Fix dup

This fixes two problems with the `dup` system calls:
1. `dup` expects that every file descriptor has a corresponding file (so pipes and (#14640)
   files that have been unlinked (#15012) cannot be duplicated ).
2. The dup'd file descriptor does not share flags and position with the original file desciptor.

This is a simplification of an upstream pull request that would fix this problem.
https://github.com/emscripten-core/emscripten/pull/9396/files
This patch is simpler than the upstream one but leaves NODERAWFS broken.

Author: Hood

src/library_fs.js

@@ -400,7 +400,7 @@ FS.staticInit();` +
     // SOCKFS is completed.
     createStream: (stream, fd_start, fd_end) => {
       if (!FS.FSStream) {
-        FS.FSStream = /** @constructor */ function(){};
+        FS.FSStream = /** @constructor */ function(){ this.shared = {}; };
         FS.FSStream.prototype = {
           object: {
             get: function() { return this.node; },
@@ -414,7 +414,11 @@ FS.staticInit();` +
           },
           isAppend: {
             get: function() { return (this.flags & {{{ cDefine('O_APPEND') }}}); }
-          }
+          },
+          get flags() { return this.shared.flags; },
+          set flags(value) { this.shared.flags = value; },
+          get position() { return this.shared.position; },
+          set position(value) { this.shared.position = value; },
         };
       }
       // clone it, so we can return an instance of FSStream

src/library_syscall.js

@@ -131,10 +131,10 @@ var SyscallsLibrary = {
       }
       return 0;
     },
-    doDup: function(path, flags, suggestFD) {
+    doDup: function(stream, flags, suggestFD) {
       var suggest = FS.getStream(suggestFD);
       if (suggest) FS.close(suggest);
-      return FS.open(path, flags, 0, suggestFD, suggestFD).fd;
+      return FS.createStream(stream, suggestFD, suggestFD).fd;
     },
     doReadv: function(stream, iov, iovcnt, offset) {
       var ret = 0;
@@ -292,7 +292,7 @@ var SyscallsLibrary = {
   },
   __syscall_dup: function(fd) {
     var old = SYSCALLS.getStreamFromFD(fd);
-    return FS.open(old.path, old.flags, 0).fd;
+    return FS.createStream(old, 0).fd;
   },
   __syscall_pipe__deps: ['$PIPEFS'],
   __syscall_pipe: function(fdPtr) {
@@ -821,7 +821,7 @@ var SyscallsLibrary = {
           return -{{{ cDefine('EINVAL') }}};
         }
         var newStream;
-        newStream = FS.open(stream.path, stream.flags, 0, arg);
+        newStream = FS.createStream(stream, arg);
         return newStream.fd;
       }
       case {{{ cDefine('F_GETFD') }}}:
@@ -1036,7 +1036,7 @@ var SyscallsLibrary = {
     assert(!flags);
 #endif
     if (old.fd === suggestFD) return -{{{ cDefine('EINVAL') }}};
-    return SYSCALLS.doDup(old.path, old.flags, suggestFD);
+    return SYSCALLS.doDup(old, suggestFD);
   },
 };