On 10/12/2020 2:02 PM, Ken Brown via Cygwin-patches wrote:
> It looks to me like there's been a bug in the MSG_WAITALL support for
> AF_INET and AF_LOCAL sockets ever since that support was first
> introduced 13 years ago in commit 023a2fa7.  If I'm right, MSG_WAITALL
> has never worked.
> 
> This patch fixes it.  I'll push it in a few days if no one sees
> anything wrong with it.
> 
> In a followup email I'll show how I tested it.

Attached are slight variants of the server/client programs from Section 57.2 of 
Kerrisk's book, "The Linux Programming Interface".  The only essential 
difference is that I've changed the server program to (a) use a small buffer 
(size 10 instead of 100) and (b) use 'recv' with the MSG_WAITALL flag instead of 
'read'.  The 'recv' call shouldn't return until it reads 10 bytes.

To test, run waitall_sv in one terminal and waitall_cl in a second.  Type 
something in the second terminal (followed by RET), and it should be echoed in 
the first.  But because of the MSG_WAITALL flag, the echoing shouldn't occur 
until 10 bytes have been written.  For example, if I type "abcd<RET>" in the 
second terminal and then do it again, I should see the following:

# Terminal 2:
$ ./waitall_cl
abcd
abcd

# Terminal 1:
$ ./waitall_sv
abcd
abcd

Here the echoing in Terminal 1 shouldn't occur until I've typed both "abcd" 
lines in Terminal 2.

[Note that there is a newline character after each "abcd", so "abcd<RET>" is 5 
bytes long, and the two lines together are 10 bytes long.]

Before I apply my patch, each line typed in Terminal 2 is immediately echoed in 
Terminal 1.  After I apply the patch, the echoing doesn't occur until I've typed 
both lines.

Ken