[libc] Enable spawn lib in riscv

[mikhail.ramalho]

Summary

In this patch we add support for the spawn lib in riscv.

Only small changes were required, the biggest one was to use of dup3
instead of dup2, if the latter is not available. This follows our
implementation of dup2.

[sivachandra]

LGTM with one comment.

libc/src/spawn/linux/posix_spawn.cpp

22

You should include <signal.h> not the macro file unconditionally. So, no ifdef, just #include <signal.h>.

[mikhail.ramalho]

Fixed header include

[sivachandra]

We have libc RISCV64 builders now: https://lab.llvm.org/buildbot/#/workers/202
Please watch them after landing.

libc/config/linux/riscv64/entrypoints.txt

list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.signal.sigaltstack
     libc.src.signal.sigdelset
     libc.src.signal.sigaddset
     libc.src.signal.sigemptyset
     libc.src.signal.sigprocmask
     libc.src.signal.sigfillset
     libc.src.signal.signal
 
+    # spawn.h entrypoints
+    libc.src.spawn.posix_spawn
+    libc.src.spawn.posix_spawn_file_actions_addclose
+    libc.src.spawn.posix_spawn_file_actions_adddup2
+    libc.src.spawn.posix_spawn_file_actions_addopen
+    libc.src.spawn.posix_spawn_file_actions_destroy
+    libc.src.spawn.posix_spawn_file_actions_init
+
     # threads.h entrypoints
     libc.src.threads.call_once
     libc.src.threads.cnd_broadcast
     libc.src.threads.cnd_destroy
     libc.src.threads.cnd_init
     libc.src.threads.cnd_signal
     libc.src.threads.cnd_wait
     libc.src.threads.mtx_destroy

libc/config/linux/riscv64/headers.txt

 set(TARGET_PUBLIC_HEADERS
     libc.include.assert
     libc.include.ctype
     libc.include.errno
     libc.include.fcntl
     libc.include.fenv
     libc.include.inttypes
     libc.include.math
     libc.include.pthread
     libc.include.sched
     libc.include.signal
+    libc.include.spawn
     libc.include.stdio
     libc.include.stdlib
     libc.include.string
     libc.include.strings
     libc.include.termios
     libc.include.threads
     libc.include.time
     libc.include.unistd

libc/src/spawn/linux/posix_spawn.cpp

 //===-- Linux implementation of posix_spawn -------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/spawn/posix_spawn.h"
 
 #include "src/__support/CPP/optional.h"
 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
 #include "src/__support/common.h"
 #include "src/spawn/file_actions.h"
 
 #include <fcntl.h>
+#include <signal.h> // For SIGCHLD
 #include <spawn.h>
 #include <sys/syscall.h> // For syscall numbers.
 
 namespace __llvm_libc {
 
 namespace {
 
 pid_t fork() {
   // TODO: Use only the clone syscall and use a sperate small stack in the child
   // to avoid duplicating the complete stack from the parent. A new stack will
   // be created on exec anyway so duplicating the full stack is unnecessary.
 #ifdef SYS_fork
   return __llvm_libc::syscall_impl(SYS_fork);
#endif
   // The open function is called as part of the child process' preparatory
   // steps. If an open fails, the child process just exits. So, unlike
   // the public open function, we do not need to set errno here.
   return cpp::nullopt;
 }
 
 void close(int fd) { __llvm_libc::syscall_impl(SYS_close, fd); }
 
+// We use dup3 if dup2 is not available, similar to our implementation of dup2
 bool dup2(int fd, int newfd) {
+#ifdef SYS_dup2
   long ret = __llvm_libc::syscall_impl(SYS_dup2, fd, newfd);
+#elif defined(SYS_dup3)
+  long ret = __llvm_libc::syscall_impl(SYS_dup3, fd, newfd, 0);
+#else
+#error "SYS_dup2 and SYS_dup3 not available for the target."
+#endif
   return ret < 0 ? false : true;
 }
 
 // All exits from child_process are error exits. So, we use a simple
 // exit implementation which exits with code 127.
 void exit() {
   for (;;) {
     __llvm_libc::syscall_impl(SYS_exit_group, 127);