Allow stream.{read,write}s of length 0 to query/signal readiness

design/mvp/Async.md

@@ -325,7 +325,11 @@ These built-ins can either return immediately if >0 elements were able to be
 written or read immediately (without blocking) or return a sentinel "blocked"
 value indicating that the read or write will execute concurrently. The readable
 and writable ends of streams and futures can then be [waited](#waiting) on to
-make progress.
+make progress. Notification of progress signals *completion* of a read or write
+(i.e., the bytes have already been copied into the buffer). Additionally,
+*readiness* (to perform a read or write in the future) can be queried and
+signalled by performing a `0`-length read or write (see the [Stream State]
+section in the Canonical ABI explainer for details).
 
 The `T` element type of streams and futures is optional, such that `future` and
 `stream` can be written in WIT without a trailing `<T>`. In this case, the

design/mvp/CanonicalABI.md

@@ -363,8 +363,8 @@ class BufferGuestImpl(Buffer):
   length: int
 
   def __init__(self, t, cx, ptr, length):
-    trap_if(length == 0 or length > Buffer.MAX_LENGTH)
-    if t:
+    trap_if(length > Buffer.MAX_LENGTH)
+    if t and length > 0:
       trap_if(ptr != align_to(ptr, alignment(t)))
       trap_if(ptr + length * elem_size(t) > len(cx.opts.memory))
     self.cx = cx
@@ -1178,14 +1178,34 @@ but in the opposite direction. Both are implemented by a single underlying
       return 'blocked'
     else:
       ncopy = min(src.remain(), dst.remain())
-      assert(ncopy > 0)
-      dst.write(src.read(ncopy))
-      if self.pending_buffer.remain() > 0:
-        self.pending_on_partial_copy(self.reset_pending)
+      if ncopy > 0:
+        dst.write(src.read(ncopy))
+        if self.pending_buffer.remain() > 0:
+          self.pending_on_partial_copy(self.reset_pending)
+        else:
+          self.reset_and_notify_pending()
+        return 'done'
       else:
-        self.reset_and_notify_pending()
-      return 'done'
-```
+        if self.pending_buffer.remain() == 0:
+          self.reset_and_notify_pending()
+        if buffer.remain() == 0:
+          return 'done'
+        else:
+          self.pending_buffer = buffer
+          self.pending_on_partial_copy = on_partial_copy
+          self.pending_on_copy_done = on_copy_done
+          return 'blocked'
+```
+The meaning of a `read` or `write` when the length is `0` is that the caller
+wants to know when the other side is "ready". When a non-`0`-length `read` or
+`write` rendezvous with a `0`-length `write` or `read`, only the `0`-length
+operation completes, keeping the non-`0`-length pending and ready for a future
+rendezvous. In a rendezvous where *both* the `read` and `write` are `0`-length,
+both operations complete. Thus, "readiness" does not guarantee "the next
+operation will not block" and performing a `0`-length `read` or `write` doesn't
+just *query* for readiness but also *signals* readiness to the other side.
+Consequently, components should always follow a successful `0`-length `read` or
+`write` with a non-`0`-length `read` or `write`.
 
 Given the above, we can define the `{Readable,Writable}StreamEnd` classes that
 are actually stored in the `waitables` table. The classes are almost entirely

design/mvp/canonical-abi/definitions.py

@@ -311,8 +311,8 @@ class BufferGuestImpl(Buffer):
   length: int
 
   def __init__(self, t, cx, ptr, length):
-    trap_if(length == 0 or length > Buffer.MAX_LENGTH)
-    if t:
+    trap_if(length > Buffer.MAX_LENGTH)
+    if t and length > 0:
       trap_if(ptr != align_to(ptr, alignment(t)))
       trap_if(ptr + length * elem_size(t) > len(cx.opts.memory))
     self.cx = cx
@@ -702,13 +702,23 @@ def copy(self, buffer, on_partial_copy, on_copy_done, src, dst):
       return 'blocked'
     else:
       ncopy = min(src.remain(), dst.remain())
-      assert(ncopy > 0)
-      dst.write(src.read(ncopy))
-      if self.pending_buffer.remain() > 0:
-        self.pending_on_partial_copy(self.reset_pending)
+      if ncopy > 0:
+        dst.write(src.read(ncopy))
+        if self.pending_buffer.remain() > 0:
+          self.pending_on_partial_copy(self.reset_pending)
+        else:
+          self.reset_and_notify_pending()
+        return 'done'
       else:
-        self.reset_and_notify_pending()
-      return 'done'
+        if self.pending_buffer.remain() == 0:
+          self.reset_and_notify_pending()
+        if buffer.remain() == 0:
+          return 'done'
+        else:
+          self.pending_buffer = buffer
+          self.pending_on_partial_copy = on_partial_copy
+          self.pending_on_copy_done = on_copy_done
+          return 'blocked'
 
 class StreamEnd(Waitable):
   stream: ReadableStream

design/mvp/canonical-abi/run_tests.py

@@ -1457,8 +1457,19 @@ async def core_func1(task, args):
     assert(mem1[retp+0] == wsi)
     assert(mem1[retp+4] == 4)
 
+    [ret] = await canon_stream_write(U8Type(), opts1, task, wsi, 12345, 0)
+    assert(ret == definitions.BLOCKED)
+
     fut4.set_result(None)
 
+    [event] = await canon_waitable_set_wait(False, mem1, task, seti, retp)
+    assert(event == EventCode.STREAM_WRITE)
+    assert(mem1[retp+0] == wsi)
+    assert(mem1[retp+4] == 0)
+
+    [ret] = await canon_stream_write(U8Type(), opts1, task, wsi, 12345, 0)
+    assert(ret == 0)
+
     [errctxi] = await canon_error_context_new(opts1, task, 0, 0)
     [] = await canon_stream_close_writable(U8Type(), task, wsi, errctxi)
     [] = await canon_waitable_set_drop(task, seti)
@@ -1498,6 +1509,9 @@ async def core_func2(task, args):
     fut2.set_result(None)
     await task.on_block(fut3)
 
+    [ret] = await canon_stream_read(U8Type(), opts2, task, rsi, 12345, 0)
+    assert(ret == 0)
+
     mem2[0:8] = bytes(8)
     [ret] = await canon_stream_read(U8Type(), opts2, task, rsi, 0, 2)
     assert(ret == 2)
@@ -1508,9 +1522,19 @@ async def core_func2(task, args):
 
     await task.on_block(fut4)
 
-    [ret] = await canon_stream_read(U8Type(), opts2, task, rsi, 0, 2)
+    [ret] = await canon_stream_read(U8Type(), opts2, task, rsi, 12345, 0)
+    assert(ret == 0)
+
+    [ret] = await canon_stream_read(U8Type(), opts2, task, rsi, 12345, 0)
+    assert(ret == definitions.BLOCKED)
+
+    [event] = await canon_waitable_set_wait(False, mem2, task, seti, retp)
+    assert(event == EventCode.STREAM_READ)
+    assert(mem2[retp+0] == rsi)
+    p2 = int.from_bytes(mem2[retp+4 : retp+8], 'little', signed=False)
     errctxi = 1
-    assert(ret == (definitions.CLOSED | errctxi))
+    assert(p2 == (definitions.CLOSED | errctxi))
+
     [] = await canon_stream_close_readable(U8Type(), task, rsi)
     [] = await canon_waitable_set_drop(task, seti)
     [] = await canon_error_context_debug_message(opts2, task, errctxi, 0)