To POST:

http://post-office.corp.redhat.com/archives/rhvirt-patches/2020-January/msg00446.html

Tested with:
libvirt-daemon-6.0.0-1.module+el8.2.0+5453+31b2b136.x86_64
qemu-kvm-4.2.0-2.module+el8.2.0+5135+ed3b2489.x86_64

1. Download the attached bash script and compile it
$ gcc -O2 -g -Wall `pkgconf libvirt --cflags --libs` test.c -o test

2. execute the script
$ i=0
$  while true; do i=$((i+1)); echo $i; ./test; done
1
2
3
....
10212
10213
10214
^C

repeat for 10000+ times, could not reproduce

Retested with:
libvirt-daemon-6.0.0-17.module+el8.2.0+6257+0d066c28.x86_64
qemu-kvm-4.2.0-17.module+el8.2.0+6141+0f540f16.x86_64

1. Download the attached bash script and compile it
$ gcc -O2 -g -Wall `pkgconf libvirt --cflags --libs` test.c -o test

2. execute the script
$ i=0
$  while true; do i=$((i+1)); echo $i; ./test; done
1
2
3
....
3025
3026
3027
^C

repeated for 3000+ times, did not hit the error

Hi, Michal

The attached file is test.c
I tried to reproduce this bug with libvirt-daemon-6.0.0-1, and also with libvirt-daemon-5.6.0-10
can not reproduce.
Please help to check whether there is something wrong with the testing steps. Thanks very much.

Created attachment 1678916 [details]
test.c

I think that test is the one which I wrote ...?  I think Peter
had a much better program which used threads to reproduce the bug more
reliably.  Unfortunately I cannot find it in the mailing list.
(https://www.redhat.com/archives/libvir-list/2020-January/msg01006.html)

Created attachment 1679013 [details]
repro.c

Yeah,  this bug is not easy to reproduce because it is a race condition. What I did when debugging this was to put sleep(5) into a specific place and then get domcaps from two terminals at once. More details here:

https://www.redhat.com/archives/libvir-list/2020-January/msg01004.html

However, with the attached file and tuning worker pool (min_workers = 2 max_workers = 2 in libvirtd.conf) I am able to reproduce occasionally, I mean, if I revert the fix and don't put the sleep():

https://www.redhat.com/archives/libvir-list/2020-January/msg01007.html

Thanks very much to Michal and Richard, can produce this bug now

1. put a sleep in the following the lines, then build from source
# git diff 
diff --git a/src/qemu/qemu_conf.c b/src/qemu/qemu_conf.c
index 30637b21ac..f76bbe4b39 100644
--- a/src/qemu/qemu_conf.c
+++ b/src/qemu/qemu_conf.c
@@ -1372,6 +1372,7 @@ virQEMUDriverGetDomainCapabilities(virQEMUDriverPtr driver,
         key = g_strdup_printf("%d:%d:%s:%s", data.arch, data.virttype,
                               NULLSTR(data.machine), NULLSTR(data.path));
 
+       sleep(5);
         if (virHashAddEntry(domCapsCache, key, domCaps) < 0)
             return NULL;
     }

2. And then try to get domcaps from two consoles at once
Terminal 1:
# virsh domcapabilities 
<domainCapabilities>
  <path>/usr/libexec/qemu-kvm</path>
  <domain>kvm</domain>
  <machine>pc-i440fx-rhel7.6.0</machine>
  <arch>x86_64</arch>
  <vcpu max='240'/>
  <iothreads supported='yes'/>
....

Terminal 2:
# virsh domcapabilities 
error: failed to get emulator capabilities
error: internal error: Duplicate key

Verify this bug with:
libvirt-daemon-6.0.0-17.module+el8.2.0+6257+0d066c28.x86_64.rpm

1. Apply the following patch to rebuild libvirt
From 75610bfea795c4988cccb84491d718492e67474f Mon Sep 17 00:00:00 2001
From: rpm-build <rpm-build>
Date: Sun, 19 Apr 2020 10:06:33 -0400
Subject: [PATCH] Verify bug 1794691

Signed-off-by: rpm-build <rpm-build>
---
 src/qemu/qemu_capabilities.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index 0b4ed42..1e22c10 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -2108,6 +2108,7 @@ virQEMUCapsGetDomainCapsCache(virQEMUCapsPtr qemuCaps,
         key = g_strdup_printf("%d:%d:%s:%s", arch, virttype,
                               NULLSTR(machine), path);

+        sleep(5);
         if (virHashAddEntry(cache->cache, key, tempDomCaps) < 0)
             goto cleanup;

--
2.18.2

2. start libvirtd

3. try to get domcapabilities from two consoles at once
Terminal 1:
# virsh domcapabilities 
<domainCapabilities>
  <path>/usr/libexec/qemu-kvm</path>
  <domain>kvm</domain>
  <machine>pc-i440fx-rhel7.6.0</machine>
  <arch>x86_64</arch>
  <vcpu max='240'/>
  <iothreads supported='yes'/>
.....

Terminal 2:
# virsh domcapabilities 
<domainCapabilities>
  <path>/usr/libexec/qemu-kvm</path>
  <domain>kvm</domain>
  <machine>pc-i440fx-rhel7.6.0</machine>
  <arch>x86_64</arch>
  <vcpu max='240'/>
  <iothreads supported='yes'/>
  <os supported='yes'>
....

As the testing result matches with the expected result, mark the bug as verified

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2020:2017