add HH\Lib\_Private\_OS\dup()

Summary:
refs hhvm/hsl#176

This part of enabling a pattern of:
```
list($stderr_parent, $stderr_child) = OS\socketpair(); // yep, stderr is read-write
$stdin_child = OS\dup($stderr_child);
OS\shutdown($stdin_child, OS\SHUT_WR);
$stdout_child = OS\dup($stderr_child);
OS\shutdown($stdout_child, OS\SHUT_RD);
```

`dup2()` is not necessary as we support explicitly remapping FDs when creating subprocesses

`dup2()` is not desirable as:
- Hack programs do not have insights to the real FD numbers. Especially in CLI server mode, STDIN/STDOUT/STDERR may not be on the canonical STDIN_FILENO/STDOUT_FILENO/STDERR_FILENO
- Given that, there's a large risk of accidentally creating cross-request issues with `dup2()`

Reviewed By: shayne-fletcher

Differential Revision: D33166096

fbshipit-source-id: 41339a9a33c854982009086f3efdfc35277715c0

hphp/hack/hhi/hsl/ext_hsl_os_fds.hhi

@@ -157,6 +157,8 @@ function write(FileDescriptor $fd, string $data): int;
 
 function close(FileDescriptor $fd): void;
 
+function dup(FileDescriptor $fd): FileDescriptor;
+
 function pipe(): (FileDescriptor, FileDescriptor);
 
 const int AF_UNSPEC = 0;

hphp/runtime/ext/hsl/ext_hsl_os.cpp

@@ -82,10 +82,11 @@ void throw_errno_exception(int number, const String& message = String()) {
 }
 
 template<class T>
-void throw_errno_if_minus_one(T var) {
+T&& throw_errno_if_minus_one(T&& var) {
   if (var == -1) {
     throw_errno_exception(errno);
   }
+  return std::forward<T>(var);
 }
 
 template<class TRet, class ...Args, class TFn>
@@ -587,6 +588,13 @@ void HHVM_FUNCTION(HSL_os_close, const Object& obj) {
   HSLFileDescriptor::get(obj)->close();
 }
 
+Object HHVM_FUNCTION(HSL_os_dup, const Object& fd_wrapper) {
+  // Not using CLI client/server proxying as `dup()` preserves SO_PEERCRED
+  auto in = HSLFileDescriptor::fd(fd_wrapper);
+  auto out = throw_errno_if_minus_one(::dup(in));
+  return HSLFileDescriptor::newInstance(out);
+}
+
 Array HHVM_FUNCTION(HSL_os_pipe) {
   int fds[2];
   throw_errno_if_minus_one(retry_on_eintr(-1, ::pipe, fds));
@@ -1214,6 +1222,7 @@ struct OSExtension final : Extension {
     HHVM_FALIAS(HH\\Lib\\_Private\\_OS\\read, HSL_os_read);
     HHVM_FALIAS(HH\\Lib\\_Private\\_OS\\write, HSL_os_write);
     HHVM_FALIAS(HH\\Lib\\_Private\\_OS\\close, HSL_os_close);
+    HHVM_FALIAS(HH\\Lib\\_Private\\_OS\\dup, HSL_os_dup);
 
     HHVM_FALIAS(HH\\Lib\\_Private\\_OS\\request_stdio_fd, HSL_os_request_stdio_fd);
     HHVM_RC_INT(HH\\Lib\\_Private\\_OS\\STDIN_FILENO, STDIN_FILENO);

hphp/runtime/ext/hsl/ext_hsl_os.php

@@ -55,6 +55,9 @@ function write(FileDescriptor $fd, string $data): int;
 <<__Native>>
 function close(FileDescriptor $fd): void;
 
+<<__Native>>
+function dup(FileDescriptor $fd): FileDescriptor;
+
 <<__Native>>
 function pipe(): varray<FileDescriptor>;
 

hphp/test/slow/ext_hsl/os_dup.php

@@ -0,0 +1,18 @@
+<?hh // strict
+
+use namespace HH\Lib\_Private\_OS;
+
+<<__EntryPoint>>
+function main(): void {
+  list ($r, $w1) = _OS\pipe();
+  $w2 = _OS\dup($w1);
+  _OS\write($w2, "Hello, ");
+  _OS\write($w1, "world.\n");
+
+  var_dump(_OS\read($r, 0xff));
+  _OS\close($w1);
+  _OS\write($w2, "Hello again.\n");
+  _OS\close($w2);
+  var_dump(_OS\read($r, 0xff));
+  var_dump(_OS\read($r, 0xff));
+}

hphp/test/slow/ext_hsl/os_dup.php.expect

@@ -0,0 +1,5 @@
+string(14) "Hello, world.
+"
+string(13) "Hello again.
+"
+string(0) ""