Hello Oleg,

I've had a quick read through the patch, and I've made a few hopefully
helpful comments below.

On Fri, 28 Jul 2017 01:01:51 +0300
Oleg Pykhalov <go.wigust@gmail.com> wrote:

> * doc/guix.texi (Incremental file transfer): Add documentation.
> * gnu/services/rsync.scm (<rsync-configuration>): New record type.
> (rsync-accounts, rsync-shepherd-service): New service extensions.
> (rsync-service-type): New service type.
> ---
>  doc/guix.texi          |  58 ++++++++++++++++++
>  gnu/local.mk           |   1 +
>  gnu/services/rsync.scm | 162
> +++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed,
> 221 insertions(+) create mode 100644 gnu/services/rsync.scm
> 
> diff --git a/doc/guix.texi b/doc/guix.texi
> index e8c4e0eaf..a3745ae01 100644
> --- a/doc/guix.texi
> +++ b/doc/guix.texi
> @@ -15661,6 +15661,64 @@ Extra options will be passed to @code{git
> daemon}, please run @end table
>  @end deftp
>  
> +@subsubsection Incremental file transfer
> +
> +The @code{(gnu services rsync)} module provides the following
> services: +
> +@subsubheading Rsync service

It would be great to give a really short explanation of what this
service can be used for here. I know what Rsync does, but I'm not quite
sure what this service does.

> +@deffn {Scheme Variable} rsync-service-type
> +This is the type for the @uref{https://rsync.samba.org} rsync daemon,
> +@command{rsync-configuration} record as in this example:
> +
> +@example
> +(service rsync-service-type
> +         (rsync-configuration))
> +@end example
> +
> +See below for details about @code{rsync-configuration}.
> +@end deffn
> +
> +@deftp {Data Type} rsync-configuration
> +Data type representing the configuration for @code{rsync-service}.
> +
> +@table @asis
> +@item @code{package} (default: @var{rsync})
> +Package object of the Rsync utility for efficiently transferring and
> +synchronizing files.

Object doesn't really fit here, if anything its a record. Also, I don't
think this needs a description of what rsync does, it's probably
clearest to just say that this is the rsync package?

