File::dupCloseOnExec()

Summary:
Close-on-exec is an important feature to avoid leaking file descriptors to spawned processes.

This diff adds `File::dupCloseOnExec()` function which is equivalent to `File::dup` function, but sets close-on-exec flag where supported (i. e. everywhere except Windows).

Practically most users want to have `closeOnExec = true` by default, but we need to preserve backwards compatibility, thus this diff leaves `dup` function as is.

Reviewed By: yfeldblum

Differential Revision: D27495214

fbshipit-source-id: 540deb2bc6c8fec626a0120bc20c345950dc8af7

folly/File.cpp

@@ -80,6 +80,7 @@ File::~File() {
   checkFopenError(tmpFile, "tmpfile() failed");
   SCOPE_EXIT { fclose(tmpFile); };
 
+  // TODO(nga): consider setting close-on-exec for the resulting FD
   int fd = ::dup(fileno(tmpFile));
   checkUnixError(fd, "dup() failed");
 
@@ -114,6 +115,22 @@ File File::dup() const {
   return File();
 }
 
+File File::dupCloseOnExec() const {
+  if (fd_ != -1) {
+    int fd;
+#ifdef _WIN32
+    fd = ::dup(fd_);
+#else
+    fd = ::fcntl(fd_, F_DUPFD_CLOEXEC, 0);
+#endif
+    checkUnixError(fd, "dup() failed");
+
+    return File(fd, true);
+  }
+
+  return File();
+}
+
 void File::close() {
   if (!closeNoThrow()) {
     throwSystemError("close() failed");

folly/File.h

@@ -89,9 +89,20 @@ class File {
 
   /**
    * Duplicate file descriptor and return File that owns it.
+   *
+   * Duplicated file descriptor does not have close-on-exec flag set,
+   * so it is leaked to child processes. Consider using "dupCloseOnExec".
    */
   File dup() const;
 
+  /**
+   * Duplicate file descriptor and return File that owns it.
+   *
+   * This functions creates a descriptor with close-on-exec flag set
+   * (where supported, otherwise it is equivalent to "dup").
+   */
+  File dupCloseOnExec() const;
+
   /**
    * If we own the file descriptor, close the file and throw on error.
    * Otherwise, do nothing.

folly/test/FileTest.cpp

@@ -166,6 +166,26 @@ TEST(File, Truthy) {
   }
 }
 
+TEST(File, Dup) {
+  auto f = File::temporary();
+
+  auto d = f.dup();
+#ifndef _WIN32
+  EXPECT_EQ(::fcntl(d.fd(), F_GETFD, 0) & FD_CLOEXEC, 0);
+#endif
+  (void)d;
+}
+
+TEST(File, DupCloseOnExec) {
+  auto f = File::temporary();
+
+  auto d = f.dupCloseOnExec();
+#ifndef _WIN32
+  EXPECT_EQ(::fcntl(d.fd(), F_GETFD, 0) & FD_CLOEXEC, FD_CLOEXEC);
+#endif
+  (void)d;
+}
+
 TEST(File, HelperCtor) {
   TempDir tmpd;
   auto tmpf = tmpd.path / "foobar.txt";