summaryrefslogtreecommitdiff
path: root/gnu/services.scm
blob: bc866eafe3ae3d0c50c203d187abaa3ce1e7fc58 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2016 Chris Marusich <cmmarusich@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)
  #:use-module (guix gexp)
  #:use-module (guix monads)
  #:use-module (guix store)
  #:use-module (guix records)
  #:use-module (guix profiles)
  #:use-module (guix discovery)
  #:use-module (guix sets)
  #:use-module (guix ui)
  #:use-module ((guix utils) #:select (source-properties->location))
  #:use-module (guix modules)
  #:use-module (gnu packages base)
  #:use-module (gnu packages bash)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-9 gnu)
  #:use-module (srfi srfi-26)
  #:use-module (srfi srfi-34)
  #:use-module (srfi srfi-35)
  #:use-module (ice-9 vlist)
  #:use-module (ice-9 match)
  #:export (service-extension
            service-extension?
            service-extension-target
            service-extension-compute

            service-type
            service-type?
            service-type-name
            service-type-extensions
            service-type-compose
            service-type-extend
            service-type-default-value
            service-type-description
            service-type-location

            %service-type-path
            fold-service-types

            service
            service?
            service-kind
            service-value
            service-parameters                    ;deprecated

            simple-service
            modify-services
            service-back-edges
            fold-services

            service-error?
            missing-value-service-error?
            missing-value-service-error-type
            missing-value-service-error-location
            missing-target-service-error?
            missing-target-service-error-service
            missing-target-service-error-target-type
            ambiguous-target-service-error?
            ambiguous-target-service-error-service
            ambiguous-target-service-error-target-type

            system-service-type
            boot-service-type
            cleanup-service-type
            activation-service-type
            activation-service->script
            %linux-bare-metal-service
            special-files-service-type
            extra-special-file
            etc-service-type
            etc-directory
            setuid-program-service-type
            profile-service-type
            firmware-service-type
            gc-root-service-type

            %boot-service
            %activation-service
            etc-service

            directory-union))

;;; Comment:
;;;
;;; This module defines a broad notion of "service types" and "services."
;;;
;;; A service type describe how its instances extend instances of other
;;; service types.  For instance, some services extend the instance of
;;; ACCOUNT-SERVICE-TYPE by providing it with accounts and groups to create;
;;; others extend SHEPHERD-ROOT-SERVICE-TYPE by passing it instances of
;;; <shepherd-service>.
;;;
;;; When applicable, the service type defines how it can itself be extended,
;;; by providing one procedure to compose extensions, and one procedure to
;;; extend itself.
;;;
;;; A notable service type is SYSTEM-SERVICE-TYPE, which has a single
;;; instance, which is the root of the service DAG.  Its value is the
;;; derivation that produces the 'system' directory as returned by
;;; 'operating-system-derivation'.
;;;
;;; The 'fold-services' procedure can be passed a list of procedures, which it
;;; "folds" by propagating extensions down the graph; it returns the root
;;; service after the applying all its extensions.
;;;
;;; Code:

(define-record-type <service-extension>
  (service-extension target compute)
  service-extension?
  (target  service-extension-target)              ;<service-type>
  (compute service-extension-compute))            ;params -> params

(define &no-default-value
  ;; Value used to denote service types that have no associated default value.
  '(no default value))

(define-record-type* <service-type> service-type make-service-type
  service-type?
  (name       service-type-name)                  ;symbol (for debugging)

  ;; Things extended by services of this type.
  (extensions service-type-extensions)            ;list of <service-extensions>

  ;; Given a list of extensions, "compose" them.
  (compose    service-type-compose                ;list of Any -> Any
              (default #f))

  ;; Extend the services' own parameters with the extension composition.
  (extend     service-type-extend                 ;list of Any -> parameters
              (default #f))

  ;; Optional default value for instances of this type.
  (default-value service-type-default-value       ;Any
                 (default &no-default-value))

  ;; Meta-data.
  (description  service-type-description          ;string
                (default #f))
  (location     service-type-location             ;<location>
                (default (and=> (current-source-location)
                                source-properties->location))
                (innate)))

