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[Sistemi CeSIA <2003189@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

[Original Report]

While we were in the process of enabling mod_proxy_hcheck on some of our
apache2 nodes we encountered an unusual behavior: sometimes, after
rebooting a backend, its worker status remains marked as "Init Err"
(balancer manager) until another request is made to the backend, no
matter how many health checks complete successfully.

The following list shows the sequence of events leading to the problem:

1. Watchdog triggers health check, request is successful; worker status is "Init Ok"
2. HTTP request to apache2 with unreachable backend (rebooting); status becomes "Init Err"
3. Watchdog triggers another health check, request is again successful because the backend recovered; worker status remains "Init Err"
4. same as 3
5. same as 4

The only way for the worker status to recover is to wait for "hcfails"
unsuccessful health checks and then again for "hcpasses" requests to be
completed or just wait for legitimate traffic to retry the failed
worker, which may not happen for a long time for rarely used
applications.

This was surprising to us since we were expecting the worker status to
be recovered after "hcpasses" successful health checks; however this
doesn't seem to happen when the error status is triggered by ordinary
traffic to the backend (i.e not health checks).

[Test Case]
# apt update && apt dist-upgrade -y
# apt install -y apache2 python3
# cat > /etc/apache2/sites-available/httpd-hcheck-initerr.conf << '__EOF__'
<VirtualHost *:80>
    ServerAdmin webmaster@xxxxxxxxxxxxxxxxxxxxxxx
    DocumentRoot "/var/www/html"
    ServerName myapp.example.com
    ErrorLog "${APACHE_LOG_DIR}/myapp.example.com-error_log"
    CustomLog "${APACHE_LOG_DIR}/myapp.example.com-access_log" common

    ProxyPass / balancer://myapp stickysession=JSESSIONID

</VirtualHost>
__EOF__
# cat > /etc/apache2/conf-available/balancers.conf << '__EOF__'
<Proxy balancer://myapp>
    BalancerMember http://127.0.0.1:8080/ route=app-route hcmethod=GET hcinterval=5 hcpasses=1 hcfails=1
</Proxy>
__EOF__
# a2enmod status
# a2enmod proxy
# a2enmod proxy_balancer
# a2enmod proxy_http
# a2enmod proxy_hcheck
# a2enmod lbmethod_byrequests
# a2enconf balancers
# a2ensite httpd-hcheck-initerr
# python3 -m http.server --bind 127.0.0.1 8080 &
# PYTHON_PID=$!
# systemctl restart apache2
# curl -s localhost/server-status?auto | grep ProxyBalancer | grep Status
# kill -9 $PYTHON_PID
# curl -s localhost -H 'host: myapp.example.com' -o /dev/null
# curl -s localhost/server-status?auto | grep ProxyBalancer | grep Status
# python3 -m http.server --bind 127.0.0.1 8080 &
# sleep 10
# curl -s localhost/server-status?auto | grep ProxyBalancer | grep Status

Example failed output:

Serving HTTP on 127.0.0.1 port 8080 (http://127.0.0.1:8080/) ...
ProxyBalancer[0]Worker[0]Status: Init Ok 
scripts: line 6: 203609 Killed                  python3 -m http.server --bind 127.0.0.1 8080
ProxyBalancer[0]Worker[0]Status: Init Err 
Serving HTTP on 127.0.0.1 port 8080 (http://127.0.0.1:8080/) ...
127.0.0.1 - - [18/Jan/2023 12:24:05] "GET / HTTP/1.0" 200 -
127.0.0.1 - - [18/Jan/2023 12:24:10] "GET / HTTP/1.0" 200 -
ProxyBalancer[0]Worker[0]Status: Init Err

Example of (expected) successful output:

Serving HTTP on 127.0.0.1 port 8080 (http://127.0.0.1:8080/) ...
ProxyBalancer[0]Worker[0]Status: Init Ok 
scripts: line 6: 202190 Killed                  python3 -m http.server --bind 127.0.0.1 8080
ProxyBalancer[0]Worker[0]Status: Init Err 
Serving HTTP on 127.0.0.1 port 8080 (http://127.0.0.1:8080/) ...
127.0.0.1 - - [18/Jan/2023 12:23:12] "GET / HTTP/1.0" 200 -
127.0.0.1 - - [18/Jan/2023 12:23:17] "GET / HTTP/1.0" 200 -
ProxyBalancer[0]Worker[0]Status: Init Ok


Upstream bug: https://bz.apache.org/bugzilla/show_bug.cgi?id=66302
Upstream fix: https://svn.apache.org/viewvc?view=revision&revision=1906496

We would like to see this fix backported in Jammy.

** Affects: apache2 (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: patch-accepted-upstream

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https://bugs.launchpad.net/bugs/2003189

Title:
  Passing health check does not recover worker from its error state

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