[Bug testsuite/28561] [gdb/testsuite] Error due to not reading \r\n at end of mi prompt

["thiago.bauermann at linaro dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28561

Thiago Jung Bauermann <thiago.bauermann at linaro dot org> changed:

           What    |Removed                     |Added
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                 CC|                            |thiago.bauermann at linaro dot org

--- Comment #7 from Thiago Jung Bauermann <thiago.bauermann at linaro dot org> ---
This bug isn't so easy to reproduce. I've been trying for the past few days
on aarch64 machines, and I was able to observe it on these systems:

1. KVM guest running openSUSE Leap 15.3
2. Container with Ubuntu 20.04 userspace inside KVM guest with openSUSE Leap
15.3
3. Container with Ubuntu 22.04 userspace inside KVM guest with openSUSE Leap
15.3

I was not able to reproduce the problem on these systems:

4. KVM guest running Ubuntu 20.04
5. KVM guest running Ubuntu 22.04
6. KVM guest running Debian 10 (buster)
7. Bare metal Ubuntu 20.04
8. Container with Ubuntu 22.04 userspace on bare metal Ubuntu 20.04
9. Container with openSUSE Leap 15.3 userspace on bare metal Ubuntu 22.04

To me, this indicates that it's a kernel issue since there were three
kernels tested and only one of them reproduced the problem. And of the three
userspaces tested, they all reproduced the problem when used with the
problematic kernel, and not when used with the other two kernels.

Kernel I tested where the bug happens:
- openSUSE Leap 15.3 5.3.18-150300.59.68-default

Kernels I tested where the bug doesn't happen:
- Ubuntu 20.04 5.4.0-110-generic
- Ubuntu 22.04 5.15.0-30-generic
- Debian 10 5.10.0-0.bpo.12-arm64

My hypothesis is that there was some kernel patch fixing the bug that was
backported to the Ubuntu and Debian kernels but not to the openSUSE Leap
kernel.

I will try testing some upstream kernels to see if I can test the hypothesis
and narrow down upstream kernel versions.

The bug reproduces only after running the affected testcases for hundreds of
iterations. It was easiest to hit with gdb.mi/user-selected-context-sync.exp.
I also saw the problem happen with gdb.mi/mi-break.exp a couple of times.

I did see gdb.gdb/unittest.exp (which is the test that fails in the GNU
Toolchain buildbot) fail on an Ubuntu 22.04 KVM guest, so perhaps my “it's
the kernel” theory is wrong. But this failure is an outlier in the general
pattern I'm seeing since it's much rarer to see this testcase fail (it fails
once every several thousands of iterations rather than several hundreds), so
I'll keep pursuing the kernel route before digging deeper into what's going
on with gdb.gdb/unittest.exp.

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