<spawn.h>: add posix_spawn_file_actions_addchdir_np()/posix_spawn_file_actions_addfchdir_np().

The recent header nullability additions and the corresponding source
cleanup made me notice that we're missing a couple of actions that most
of the other implementations have. They've also been added to the _next_
revision of POSIX, unchanged except for the removal of the `_np` suffix.

They're trivial to implement, the testing is quite simple too, and
if they're going to be in POSIX soon, having them accessible in older
versions of Android via __RENAME() seems useful. (No-one else has shipped
the POSIX names yet.)

Bug: http://b/152414297
Test: treehugger
Change-Id: I0d2a1e47fbd2e826cff9c45038928aa1b6fcce59
diff --git a/libc/bionic/spawn.cpp b/libc/bionic/spawn.cpp
index 5cf95d8..5d76f77 100644
--- a/libc/bionic/spawn.cpp
+++ b/libc/bionic/spawn.cpp
@@ -68,7 +68,9 @@
 enum Action {
   kOpen,
   kClose,
-  kDup2
+  kDup2,
+  kChdir,
+  kFchdir,
 };
 
 struct __posix_spawn_file_action {
@@ -93,6 +95,10 @@
     } else if (what == kClose) {
       // Failure to close is ignored.
       close(fd);
+    } else if (what == kChdir) {
+      if (chdir(path) == -1) _exit(127);
+    } else if (what == kFchdir) {
+      if (fchdir(fd) == -1) _exit(127);
     } else {
       // It's a dup2.
       if (fd == new_fd) {
@@ -340,7 +346,7 @@
   if (action == nullptr) return errno;
 
   action->next = nullptr;
-  if (what == kOpen) {
+  if (what == kOpen || what == kChdir) {
     action->path = strdup(path);
     if (action->path == nullptr) {
       free(action);
@@ -380,3 +386,12 @@
   if (fd < 0 || new_fd < 0) return EBADF;
   return posix_spawn_add_file_action(actions, kDup2, fd, new_fd, nullptr, 0, 0);
 }
+
+int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t* actions, const char* path) {
+  return posix_spawn_add_file_action(actions, kChdir, -1, -1, path, 0, 0);
+}
+
+int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t* actions, int fd) {
+  if (fd < 0) return EBADF;
+  return posix_spawn_add_file_action(actions, kFchdir, fd, -1, nullptr, 0, 0);
+}
diff --git a/libc/include/spawn.h b/libc/include/spawn.h
index 8bb34d0..6c34b98 100644
--- a/libc/include/spawn.h
+++ b/libc/include/spawn.h
@@ -87,6 +87,9 @@
 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd) __INTRODUCED_IN(28);
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd, int __new_fd) __INTRODUCED_IN(28);
 
+int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, const char* _Nonnull __path) __INTRODUCED_IN(34);
+int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd) __INTRODUCED_IN(34);
+
 __END_DECLS
 
 #endif
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 4a4e607..c75b13a 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1580,6 +1580,8 @@
     close_range;
     copy_file_range;
     memset_explicit;
+    posix_spawn_file_actions_addchdir_np;
+    posix_spawn_file_actions_addfchdir_np;
 } LIBC_T;
 
 LIBC_PRIVATE {