Subprocess fd-action to keep open

Summary:
A way to ask `Subprocess` to keep a specific fd open in child process, even if it might be marked cloexec in the parent. Two options:
* The value is the new constant `NO_CLOEXEC`.
* The value is equal to the fd key. As if doing a `dup2` of the key, which is a no-op, but with the added behavior of removing cloexec.

Reviewed By: dmm-fb

Differential Revision: D70514691

fbshipit-source-id: 8065c2de654a9f52e5fe47b94369647bdcba880a

Author: yfeldblum

folly/Subprocess.cpp

@@ -790,10 +790,20 @@ int Subprocess::prepareChild(SpawnRawArgs const& args) {
         return errno;
       }
       detail::subprocess_libc::close(devNull);
-    } else if (p.second != p.first) {
+    } else if (p.second != p.first && p.second != NO_CLOEXEC) {
       if (detail::subprocess_libc::dup2(p.second, p.first) == -1) {
         return errno;
       }
+    } else if (p.second == p.first || p.second == NO_CLOEXEC) {
+      int flags = detail::subprocess_libc::fcntl(p.first, F_GETFD);
+      if (flags == -1) {
+        return errno;
+      }
+      if (int newflags = flags & ~FD_CLOEXEC; newflags != flags) {
+        if (detail::subprocess_libc::fcntl(p.first, F_SETFD, newflags) == -1) {
+          return errno;
+        }
+      }
     }
   }
 

folly/Subprocess.h

@@ -299,6 +299,7 @@ class Subprocess {
   static const int PIPE_IN = -3;
   static const int PIPE_OUT = -4;
   static const int DEV_NULL = -5;
+  static const int NO_CLOEXEC = -6;
 
   /**
    * See Subprocess::Options::dangerousPostForkPreExecCallback() for usage.

folly/test/SubprocessTest.cpp

@@ -897,3 +897,26 @@ TEST(CloseOtherDescriptorsSubprocessTest, ClosesFileDescriptors) {
   EXPECT_EQ("0\n1\n2\n3\n", p.first);
   proc.wait();
 }
+
+TEST(KeepFileOpenSubprocessTest, KeepsFileOpen) {
+  auto f0 = folly::File{"/dev/null"};
+  auto f1 = f0.dup();
+  auto f2 = f0.dup();
+  auto f3 = f0.dup();
+
+  f0.close(); // make space for fd 3, for ls to open /proc/self/fd
+
+  auto options =
+      Subprocess::Options()
+          .closeOtherFds()
+          .pipeStdout()
+          .fd(f1.fd(), Subprocess::NO_CLOEXEC)
+          .fd(f2.fd(), f2.fd());
+  Subprocess proc(
+      std::vector<std::string>{"/bin/ls", "/proc/self/fd"}, options);
+  auto p = proc.communicate();
+  proc.wait();
+  int fds[] = {0, 1, 2, 3, f1.fd(), f2.fd()};
+  std::sort(std::begin(fds), std::end(fds));
+  EXPECT_EQ(fmt::format("{}\n", fmt::join(fds, "\n")), p.first);
+}