caddy-frontend: Make sure etc/log-access.conf is never empty

When there are no shared instances, the file was empty, but caddy
refuses to start when using an import statement on an empty file, with
this error:

```
Error during parsing: Could not read tokens while importing .../etc/log-access.conf: EOF
```

/reviewed-on nexedi/slapos!545

software/caddy-frontend/buildout.hash.cfg

@@ -30,7 +30,7 @@ md5sum = 37edefdb9963daa67b01e5d55d97c17d
 
 [template-slave-list]
 filename = templates/apache-custom-slave-list.cfg.in
-md5sum = 61919290d810518eba2e74e821dc9de8
+md5sum = f9efdfe7a7e3a78f0b15f414b5469316
 
 [template-slave-configuration]
 filename = templates/custom-virtualhost.conf.in

software/caddy-frontend/templates/apache-custom-slave-list.cfg.in

@@ -475,6 +475,13 @@ template = inline:
 
 rendered = ${:file}
 
+[caddy-log-access-header]
+# Caddy refuse to start if an `import`ed file is empty, so we prepend a header
+# so that the file is never empty.
+< = jinja2-template-base
+template = inline: # This file contain directives to serve directories with log files
+rendered = {{frontend_configuration.get('log-access-configuration')}}
+
 [buildout]
 extends =
   {{ common_profile }}
@@ -482,6 +489,7 @@ extends =
 
 parts +=
     kedifa-updater
+    caddy-log-access-header
 {% for part in part_list %}
 {{ '    %s' % part }}
 {% endfor %}

software/caddy-frontend/test/test_data/test.TestMasterRequest.test_file_list_log-CADDY.txt

 T-0/var/log/monitor-httpd-error.log
 T-0/var/log/slapgrid-T-0-error.log
 T-1/var/log/expose-csr_id.log
+T-2/var/log/frontend-access.log
+T-2/var/log/frontend-error.log
 T-2/var/log/httpd-csr_id/expose-csr_id.log
 T-2/var/log/nginx-access.log
 T-2/var/log/nginx-error.log

software/caddy-frontend/test/test_data/test.TestMasterRequest.test_file_list_run-CADDY.txt

@@ -4,6 +4,7 @@ T-1/var/run/kedifa.pid
 T-2/var/run/caddy_graceful_signature
 T-2/var/run/caddy_validate_signature
 T-2/var/run/caddy_validate_signature.status
+T-2/var/run/httpd.pid
 T-2/var/run/monitor/monitor-bootstrap.pid
 T-2/var/run/nginx.pid
 T-2/var/run/nginx_graceful_signature

software/caddy-frontend/test/test_data/test.TestMasterRequest.test_promise_run_plugin-CADDY.txt

@@ -4,13 +4,13 @@ T-0/etc/plugin/monitor-bootstrap-status.py: OK
 T-0/etc/plugin/monitor-httpd-listening-on-tcp.py: OK
 T-2/etc/plugin/buildout-T-2-status.py: OK
 T-2/etc/plugin/caddy_cached.py: ERROR
-T-2/etc/plugin/caddy_frontend_ipv4_http.py: ERROR
-T-2/etc/plugin/caddy_frontend_ipv4_https.py: ERROR
+T-2/etc/plugin/caddy_frontend_ipv4_http.py: OK
+T-2/etc/plugin/caddy_frontend_ipv4_https.py: OK
 T-2/etc/plugin/caddy_frontend_ipv6_http.py: OK
 T-2/etc/plugin/caddy_frontend_ipv6_https.py: OK
 T-2/etc/plugin/caddy_ssl_cached.py: ERROR
 T-2/etc/plugin/check-free-disk-space.py: OK
-T-2/etc/plugin/frontend-caddy-configuration-promise.py: ERROR
+T-2/etc/plugin/frontend-caddy-configuration-promise.py: OK
 T-2/etc/plugin/monitor-bootstrap-status.py: OK
 T-2/etc/plugin/monitor-httpd-listening-on-tcp.py: OK
 T-2/etc/plugin/nginx-configuration-promise.py: OK

software/caddy-frontend/test/test_data/test.TestMasterRequest.test_supervisor_state-CADDY.txt

