io.write returns number of written bytes on error

testes/files.lua

@@ -696,6 +696,37 @@ do
 end
 
 
+if T and T.nonblock then
+  print("testing failed write")
+
+  -- unable to write anything to /dev/full
+  local f = io.open("/dev/full", "w")
+  assert(f:setvbuf("no"))
+  local _, _, err, count = f:write("abcd")
+  assert(err > 0 and count == 0)
+  assert(f:close())
+
+  -- receiver will read a "few" bytes (enough to empty a large buffer)
+  local receiver = [[
+    lua -e 'assert(io.stdin:setvbuf("no")); assert(#io.read(1e4) == 1e4)' ]]
+
+  local f = io.popen(receiver, "w")
+  assert(f:setvbuf("no"))
+  T.nonblock(f)
+
+  -- able to write a few bytes
+  assert(f:write(string.rep("a", 1e2)))
+
+  -- Unable to write more bytes than the pipe buffer supports.
+  -- (In Linux, the pipe buffer size is 64K (2^16). Posix requires at
+  -- least 512 bytes.)
+  local _, _, err, count = f:write("abcd", string.rep("a", 2^17))
+  assert(err > 0 and count >= 512 and count < 2^17)
+
+  assert(f:close())
+end
+
+
 if not _soft then
   print("testing large files (> BUFSIZ)")
   io.output(file)