Make splice atomic.

imports.md

@@ -389,6 +389,9 @@ let _ = this.check-write();         // eliding error handling
 <code>check-write</code> permitted length and the <code>len</code> provided to <code>splice</code></li>
 <li>calling <code>write</code> on the <a href="#output_stream"><code>output-stream</code></a> with that read data.</li>
 </ol>
+<p>This function behaves as-if these steps are performed atomically.
+I.e. if the read (in step 2) succeeds, but the write (in step 3) fails,
+future reads will first read from the buffer alread retrieved in step 2.</p>
 <p>Any error reported by the call to <code>check-write</code>, <code>read</code>, or
 <code>write</code> ends the splice and reports that error.</p>
 <p>This function returns the number of bytes transferred; it may be less

wit/streams.wit

@@ -235,6 +235,10 @@ interface streams {
         /// `check-write` permitted length and the `len` provided to `splice`
         /// 3. calling `write` on the `output-stream` with that read data.
         ///
+        /// This function behaves as-if these steps are performed atomically.
+        /// I.e. if the read (in step 2) succeeds, but the write (in step 3) fails,
+        /// future reads will first read from the buffer alread retrieved in step 2.
+        ///
         /// Any error reported by the call to `check-write`, `read`, or
         /// `write` ends the splice and reports that error.
         ///