@@ -20,7 +20,7 @@ T-2:crond-{hash}-on-watch RUNNING
 T-2:expose-csr_id-{hash}-on-watch RUNNING
 T-2:frontend-caddy-safe-graceful EXITED
 T-2:frontend-nginx-safe-graceful EXITED
-T-2:frontend_caddy-{hash}-on-watch EXITED
+T-2:frontend_caddy-{hash}-on-watch RUNNING
 T-2:frontend_nginx-{hash}-on-watch RUNNING
 T-2:kedifa-login-certificate-caucase-updater-on-watch RUNNING
 T-2:kedifa-updater-{hash}-on-watch RUNNING

software/caddy-frontend/test/test_data/test.TestMasterRequestDomain.test_file_list_log-CADDY.txt

 T-0/var/log/monitor-httpd-error.log
 T-0/var/log/slapgrid-T-0-error.log
 T-1/var/log/expose-csr_id.log
+T-2/var/log/frontend-access.log
+T-2/var/log/frontend-error.log
 T-2/var/log/httpd-csr_id/expose-csr_id.log
 T-2/var/log/nginx-access.log
 T-2/var/log/nginx-error.log

software/caddy-frontend/test/test_data/test.TestMasterRequestDomain.test_file_list_run-CADDY.txt

@@ -4,6 +4,7 @@ T-1/var/run/kedifa.pid
 T-2/var/run/caddy_graceful_signature
 T-2/var/run/caddy_validate_signature
 T-2/var/run/caddy_validate_signature.status
+T-2/var/run/httpd.pid
 T-2/var/run/monitor/monitor-bootstrap.pid
 T-2/var/run/nginx.pid
 T-2/var/run/nginx_graceful_signature

software/caddy-frontend/test/test_data/test.TestMasterRequestDomain.test_promise_run_plugin-CADDY.txt

@@ -4,13 +4,13 @@ T-0/etc/plugin/monitor-bootstrap-status.py: OK
 T-0/etc/plugin/monitor-httpd-listening-on-tcp.py: OK
 T-2/etc/plugin/buildout-T-2-status.py: OK
 T-2/etc/plugin/caddy_cached.py: ERROR
-T-2/etc/plugin/caddy_frontend_ipv4_http.py: ERROR
-T-2/etc/plugin/caddy_frontend_ipv4_https.py: ERROR
+T-2/etc/plugin/caddy_frontend_ipv4_http.py: OK
+T-2/etc/plugin/caddy_frontend_ipv4_https.py: OK
 T-2/etc/plugin/caddy_frontend_ipv6_http.py: OK
 T-2/etc/plugin/caddy_frontend_ipv6_https.py: OK
 T-2/etc/plugin/caddy_ssl_cached.py: ERROR
 T-2/etc/plugin/check-free-disk-space.py: OK
-T-2/etc/plugin/frontend-caddy-configuration-promise.py: ERROR
+T-2/etc/plugin/frontend-caddy-configuration-promise.py: OK
 T-2/etc/plugin/monitor-bootstrap-status.py: OK
 T-2/etc/plugin/monitor-httpd-listening-on-tcp.py: OK
 T-2/etc/plugin/nginx-configuration-promise.py: OK

software/caddy-frontend/test/test_data/test.TestMasterRequestDomain.test_supervisor_state-CADDY.txt

@@ -20,7 +20,7 @@ T-2:crond-{hash}-on-watch RUNNING
 T-2:expose-csr_id-{hash}-on-watch RUNNING
 T-2:frontend-caddy-safe-graceful EXITED
 T-2:frontend-nginx-safe-graceful EXITED
-T-2:frontend_caddy-{hash}-on-watch EXITED
+T-2:frontend_caddy-{hash}-on-watch RUNNING
 T-2:frontend_nginx-{hash}-on-watch RUNNING
 T-2:kedifa-login-certificate-caucase-updater-on-watch RUNNING
 T-2:kedifa-updater-{hash}-on-watch RUNNING