[Support] Add sys::fs::is_tty() wrapper around isatty(3)

[abrachet]

Summary

Adds a wrapper in Support/Filesystem.h for determining if a file descriptor refers to a terminal.

This patch is needed by an upcoming patch which needs to know if stdin has redirected input or not.

[jhenderson]

Code change looks good, but one very weird thing I noticed is that your change in the header is in "FileSystem.h" but the definitions are in "Path.inc". This seems wrong, and should probably be fixed, possibly by adding FileSystem.inc?

Also, you should definitely get some more reviewers to look at this, as I don't want to be making the decision on that alone. Take a look at the commits history for the files you touched.

llvm/include/llvm/Support/FileSystem.h

640

What about a) if the file descriptor is invalid (doesn't refer to anything) or b) doesn't refer to a terminal (in other words, be explicit with the false as well as the true)?

[abrachet]

Already exists in sys::Process::StandardInIsUserInput.

llvm/include/llvm/Support/FileSystem.h

 ///
 /// @param path Input path.
 /// @param result Set to true if \a path exists, but is not a directory, regular
 ///               file, or a symlink, false if it does not. Undefined otherwise.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code is_other(const Twine &path, bool &result);
 
+/// Does FD refer to a terminal?
+///
+/// @param FD Input file descriptor.
+/// @returns true if the file descriptor refers to a terminal.
+bool is_tty(int FD);
+
 /// Get file status as if by POSIX stat().
 ///
 /// @param path Input path.
 /// @param result Set to the file status.
 /// @param follow When true, follows symlinks.  Otherwise, the symlink itself is
 ///               statted.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.

llvm/lib/Support/Unix/Path.inc

 }
 
 std::error_code status(int FD, file_status &Result) {
   struct stat Status;
   int StatRet = ::fstat(FD, &Status);
   return fillStatus(StatRet, Status, Result);
 }
 
+bool is_tty(int FD) {
+  return isatty(FD);
+}
+
 std::error_code setPermissions(const Twine &Path, perms Permissions) {
   SmallString<128> PathStorage;
   StringRef P = Path.toNullTerminatedStringRef(PathStorage);
 
   if (::chmod(P.begin(), Permissions))
     return std::error_code(errno, std::generic_category());
   return std::error_code();
 }

llvm/lib/Support/Windows/Path.inc

std::error_code status(const Twine &path, file_status &result, bool Follow) {
   return getStatus(h, result);
 }
 
 std::error_code status(int FD, file_status &Result) {
   HANDLE FileHandle = reinterpret_cast<HANDLE>(_get_osfhandle(FD));
   return getStatus(FileHandle, Result);
 }
 
+bool is_tty(int FD) {
+  return _isatty(FD);
+}
+
 std::error_code setPermissions(const Twine &Path, perms Permissions) {
   SmallVector<wchar_t, 128> PathUTF16;
   if (std::error_code EC = widenPath(Path, PathUTF16))
     return EC;
 
   DWORD Attributes = ::GetFileAttributesW(PathUTF16.begin());
   if (Attributes == INVALID_FILE_ATTRIBUTES)
     return mapWindowsError(GetLastError());