Document atomicity of write and pwrite, and technical fix about POSIX (#257)

* Techinically speaking, preadv and pwritev are not in POSIX.

Ref: https://pubs.opengroup.org/onlinepubs/9699919799/idx/ip.html

* Document about atomicity of `write` and `pwrite`

Related thread of ZULIP: https://bytecodealliance.zulipchat.com/#narrow/stream/219900-wasi/topic/Why.20is.20.60iovecs.60.20an.20array.20of.20strings.3F

The thread refers atomicity of `read`, but I couldn't make a good expression
for the atomicity of `read`.
So I add document only in `write` and `pwrite`.

* Limit the scope of atomicity

The related POSIX document refers only to threads. https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_07

So the atomicity is limited in the process

* Fix test

* Fix after review

#257 (comment)

The atomicity is guaranteed only for regular files or symbolic links (only symbolic links for a regular files, right?)

Delete expressions indicating 'all or nothing' to reflect the actual behavior of POSIX's`write` etc.

* Fix after review

#257 (comment)

Delete the sentences related to 'all data' completely

Don't refer to symlink

* Fix after review

#257 (review)

Author: YAMAMOTO Yuji

phases/ephemeral/docs.md

@@ -1415,7 +1415,7 @@ The permissions associated with the file.
 
 #### <a href="#pread" name="pread"></a> `pread(fd: fd, iovs: iovec_array, offset: filesize) -> (errno, size)`
 Read from a file descriptor, without using and updating the file descriptor's offset.
-Note: This is similar to `preadv` in POSIX.
+Note: This is similar to `preadv` in Linux (and other Unix-es).
 
 ##### Params
 - <a href="#pread.fd" name="pread.fd"></a> `fd`: [`fd`](#fd)
@@ -1469,7 +1469,11 @@ A buffer into which to write the preopened directory name.
 
 #### <a href="#pwrite" name="pwrite"></a> `pwrite(fd: fd, iovs: ciovec_array, offset: filesize) -> (errno, size)`
 Write to a file descriptor, without using and updating the file descriptor's offset.
-Note: This is similar to `pwritev` in POSIX.
+Note: This is similar to `pwritev` in Linux (and other Unix-es).
+
+Like Linux (and other Unix-es), any calls of [`pwrite`](#pwrite) (and other
+functions to read or write) for a regular file by other threads in the
+WASI process should not be interleaved while [`pwrite`](#pwrite) is executed.
 
 ##### Params
 - <a href="#pwrite.fd" name="pwrite.fd"></a> `fd`: [`fd`](#fd)
@@ -1616,6 +1620,10 @@ The current offset of the file descriptor, relative to the start of the file.
 Write to a file descriptor.
 Note: This is similar to `writev` in POSIX.
 
+Like POSIX, any calls of [`write`](#write) (and other functions to read or write)
+for a regular file by other threads in the WASI process should not be
+interleaved while [`write`](#write) is executed.
+
 ##### Params
 - <a href="#write.fd" name="write.fd"></a> `fd`: [`fd`](#fd)
 

phases/ephemeral/witx/wasi_ephemeral_fd.witx

@@ -127,7 +127,7 @@
   )
 
   ;;; Read from a file descriptor, without using and updating the file descriptor's offset.
-  ;;; Note: This is similar to `preadv` in POSIX.
+  ;;; Note: This is similar to `preadv` in Linux (and other Unix-es).
   (@interface func (export "pread")
     (param $fd $fd)
     ;;; List of scatter/gather vectors in which to store data.
@@ -157,7 +157,11 @@
   )
 
   ;;; Write to a file descriptor, without using and updating the file descriptor's offset.
-  ;;; Note: This is similar to `pwritev` in POSIX.
+  ;;; Note: This is similar to `pwritev` in Linux (and other Unix-es).
+  ;;;
+  ;;; Like Linux (and other Unix-es), any calls of `pwrite` (and other
+  ;;; functions to read or write) for a regular file by other threads in the
+  ;;; WASI process should not be interleaved while `pwrite` is executed.
   (@interface func (export "pwrite")
     (param $fd $fd)
     ;;; List of scatter/gather vectors from which to retrieve data.
@@ -250,6 +254,10 @@
 
   ;;; Write to a file descriptor.
   ;;; Note: This is similar to `writev` in POSIX.
+  ;;;
+  ;;; Like POSIX, any calls of `write` (and other functions to read or write)
+  ;;; for a regular file by other threads in the WASI process should not be
+  ;;; interleaved while `write` is executed.
   (@interface func (export "write")
     (param $fd $fd)
     ;;; List of scatter/gather vectors from which to retrieve data.