(define (write-service-type type port)
  (format port "#<service-type ~a ~a>"
          (service-type-name type)
          (number->string (object-address type) 16)))

(set-record-type-printer! <service-type> write-service-type)

(define %distro-root-directory
  ;; Absolute file name of the module hierarchy.
  (dirname (search-path %load-path "guix.scm")))

(define %service-type-path
  ;; Search path for service types.
  (make-parameter `((,%distro-root-directory . "gnu/services")
                    (,%distro-root-directory . "gnu/system"))))

(define* (fold-service-types proc seed
                             #:optional
                             (modules (all-modules (%service-type-path))))
  "For each service type exported by one of MODULES, call (PROC RESULT).  SEED
is used as the initial value of RESULT."
  (fold-module-public-variables (lambda (object result)
                                  (if (service-type? object)
                                      (proc object result)
                                      result))
                                '()
                                modules))

;; Services of a given type.
(define-record-type <service>
  (make-service type value)
  service?
  (type       service-kind)
  (value      service-value))

(define-syntax service
  (syntax-rules ()
    "Return a service instance of TYPE.  The service value is VALUE or, if
omitted, TYPE's default value."
    ((_ type value)
     (make-service type value))
    ((_ type)
     (%service-with-default-value (current-source-location)
                                  type))))

(define (%service-with-default-value location type)
  "Return a instance of service type TYPE with its default value, if any.  If
TYPE does not have a default value, an error is raised."
  ;; TODO: Currently this is a run-time error but with a little bit macrology
  ;; we could turn it into an expansion-time error.
  (let ((default (service-type-default-value type)))
    (if (eq? default &no-default-value)
        (let ((location (source-properties->location location)))
          (raise
           (condition
            (&missing-value-service-error (type type) (location location))
            (&message
             (message (format #f (G_ "~a: no value specified \
for service of type '~a'")
                              (location->string location)
                              (service-type-name type)))))))
        (service type default))))

(define-condition-type &service-error &error
  service-error?)

(define-condition-type &missing-value-service-error &service-error
  missing-value-service-error?
  (type     missing-value-service-error-type)
  (location missing-value-service-error-location))



;;;
;;; Helpers.
;;;

(define service-parameters
  ;; Deprecated alias.
  service-value)

(define (simple-service name target value)
  "Return a service that extends TARGET with VALUE.  This works by creating a
singleton service type NAME, of which the returned service is an instance."
  (let* ((extension (service-extension target identity))
         (type      (service-type (name name)
                                  (extensions (list extension)))))
    (service type value)))

(define-syntax %modify-service
  (syntax-rules (=>)
    ((_ service)
     service)
    ((_ svc (kind param => exp ...) clauses ...)
     (if (eq? (service-kind svc) kind)
         (let ((param (service-value svc)))
           (service (service-kind svc)
                    (begin exp ...)))
         (%modify-service svc clauses ...)))))

(define-syntax modify-services
  (syntax-rules ()
    "Modify the services listed in SERVICES according to CLAUSES and return
the resulting list of services.  Each clause must have the form:

  (TYPE VARIABLE => BODY)

where TYPE is a service type, such as 'guix-service-type', and VARIABLE is an
identifier that is bound within BODY to the value of the service of that
TYPE.  Consider this example:

  (modify-services %base-services
    (guix-service-type config =>
                       (guix-configuration
                        (inherit config)
                        (use-substitutes? #f)
                        (extra-options '(\"--gc-keep-derivations\"))))
    (mingetty-service-type config =>
                           (mingetty-configuration
                            (inherit config)
                            (motd (plain-file \"motd\" \"Hi there!\")))))

It changes the configuration of the GUIX-SERVICE-TYPE instance, and that of
all the MINGETTY-SERVICE-TYPE instances.