> +@item @code{port-number} (default: @code{873})
> +TCP port on which @command{rsync} listens for incoming connections.
> If +port is less than @code{1024} @command{rsync} will be started as
> the +@code{root} user and group.
> +
> +@item @code{pid-file} (default: @code{"/var/run/rsyncd.pid"})
> +Name of the file where @command{rsync} writes its PID.
> +
> +@item @code{lock-file} (default: @code{"/var/run/rsyncd.lock"})
> +Name of the file where @command{rsync} writes its lock file.
> +
> +@item @code{log-file} (default: @code{"/var/log/rsyncd.log"})
> +Name of the file where @command{rsync} writes its log file.
> +
> +@item @code{use-choot?} (default: @var{#f})
> +Whether to use chroot for @command{rsync} shared directory.
> +
> +@item @code{share-path} (default: @file{/srv/rsync})
> +Location of the @command{rsync} shared directory.
> +
> +@item @code{share-comment} (default: @code{"Rsync share"})
> +Comment of the @command{rsync} shared directory.
> +
> +@item @code{read-only?} (default: @var{#f})
> +Read-write permissions to shared directory.
> +
> +@item @code{timeout} (default: @code{300})
> +I/O timeout in seconds.
> +
> +@end table
> +@end deftp
> +
>  @node Setuid Programs
>  @subsection Setuid Programs
>  
> diff --git a/gnu/local.mk b/gnu/local.mk
> index 724c6b675..fa514b278 100644
> --- a/gnu/local.mk
> +++ b/gnu/local.mk
> @@ -444,6 +444,7 @@ GNU_SYSTEM_MODULES
> =				\
> %D%/services/shepherd.scm			\
> %D%/services/herd.scm				\
> %D%/services/pm.scm				\
> +  %D%/services/rsync.scm			\
>    %D%/services/sddm.scm				\
>    %D%/services/spice.scm				\
>    %D%/services/ssh.scm				\
> diff --git a/gnu/services/rsync.scm b/gnu/services/rsync.scm
> new file mode 100644
> index 000000000..49c4cb7e2
> --- /dev/null
> +++ b/gnu/services/rsync.scm
> @@ -0,0 +1,162 @@
> +;;; GNU Guix --- Functional package management for GNU
> +;;; Copyright © 2017 Oleg Pykhalov <go.wigust@gmail.com>
> +;;;
> +;;; This file is part of GNU Guix.
> +;;;
> +;;; GNU Guix is free software; you can redistribute it and/or modify
> it +;;; under the terms of the GNU General Public License as
> published by +;;; the Free Software Foundation; either version 3 of
> the License, or (at +;;; your option) any later version.
> +;;;
> +;;; GNU Guix is distributed in the hope that it will be useful, but
> +;;; WITHOUT ANY WARRANTY; without even the implied warranty of
> +;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +;;; GNU General Public License for more details.
> +;;;
> +;;; You should have received a copy of the GNU General Public License
> +;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
> +
> +(define-module (gnu services rsync)
> +  #:use-module (gnu services)
> +  #:use-module (gnu services base)
> +  #:use-module (gnu services shepherd)
> +  #:use-module (gnu system shadow)
> +  #:use-module (gnu packages rsync)
> +  #:use-module (gnu packages admin)
> +  #:use-module (guix records)
> +  #:use-module (guix gexp)
> +  #:use-module (srfi srfi-1)
> +  #:use-module (srfi srfi-26)
> +  #:use-module (ice-9 match)
> +  #:export (rsync-configuration
> +            rsync-configuration?
> +            rsync-service-type))
> +
> +;;;
> +;;; Rsync.
> +;;;
> +
> +(define-record-type* <rsync-configuration>
> +  rsync-configuration make-rsync-configuration
> +  rsync-configuration?
> +  ;; <package>
> +  (package rsync-configuration-package
> +           (default rsync))
> +  ;; integer
> +  (port-number rsync-configuration-port-number
> +               (default 873))
> +  ;; string
> +  (pid-file rsync-configuration-pid-file
> +            (default "/var/run/rsyncd.pid"))
> +  ;; string
> +  (lock-file rsync-configuration-lock-file
> +             (default "/var/run/rsyncd.lock"))
> +  ;; string
> +  (log-file rsync-configuration-log-file
> +            (default "/var/log/rsyncd.log"))
> +  ;; Boolean
> +  (use-chroot? rsync-configuration-use-chroot?
> +               (default #f))
> +  ;; string
> +  (share-path rsync-configuration-share-path
> +              (default "/srv/rsync"))
> +  ;; string
> +  (share-comment rsync-configuration-share-comment
> +                 (default "Rsync share"))
> +  ;; Boolean
> +  (read-only? rsync-configuration-read-only?
> +              (default #f))
> +  ;; integer
> +  (timeout rsync-configuration-timeout
> +           (default 300)))
> +
> +(define %rsync-accounts
> +  ;; User account and group for rsync.
> +  (list (user-group (name "rsyncd") (system? #t))
> +        (user-account
> +         (name "rsyncd")
> +         (system? #t)
> +         (group "rsyncd")
> +         (comment "rsyncd privilege separation user")
> +         (home-directory "/var/run/rsyncd")
> +         (shell #~(string-append #$shadow "/sbin/nologin")))))
> +
> +(define (rsync-activation config)
> +  "Return the activation GEXP for CONFIG."
> +  #~(begin
> +      (use-modules (guix build utils))
> +      (let ((share-directory #$(rsync-configuration-share-path
> config))
> +            (user (getpw "rsyncd")))
> +        (and=> share-directory mkdir-p)
> +        (chown share-directory
> +               (passwd:uid user)
> +               (group:gid user)))))
> +
> +(define (rsync-config-file config)
> +  "Return the rsync configuration file corresponding to CONFIG."
> +  (computed-file
> +   "rsync.conf"
> +   #~(begin
> +       (call-with-output-file #$output
> +         (lambda (port)
> +           (display "# Generated by 'rsync-service'.\n" port)
> +           (format port "pid file = ~a\n"
> +                   #$(rsync-configuration-pid-file config))
> +           (format port "lock file = ~a\n"
> +                   #$(rsync-configuration-lock-file config))
> +           (format port "log file = ~a\n"
> +                   #$(rsync-configuration-log-file config))
> +           (format port "port = ~a\n"
> +                   #$(number->string
> +                      (rsync-configuration-port-number config)))
> +           (format port "use chroot = ~a\n"
> +                   #$(if (rsync-configuration-use-chroot? config)
> +                         "true" "false"))
> +           (display "[files]\n" port)
> +           (format port "path = ~a\n"
> +                   #$(rsync-configuration-share-path config))
> +           (format port "comment = ~a\n"
> +                   #$(rsync-configuration-share-comment config))
> +           (format port "read only = ~a\n"
> +                   #$(if (rsync-configuration-read-only? config)
> +                         "true" "false"))
> +           (format port "timeout = ~a\n"
> +                   #$(number->string
> +                      (rsync-configuration-timeout config)))
> +           #t)))))

