[libc] Add a simple implementation of the posix_spawn function.

The implementation currently ignores all spawn attributes. Support for
them will be added in future changes.

A simple allocator for integration tests has been added so that the
integration test for posix_spawn can use the
posix_spawn_file_actions_add* functions.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D135752

4 files changed, 187 insertions, 0 deletions

diff --git a/libc/src/spawn/CMakeLists.txt b/libc/src/spawn/CMakeLists.txt
index 95a0c7436d26..fd929fa0cee1 100644
--- a/libc/src/spawn/CMakeLists.txt
+++ b/libc/src/spawn/CMakeLists.txt
@@ -1,3 +1,7 @@
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+endif()
+
 add_header_library(
   file_actions
   HDRS
@@ -63,3 +67,10 @@ add_entrypoint_object(
     libc.include.errno
     libc.include.spawn
 )
+
+add_entrypoint_object(
+  posix_spawn
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.posix_spawn
+)
diff --git a/libc/src/spawn/linux/CMakeLists.txt b/libc/src/spawn/linux/CMakeLists.txt
new file mode 100644
index 000000000000..9bd3ac50415b
--- /dev/null
+++ b/libc/src/spawn/linux/CMakeLists.txt
@@ -0,0 +1,14 @@
+add_entrypoint_object(
+  posix_spawn
+  SRCS
+    posix_spawn.cpp
+  HDRS
+    ../posix_spawn.h
+  DEPENDS
+    libc.include.fcntl
+    libc.include.spawn
+    libc.include.sys_syscall
+    libc.src.__support.CPP.optional
+    libc.src.__support.OSUtil.osutil
+    libc.src.spawn.file_actions
+)
diff --git a/libc/src/spawn/linux/posix_spawn.cpp b/libc/src/spawn/linux/posix_spawn.cpp
new file mode 100644
index 000000000000..2f8287238ec8
--- /dev/null
+++ b/libc/src/spawn/linux/posix_spawn.cpp
@@ -0,0 +1,139 @@
+//===-- 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 <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);
+#elif defined(SYS_clone)
+  return __llvm_libc::syscall_impl(SYS_clone, SIGCHLD, 0);
+#else
+#error "SYS_fork or SYS_clone not available."
+#endif
+}
+
+cpp::optional<int> open(const char *path, int oflags, mode_t mode) {
+#ifdef SYS_open
+  int fd = __llvm_libc::syscall_impl(SYS_open, path, oflags, mode);
+#else
+  int fd = __llvm_libc::syscall_impl(SYS_openat, AT_FDCWD, path, oflags, mode);
+#endif
+  if (fd > 0)
+    return fd;
+  // 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); }
+
+bool dup2(int fd, int newfd) {
+  long ret = __llvm_libc::syscall_impl(SYS_dup2, fd, newfd);
+  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);
+    __llvm_libc::syscall_impl(SYS_exit, 127);
+  }
+}
+
+void child_process(const char *__restrict path,
+                   const posix_spawn_file_actions_t *file_actions,
+                   const posix_spawnattr_t *__restrict, // For now unused
+                   char *const *__restrict argv, char *const *__restrict envp) {
+  // TODO: In the code below, the child_process just exits on error during
+  // processing |file_actions| and |attr|. The correct way would be to exit
+  // after conveying the information about the failure to the parent process
+  // (via a pipe for example).
+  // TODO: Handle |attr|.
+
+  if (file_actions != nullptr) {
+    auto *act = reinterpret_cast<BaseSpawnFileAction *>(file_actions->__front);
+    while (act != nullptr) {
+      switch (act->type) {
+      case BaseSpawnFileAction::OPEN: {
+        auto *open_act = reinterpret_cast<SpawnFileOpenAction *>(act);
+        auto fd = open(open_act->path, open_act->oflag, open_act->mode);
+        if (!fd)
+          exit();
+        int actual_fd = *fd;
+        if (actual_fd != open_act->fd) {
+          bool dup2_result = dup2(actual_fd, open_act->fd);
+          close(actual_fd); // The old fd is not needed anymore.
+          if (!dup2_result)
+            exit();
+        }
+        break;
+      }
+      case BaseSpawnFileAction::CLOSE: {
+        auto *close_act = reinterpret_cast<SpawnFileCloseAction *>(act);
+        close(close_act->fd);
+        break;
+      }
+      case BaseSpawnFileAction::DUP2: {
+        auto *dup2_act = reinterpret_cast<SpawnFileDup2Action *>(act);
+        if (!dup2(dup2_act->fd, dup2_act->newfd))
+          exit();
+        break;
+      }
+      }
+      act = act->next;
+    }
+  }
+
+  if (__llvm_libc::syscall_impl(SYS_execve, path, argv, envp) < 0)
+    exit();
+}
+
+} // anonymous namespace
+
+LLVM_LIBC_FUNCTION(int, posix_spawn,
+                   (pid_t *__restrict pid, const char *__restrict path,
+                    const posix_spawn_file_actions_t *file_actions,
+                    const posix_spawnattr_t *__restrict attr,
+                    char *const *__restrict argv,
+                    char *const *__restrict envp)) {
+  pid_t cpid = fork();
+  if (cpid == 0)
+    child_process(path, file_actions, attr, argv, envp);
+  else if (cpid < 0)
+    return -cpid;
+
+  if (pid != nullptr)
+    *pid = cpid;
+
+  // TODO: Before returning, one should wait for the child_process to startup
+  // successfully. For now, we will just return. Future changes will add proper
+  // wait (using pipes for example).
+
+  return 0;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/src/spawn/posix_spawn.h b/libc/src/spawn/posix_spawn.h
new file mode 100644
index 000000000000..721462701b25
--- /dev/null
+++ b/libc/src/spawn/posix_spawn.h
@@ -0,0 +1,23 @@
+//===-- Implementation header for posix_spawn -------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SPAWN_POSIX_SPAWN_H
+#define LLVM_LIBC_SRC_SPAWN_POSIX_SPAWN_H
+
+#include <spawn.h>
+
+namespace __llvm_libc {
+
+int posix_spawn(pid_t *__restrict pid, const char *__restrict path,
+                const posix_spawn_file_actions_t *file_actions,
+                const posix_spawnattr_t *__restrict attr,
+                char *const *__restrict argv, char *const *__restrict envp);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SPAWN_POSIX_SPAWN_H