gnu: services: Create logs directory.

* gnu/services/web.scm (nginx-activation): Create logs directory so nginx can
log its startup messages before it loads its configuration.

doc/guix.texi

@@ -13316,6 +13316,15 @@ used to specify the list of @dfn{server blocks} required on the host and
 blocks} to configure.  For this to work, use the default value for
 @var{config-file}.
 
+At startup, @command{nginx} has not yet read its configuration file, so it
+uses a default file to log error messages.  If it fails to load its
+configuration file, that is where error messages are logged.  After the
+configuration file is loaded, the default error log file changes as per
+configuration.  In our case, startup error messages can be found in
+@file{/var/run/nginx/logs/error.log}, and after configuration in
+@file{/var/log/nginx/error.log}.  The second location can be changed with the
+@var{log-directory} configuration option.
+
 @end deffn
 
 @deffn {Scheme Variable} nginx-service-type

gnu/services/web.scm

@@ -237,6 +237,9 @@ of index files."
          (mkdir-p (string-append #$run-directory "/fastcgi_temp"))
          (mkdir-p (string-append #$run-directory "/uwsgi_temp"))
          (mkdir-p (string-append #$run-directory "/scgi_temp"))
+         ;; Start-up logs. Once configuration is loaded, nginx switches to
+         ;; log-directory.
+         (mkdir-p (string-append #$run-directory "/logs"))
          ;; Check configuration file syntax.
          (system* (string-append #$nginx "/sbin/nginx")
                   "-c" #$(or config-file