This is a shorthand for (map (lambda (svc) ...) %base-services)."
    ((_ services clauses ...)
     (map (lambda (service)
            (%modify-service service clauses ...))
          services))))


;;;
;;; Core services.
;;;

(define (system-derivation mentries mextensions)
  "Return as a monadic value the derivation of the 'system' directory
containing the given entries."
  (mlet %store-monad ((entries    mentries)
                      (extensions (sequence %store-monad mextensions)))
    (lower-object
     (file-union "system"
                 (append entries (concatenate extensions))))))

(define system-service-type
  ;; This is the ultimate service type, the root of the service DAG.  The
  ;; service of this type is extended by monadic name/item pairs.  These items
  ;; end up in the "system directory" as returned by
  ;; 'operating-system-derivation'.
  (service-type (name 'system)
                (extensions '())
                (compose identity)
                (extend system-derivation)))

(define (compute-boot-script _ mexps)
  (mlet %store-monad ((gexps (sequence %store-monad mexps)))
    (gexp->file "boot"
                ;; Clean up and activate the system, then spawn shepherd.
                #~(begin #$@gexps))))

(define (boot-script-entry mboot)
  "Return, as a monadic value, an entry for the boot script in the system
directory."
  (mlet %store-monad ((boot mboot))
    (return `(("boot" ,boot)))))

(define boot-service-type
  ;; The service of this type is extended by being passed gexps as monadic
  ;; values.  It aggregates them in a single script, as a monadic value, which
  ;; becomes its 'parameters'.  It is the only service that extends nothing.
  (service-type (name 'boot)
                (extensions
                 (list (service-extension system-service-type
                                          boot-script-entry)))
                (compose append)
                (extend compute-boot-script)))

(define %boot-service
  ;; The service that produces the boot script.
  (service boot-service-type #t))

(define (cleanup-gexp _)
  "Return as a monadic value a gexp to clean up /tmp and similar places upon
boot."
  (with-monad %store-monad
    (with-imported-modules '((guix build utils))
      (return #~(begin
                  (use-modules (guix build utils))

                  ;; Clean out /tmp and /var/run.
                  ;;
                  ;; XXX This needs to happen before service activations, so it
                  ;; has to be here, but this also implicitly assumes that /tmp
                  ;; and /var/run are on the root partition.
                  (letrec-syntax ((fail-safe (syntax-rules ()
                                               ((_ exp rest ...)
                                                (begin
                                                  (catch 'system-error
                                                    (lambda () exp)
                                                    (const #f))
                                                  (fail-safe rest ...)))
                                               ((_)
                                                #t))))
                    ;; Ignore I/O errors so the system can boot.
                    (fail-safe
                     ;; Remove stale Shadow lock files as they would lead to
                     ;; failures of 'useradd' & co.
                     (delete-file "/etc/group.lock")
                     (delete-file "/etc/passwd.lock")
                     (delete-file "/etc/.pwd.lock") ;from 'lckpwdf'

                     (delete-file-recursively "/tmp")
                     (delete-file-recursively "/var/run")
                     (mkdir "/tmp")
                     (chmod "/tmp" #o1777)
                     (mkdir "/var/run")
                     (chmod "/var/run" #o755))))))))

(define cleanup-service-type
  ;; Service that cleans things up in /tmp and similar.
  (service-type (name 'cleanup)
                (extensions
                 (list (service-extension boot-service-type
                                          cleanup-gexp)))))

(define (directory-union name things)
  "Return a directory that is the union of THINGS."
  (match things
    ((one)
     ;; Only one thing; return it.
     one)
    (_
     (computed-file name
                    (with-imported-modules '((guix build union))
                      #~(begin
                          (use-modules (guix build union))
                          (union-build #$output '#$things)))))))

(define* (activation-service->script service)
  "Return as a monadic value the activation script for SERVICE, a service of
ACTIVATION-SCRIPT-TYPE."
  (activation-script (service-value service)))

(define (activation-script gexps)
  "Return the system's activation script, which evaluates GEXPS."
  (define (service-activations)
    ;; Return the activation scripts for SERVICES.
    (mapm %store-monad
          (cut gexp->file "activate-service" <>)
          gexps))

  (mlet* %store-monad ((actions (service-activations)))
    (gexp->file "activate"
                (with-imported-modules (source-module-closure
                                        '((gnu build activation)
                                          (guix build utils)))
                  #~(begin
                      (use-modules (gnu build activation)
                                   (guix build utils))

