.5.0) on Apple Silicon. QEMU 11.0.3, xPack riscv-none-elf-gcc 14.2.0-3, Arm GNU arm-none-eabi-gcc 14.2.Rel1.

Style

../nuttx/tools/checkpatch.sh -c -u -m -g <base>..HEAD, the exact command .github/workflows/check.yml runs — ✔️ All checks pass, with codespell, cvt2utf, cmake-format and nxstyle all installed.

Verification

This PR was restructured after review: the compatibility layer is gone entirely, each test keys on the primitive it tests, and Depends-On: now carries the relationship to apache/nuttx#19562 that the fallbacks used to stand in for. The branch has been rebased onto current master.

The functional matrix is being re-run against the restructured branch and will be posted here — full ostest to exit status 0 on rv-virt:nsh64, rv-virt:pnsh64, rv-virt:knsh64, qemu-armv7a:nsh, qemu-armv8a:nsh, qemu-intel64:nsh and sim:ostest, plus lm3s6965-ek:qemu-protected free bytes against the apps master baseline. The previous revision's results are in this PR's history.

[xiaoxiang781216]

why not move to #3673

[xiaoxiang781216]

why not move to #3673

[xiaoxiang781216]

why not move to #3673