It might be neater here to use mixed-text-file here. It might look
something like...

(define (rsync-config-file config)
  "Return the rsync configuration file corresponding to CONFIG."
  (match config
    (($ <rsync-configuration> package port-number pid-file lock-file
                              log-file use-chroot? share-path
                              share-comment read-only? timeout)
     (mixed text-file "rsync.conf"
      "# Generated by 'rsync-service'.\n"
      "pid file = " pid-file "\n"
      "lock file = " lock-file "\n"
      "log file = " log-file "\n"
      "port = " (number->string port-number) "\n"
      "use chroot = " (if use-chroot? "true" "false") "\n"
      "[files]\n"
      "path = " share-path "\n"
      "comment = " share-comment "\n"
      "read only = " (if read-only? "true" "false") "\n"
      "timeout = " (number->string timeout) "\n"))))
     

One thing I tried with the Tailon service, was to define a
gexp-compiler for the record type representing the configuration file.
One really big advantage in the case of Tailon is that it easily allows
the use of a custom file, just by providing a different gexp.

Here however, I can see that you are using some values from the
configuration (the port-number, package and share-path), which don't
make that possible. The use of the port-number and package could
probably be elegantly separated out, but I can't figure out what could
be done about the share path, and its use in the service activation.

> +
> +(define (rsync-shepherd-service config)
> +  "Return a <shepherd-service> for rsync with CONFIG."
> +
> +  (define rsync-command
> +    #~(list (string-append #$(rsync-configuration-package config)
> "/bin/rsync")
> +            "--daemon" "--config" #$(rsync-config-file config)))
> +
> +  (define pid-file
> +    (rsync-configuration-pid-file config))
> +
> +  (define user
> +    (let ((port (rsync-configuration-port-number config)))
> +      (if (> port  1024)
> +          "rsyncd"
> +          "root")))
> +
> +  (list (shepherd-service
> +         (provision '(rsync))
> +         (documentation "Run rsync daemon.")
> +         (start #~(make-forkexec-constructor #$rsync-command
> +                                             #:pid-file #$pid-file
> +                                             #:user #$user
> +                                             #:group #$user))
> +         (stop #~(make-kill-destructor)))))
> +
> +(define rsync-service-type
> +  (service-type
> +   (name 'rsync)
> +   (extensions
> +    (list (service-extension shepherd-root-service-type
> +                             rsync-shepherd-service)
> +          (service-extension account-service-type
> +                             (const %rsync-accounts))
> +          (service-extension activation-service-type
> +                             rsync-activation)))))

As I understand it, the service might not use the rsyncd user, if it is
configured to bind to a low port? If that is the case, then it might be
neater to include a check for this in the account-service-type
extension, so that these only get created if they are needed?