services: dovecot: Provide plugins through a /gnu/store directory.

* gnu/services/mail.scm (package-list?, serialize-package-list): New
procedures.
* gnu/services/mail.scm (dovecot-configuration)[extensions]: New field. The
field lets you provide a list of dovecot plugins that need to be available
during the runtime. A union of the set of modules will be created on the
activation time.
* gnu/services/mail.scm (opaque-dovecot-configuration)[extensions]: Likewise.
* gnu/services/mail.scm (make-dovecot-moduledir): New function.
* gnu/services/mail.scm (%dovecot-activation): Add step to compute a set of
modules, and provide them over the shared link at /usr/lib/dovecot.
* doc/guix.texi (Mail Services)[extension]: Add documentation. Clarify the
purpose and usage of the extensions parameter. Add an example showing how to
enable Sieve filtering using dovecot-pigeonhole. Better explain the module
directory structure and requirements.

Change-Id: I3c3955bb04b09d245242112f6810ecc0558109a1
Signed-off-by: Christopher Baines <mail@cbaines.net>

gnu/services/mail.scm

@@ -521,11 +521,21 @@ as @code{#t}.)")
               (serialize-namespace-configuration field-name val))
             val))
 
+(define (package-list? val)
+  (and (list? val) (and-map package? val)))
+(define (serialize-package-list field-name val)
+  #f)
+
 (define-configuration dovecot-configuration
   (dovecot
    (file-like dovecot)
    "The dovecot package.")
 
+  (extensions
+   (package-list '())
+   "Plugins and extensions to the Dovecot package. Specify a list of dovecot
+plugins that needs to be available for dovecot and its modules.")
+
   (listen
    (comma-separated-string-list '("*" "::"))
    "A list of IPs or hosts where to listen in for connections.  @samp{*}
@@ -1109,7 +1119,7 @@ isn't enough disk space, just skip it.
 @item fcntl
 Use this if possible.  Works with NFS too if lockd is used.
 @item flock
-May not exist in all systems.  Doesn't work with NFS. 
+May not exist in all systems.  Doesn't work with NFS.
 @item lockf
 May not exist in all systems.  Doesn't work with NFS.
 @end table
@@ -1500,6 +1510,11 @@ greyed out, instead of only later giving \"not selectable\" popup error.
    (file-like dovecot)
    "The dovecot package.")
 
+  (extensions
+   (package-list '())
+   "Plugins and extensions to the Dovecot package. Specify a list of dovecot
+plugins that needs to be available for dovecot and its modules.")
+
   (string
    (string (configuration-missing-field 'opaque-dovecot-configuration
                                         'string))
@@ -1525,6 +1540,21 @@ greyed out, instead of only later giving \"not selectable\" popup error.
          (home-directory "/var/empty")
          (shell (file-append shadow "/sbin/nologin")))))
 
+(define (make-dovecot-moduledir packages)
+  "Return a computed file containing a union of Dovecot module directories from PACKAGES.
+Each package's '/lib/dovecot' directory is combined into a single location."
+  ;; Create a union of the set of modules and dovecot itself.
+  (with-imported-modules '((guix build union))
+    (computed-file
+     "dovecot-moduledir"
+     #~(begin
+         (use-modules (guix build union) (srfi srfi-26))
+
+         (union-build #$output
+                      (map (cut string-append <>
+                                "/lib/dovecot")
+                           (list #$@packages)))))))
+
 (define (%dovecot-activation config)
   ;; Activation gexp.
   (let ((config-str
@@ -1535,7 +1565,15 @@ greyed out, instead of only later giving \"not selectable\" popup error.
            (with-output-to-string
              (lambda ()
                (serialize-configuration config
-                                        dovecot-configuration-fields)))))))
+                                        dovecot-configuration-fields))))))
+        (moduledir-directory
+         (cond
+          ((opaque-dovecot-configuration? config)
+           (make-dovecot-moduledir (cons* (opaque-dovecot-configuration-dovecot config)
+                                          (opaque-dovecot-configuration-extensions config))))
+          (else
+           (make-dovecot-moduledir (cons* (dovecot-configuration-dovecot config)
+                                          (dovecot-configuration-extensions config)))))))
     (with-imported-modules (source-module-closure '((gnu build activation)))
       #~(begin
           (use-modules (guix build utils) (gnu build activation))
@@ -1586,6 +1624,8 @@ greyed out, instead of only later giving \"not selectable\" popup error.
             (copy-file #$(plain-file "dovecot.conf" config-str)
                        "/etc/dovecot/dovecot.conf")
             (mkdir-p/perms "/etc/dovecot/private" user #o700)
+            (mkdir-p "/usr/lib")
+            (switch-symlinks "/usr/lib/dovecot" #$moduledir-directory)
             (create-self-signed-certificate-if-absent
              #:private-key "/etc/dovecot/private/default.pem"
              #:public-key "/etc/dovecot/default.pem"