Comment 0 for bug 1961697

When resolving DNS names with getaddrinfo(), I have seen this hang for 5 seconds and then retry and succeed. The issue is that glibc will issue a both an A and AAAA query on the same socket, and in some circumstances they can be sent with the same DNS transaction ID as well.

I verified this with a packet capture; in the packet capture, I saw the A and AAAA queries for a name be made with the same DNS transaction ID, get responses, do nothing for five seconds, and then send the same DNS query again. On the glibc side, I confirmed that it's blocked waiting for the DNS response by interrupting it with gdb, even though the packet capture shows the response has well and truly arrived. I've attached a packet capture & a backtrace of the glibc hang.

I believe this is the same issue reported in these places:
    * In RHEL: https://bugzilla.redhat.com/show_bug.cgi?id=1904153
    * Also RHEL: https://bugzilla.redhat.com/show_bug.cgi?id=1903880
    * Upstream: https://sourceware.org/bugzilla/show_bug.cgi?id=26600

The environment I noticed this bug in was:
    * Docker for Mac on an arm64 m1 Macbook
    * Docker for Mac Linux kernel version is 5.10.76-linuxkit
    * Linux is also arm64, not emulated
    * Container with the buggy DNS environment is Ubuntu bionic (also arm64, not emulated)
    * Glibc 2.27-3ubuntu1.4

However one of the redhat reporters noticed this issue in m6 series EC2 instances in AWS.

A patch has been provided upstream for this issue: https://sourceware.org/pipermail/libc-alpha/2020-September/117547.html

I applied the upstream patch to glibc 2.27-3ubuntu1.4 and rebuilt the package, and the problem went away. I've attached the exact patch I applied, since I had to work through some conflicts.

So, I think that patch just needs to be backported to Bionic and (I think) Focal as well. Is that reasonable?

Thanks!