[casaroli]

Summary

libc has popen(), pclose(), dpopen() and dpclose() ([1/5] apache/nuttx#20778). The users of popen() in apps accept the libc copy ([2/5] #3877), and no nuttx configuration enables CONFIG_SYSTEM_POPEN any more ([3/5] apache/nuttx#20792).

This PR removes the code of system/popen. The fastboot shell command, the popen test, the popen example, dhcpd and gdbstub now depend only on the libc conditions: CONFIG_PIPES, CONFIG_LIBC_EXECFUNCS and CONFIG_SCHED_WAITPID. CONFIG_SYSTEM_POPEN stays as a deprecated option that only selects CONFIG_LIBC_EXECFUNCS and CONFIG_SCHED_WAITPID, so configurations that set it keep popen().

[5/5] apache/nuttx#20839 removes the CONFIG_SYSTEM_POPEN check from the libc build and moves the documentation page of system/popen to the libc documentation.

Impact

No nuttx configuration changes: the 15 configurations that used system/popen already use the libc copy. The libc copy has all the functions of the apps copy. The options CONFIG_SYSTEM_POPEN_SHPATH, CONFIG_SYSTEM_POPEN_STACKSIZE, CONFIG_SYSTEM_POPEN_PRIORITY and CONFIG_SYSTEM_POPEN_MAXARGUMENTS are removed: libc starts sh from PATH (or /bin/sh) with the posix_spawn() defaults, and has no limit on the number of arguments. popen() of libc accepts only the POSIX modes "r" and "w"; the apps copy also accepted "r+" and "w+". dpopen() with O_RDWR is still available.

A configuration out of the tree that sets CONFIG_SYSTEM_POPEN=y keeps building and gets the libc copy. CONFIG_SYSTEM_POPEN selected CONFIG_SCHED_WAITPID, so a defconfig from make savedefconfig does not contain CONFIG_SCHED_WAITPID; without the deprecated option, such a configuration would lose it, popen() would not be built, the popen users would be disabled without a message, and programs that call waitpid() (for example system/nxinit) would not compile. Tested with defconfigs from make savedefconfig of qemu-armv8a:nsh and :netnsh with CONFIG_SYSTEM_POPEN=y, with and without CONFIG_SYSTEM_SYSTEM: all build with the libc copy and lose no option.

Testing

Built with [5/5], all with CONFIG_SYSTEM_POPEN gone and the libc lib_popen.o and lib_dpopen.o built. After make olddefconfig, no option of the defconfig was lost: CONFIG_SYSTEM_FASTBOOTD_SHELL=y and CONFIG_EXAMPLES_POPEN=y stay set.

Configuration	Result
qemu-armv8a:fastboot	builds, CONFIG_SYSTEM_FASTBOOTD_SHELL=y
sabre-6quad:citest, rv-virt:citest, rv-virt:citest64	build, CONFIG_EXAMPLES_POPEN=y
lx_cpu:nsh	builds
esp32s3-devkit:fastboot, :fastboot_tcp, :fastboot_usb, lckfb-szpi-esp32s3:fastboot, :fastboot_tcp, :fastboot_usb	build, CONFIG_SYSTEM_FASTBOOTD_SHELL=y
sim:citest, sim:quickjs (x86_64 Linux in Docker)	build, CONFIG_EXAMPLES_POPEN=y in sim:citest
qemu-intel64:citest_smp (x86_64 Linux in Docker)	builds, CONFIG_EXAMPLES_POPEN=y

sim:sqlite was not built: the build host has no tclsh, which the SQLite build needs.

Runtime on qemu-armv8a:knsh (kernel build) and qemu-armv8a:nsh (flat build), QEMU with HVF, with CONFIG_TESTING_POPEN, CONFIG_EXAMPLES_POPEN and CONFIG_SCHED_CHILD_STATUS: popen_test passes all 8 tests (=== Results: 0 failed ===), incl. test 8 (pclose() returns the failure status of the command). popen (the example) reads the output of help to End of data and exits with 0.

tools/checkpatch.sh -c -u -m -g passes.