Merge recv/send into read/write.

Interesting bits:

 - End-of-stream (EOS, aka EOF) is indicated by returning the
  `$success_eos` errno value.

     - This differs from POSIX, where EOS is indicated by having `read`
       return 0, which is ambiguous in several cases. Of course, WASI
       libc will continue to provide POSIX semantics to C users.

     - This is a success condition being indicated with a non-zero errno
       value, which does not have precedent in POSIX.

 - `$fdstat`'s `$filetype` field is removed. The `$filetype` is still
   available through the `$filestat` type, though note that it's now
   protected by a rights flag. The goal is to start abstracting away more
   of the differences between files and sockets.

 - The `$peek` and `$waitall` flags are now available on `$read`, and can
   be used on files. This differs from POSIX.

 - POSIX says that `recv` and `send` fail if the stream is not a socket,
   but with `recv`/`send` merged into `read`/`write`, this restriction
   will no longer be automatically enforced. It could be manually enforced
   in WASI libc if needed, but in most cases, it's probably not important.

 - There is a new `$fd_line_oriented_terminal` right, to support
   `isatty`. I also anticipate expanding on this right in my upcoming
   terminal I/O proposal.

 - `$read`'s `$nread` result can be larger than the provided buffer to
   indicate a truncate message on a message-oriented socket (see also
   `$recv_data_truncated` in the current API, or `MSG_TRUNC` in POSIX
   `recv`. This avoids the need for a separate output value, but is
   it too surprising?

Out of scope for this proposal (but maybe in scope in the future!):

 - Streams vs datagrams - Having these merged is tricky, but this isn't
   new here; both `read` and `recv` already support both streams and
   datagrams.

 - Streams vs arrays - POSIX unifies the concept of a file as an array
   of bytes with the concept of file as a stream of data, which creates
   some tricky situations, but this also isn't new here.

 - Out-of-band messages and connectionless sockets aren't in WASI, and
   this proposal doesn't change that.

This implements WebAssembly#4.

phases/ephemeral/witx/typenames.witx

@@ -38,6 +38,8 @@
   (enum u16
     ;;; No error occurred. System call completed successfully.
     $success
+    ;;; End of stream was reached successfully.
+    $success_eos
     ;;; Argument list too long.
     $2big
     ;;; Permission denied.
@@ -201,7 +203,7 @@
     ;;; If `path_open` is set, includes the right to invoke
     ;;; `path_open` with `fdflags::dsync`.
     $fd_datasync
-    ;;; The right to invoke `fd_read` and `sock_recv`.
+    ;;; The right to invoke `fd_read`.
     ;;
     ;;; If `rights::fd_seek` is set, includes the right to invoke `fd_pread`.
     $fd_read
@@ -218,7 +220,7 @@
     ;;; remains unaltered (i.e., `whence::cur` with offset zero), or to
     ;;; invoke `fd_tell`.
     $fd_tell
-    ;;; The right to invoke `fd_write` and `sock_send`.
+    ;;; The right to invoke `fd_write`.
     ;;; If `rights::fd_seek` is set, includes the right to invoke `fd_pwrite`.
     $fd_write
     ;;; The right to invoke `fd_advise`.
@@ -271,8 +273,10 @@
     ;;; If `rights::fd_read` is set, includes the right to invoke `poll_oneoff` to subscribe to `eventtype::fd_read`.
     ;;; If `rights::fd_write` is set, includes the right to invoke `poll_oneoff` to subscribe to `eventtype::fd_write`.
     $poll_fd_readwrite
-    ;;; The right to invoke `sock_shutdown`.
-    $sock_shutdown
+    ;;; The right to invoke `fd_shutdown`.
+    $fd_shutdown
+    ;;; The output supports line-oriented terminal output.
+    $fd_line_oriented_terminal
   )
 )
 
@@ -408,8 +412,6 @@
 ;;; File descriptor attributes.
 (typename $fdstat
   (struct
-    ;;; File type.
-    (field $fs_filetype $filetype)
     ;;; File descriptor flags.
     (field $fs_flags $fdflags)
     ;;; Rights that apply to this file descriptor.
@@ -638,25 +640,23 @@
 ;;; Exit code generated by a process when exiting.
 (typename $exitcode u32)
 
-;;; Flags provided to `sock_recv`.
+;;; Flags provided to `fd_read`.
 (typename $riflags
   (flags u16
     ;;; Returns the message without removing it from the socket's receive queue.
-    $recv_peek
-    ;;; On byte-stream sockets, block until the full amount of data can be returned.
-    $recv_waitall
-  )
-)
+    ;;;
+    ;;; Note: This is similar to the `MSG_PEEK` flag in `recv` in POSIX.
+    $read_peek
 
-;;; Flags returned by `sock_recv`.
-(typename $roflags
-  (flags u16
-    ;;; Returned by `sock_recv`: Message data has been truncated.
-    $recv_data_truncated
+    ;;; On bytestream sockets, block until the full amount of data can be returned,
+    ;;; unless an error or disconnect occurs.
+    ;;;
+    ;;; Note: This is similar to the `MSG_WAITALL` flag in `recv` in POSIX.
+    $read_waitall
   )
 )
 
-;;; Flags provided to `sock_send`. As there are currently no flags
+;;; Flags provided to `fd_write`. As there are currently no flags
 ;;; defined, it must be set to zero.
 (typename $siflags u16)
 

phases/ephemeral/witx/wasi_ephemeral_fd.witx

@@ -170,13 +170,25 @@
   )
 
   ;;; Read from a file descriptor.
-  ;;; Note: This is similar to `readv` in POSIX.
+  ;;;
+  ;;; When reading from a datagram socket, returns `$msgsize` if a datagram
+  ;;; is larger than the provided buffer
+  ;;;
+  ;;; When the end of the file or stream is reached, this function returns
+  ;;; `$success_eos`.
+  ;;;
+  ;;; Note: This is similar to `readv` and `recv` in POSIX, except that a return
+  ;;; value of 0 does not necessarily indicate end of file/stream.
   (@interface func (export "read")
     (param $fd $fd)
     ;;; List of scatter/gather vectors to which to store data.
     (param $iovs $iovec_array)
+    ;;; Message flags.
+    (param $ri_flags $riflags)
     (result $error $errno)
-    ;;; The number of bytes read.
+    ;;; The number of bytes read. If this is greater than the total size of the
+    ;;; provided buffer, it indicates the length of a message on a message-based
+    ;;; socket, and data beyond the size of the buffer is discarded.
     (result $nread $size)
   )
 
@@ -249,13 +261,24 @@
   )
 
   ;;; Write to a file descriptor.
-  ;;; Note: This is similar to `writev` in POSIX.
+  ;;; Note: This is similar to `writev` and `send` in POSIX.
   (@interface func (export "write")
     (param $fd $fd)
     ;;; List of scatter/gather vectors from which to retrieve data.
     (param $iovs $ciovec_array)
+    ;;; Message flags.
+    (param $si_flags $siflags)
     (result $error $errno)
     ;;; The number of bytes written.
     (result $nwritten $size)
   )
+
+  ;;; Shut down socket send and receive channels.
+  ;;; Note: This is similar to `shutdown` in POSIX.
+  (@interface func (export "shutdown")
+    (param $fd $fd)
+    ;;; Which channels on the socket to shut down.
+    (param $how $sdflags)
+    (result $error $errno)
+  )
 )