bug#62990: 30.0.50; UDP server closes connection upon receiving an empty packet

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Wed, 26 Apr 2023 13:38:47 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Robert Pluim <rpluim@gmail.com>
    >> Cc: 62990@debbugs.gnu.org,  mail@vasilij.de
    >> Date: Wed, 26 Apr 2023 12:35:21 +0200
    >> 
    >> >>>>> On Wed, 26 Apr 2023 13:30:32 +0300, Eli Zaretskii <eliz@gnu.org> said:
    >> 
    Eli> Do we want some variable to get back the old behavior, in case some
    Eli> application out there depends on that?
    >> 
    >> For which one?

    Eli> The 2nd one.

I guess we could, but if the application asks us to send an empty
packet we should do it.

    >> Does it help if I say that [1] is a security fix? 😜

    Eli> No.  But it will help if you convince me that it's a bug-fix.

It transforms this:

    recvfrom(5, "hello", 4096, 0, {sa_family=AF_INET, sin_port=htons(56845), sin_addr=inet_addr("127.0.0.1")}, [16]) = 5
    Received string (5): "hello"
    recvfrom(5, "", 4096, 0, {sa_family=AF_INET, sin_port=htons(56845), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
    Received event: "connection broken by remote peer

into this:

    recvfrom(5, "hello", 4096, 0, {sa_family=AF_INET, sin_port=htons(55486), sin_addr=inet_addr("127.0.0.1")}, [16]) = 5
    Received string (5): "hello"
    recvfrom(5, "", 4096, 0, {sa_family=AF_INET, sin_port=htons(55486), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
    recvfrom(5, "world", 4096, 0, {sa_family=AF_INET, sin_port=htons(55486), sin_addr=inet_addr("127.0.0.1")}, [16]) = 5
    Received string (5): "world"

ie the UDP process is still running and can still receive messages

Robert
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