fcntl DUPFD doesn't work on pipes

[hoodmane]

There is a problem in the implementation of DUPFD for pipes. The implementation assumes that stream.path is a valid path. However, for pipes, stream.path looks like pipe[n].
The issue is here:

emscripten/src/library_syscall.js

Line 1111 in fea2ac8

newStream = FS.open(stream.path, stream.flags, 0, arg);

I think if instead it said:

newStream=FS.createStream(stream, arg);

that should fix the problem? Or maybe it should say Object.assign({}, stream) instead of stream as the argument? This seems less brittle to me than the current approach.

[hoodmane]

There's another bug here which is that instead of 0, arg the line quoted above should just have arg:

newStream = FS.open(stream.path, stream.flags, arg); 

From man fcntl:

Duplicate the file descriptor fd using the lowest-numbered available file descriptor greater than or equal to arg

Existing behavior uses the lowest-numbered available file descriptor less than arg.