                      ;; Make sure the user accounting database exists.  If it
                      ;; does not exist, 'setutxent' does not create it and
                      ;; thus there is no accounting at all.
                      (close-port (open-file "/var/run/utmpx" "a0"))

                      ;; Same for 'wtmp', which is populated by mingetty et
                      ;; al.
                      (mkdir-p "/var/log")
                      (close-port (open-file "/var/log/wtmp" "a0"))

                      ;; Set up /run/current-system.  Among other things this
                      ;; sets up locales, which the activation snippets
                      ;; executed below may expect.
                      (activate-current-system)

                      ;; Run the services' activation snippets.
                      ;; TODO: Use 'load-compiled'.
                      (for-each primitive-load '#$actions))))))

(define (gexps->activation-gexp gexps)
  "Return a gexp that runs the activation script containing GEXPS."
  (mlet %store-monad ((script (activation-script gexps)))
    (return #~(primitive-load #$script))))

(define (second-argument a b) b)

(define activation-service-type
  (service-type (name 'activate)
                (extensions
                 (list (service-extension boot-service-type
                                          gexps->activation-gexp)))
                (compose append)
                (extend second-argument)))

(define %activation-service
  ;; The activation service produces the activation script from the gexps it
  ;; receives.
  (service activation-service-type #t))

(define %modprobe-wrapper
  ;; Wrapper for the 'modprobe' command that knows where modules live.
  ;;
  ;; This wrapper is typically invoked by the Linux kernel ('call_modprobe',
  ;; in kernel/kmod.c), a situation where the 'LINUX_MODULE_DIRECTORY'
  ;; environment variable is not set---hence the need for this wrapper.
  (let ((modprobe "/run/current-system/profile/bin/modprobe"))
    (program-file "modprobe"
                  #~(begin
                      (setenv "LINUX_MODULE_DIRECTORY"
                              "/run/booted-system/kernel/lib/modules")
                      (apply execl #$modprobe
                             (cons #$modprobe (cdr (command-line))))))))

(define %linux-kernel-activation
  ;; Activation of the Linux kernel running on the bare metal (as opposed to
  ;; running in a container.)
  #~(begin
      ;; Tell the kernel to use our 'modprobe' command.
      (activate-modprobe #$%modprobe-wrapper)

      ;; Let users debug their own processes!
      (activate-ptrace-attach)))

(define %linux-bare-metal-service
  ;; The service that does things that are needed on the "bare metal", but not
  ;; necessary or impossible in a container.
  (simple-service 'linux-bare-metal
                  activation-service-type
                  %linux-kernel-activation))


(define special-files-service-type
  ;; Service to install "special files" such as /bin/sh and /usr/bin/env.
  (service-type
   (name 'special-files)
   (extensions
    (list (service-extension activation-service-type
                             (lambda (files)
                               #~(activate-special-files '#$files)))))
   (compose concatenate)
   (extend append)))

(define (extra-special-file file target)
  "Use TARGET as the \"special file\" FILE.  For example, TARGET might be
  (file-append coreutils \"/bin/env\")
and FILE could be \"/usr/bin/env\"."
  (simple-service (string->symbol (string-append "special-file-" file))
                  special-files-service-type
                  `((,file ,target))))

(define (etc-directory service)
  "Return the directory for SERVICE, a service of type ETC-SERVICE-TYPE."
  (files->etc-directory (service-value service)))

(define (files->etc-directory files)
  (file-union "etc" files))

(define (etc-entry files)
  "Return an entry for the /etc directory consisting of FILES in the system
directory."
  (with-monad %store-monad
    (return `(("etc" ,(files->etc-directory files))))))

(define etc-service-type
  (service-type (name 'etc)
                (extensions
                 (list
                  (service-extension activation-service-type
                                     (lambda (files)
                                       (let ((etc
                                              (files->etc-directory files)))
                                         #~(activate-etc #$etc))))
                  (service-extension system-service-type etc-entry)))
                (compose concatenate)
                (extend append)))

(define (etc-service files)
  "Return a new service of ETC-SERVICE-TYPE that populates /etc with FILES.
FILES must be a list of name/file-like object pairs."
  (service etc-service-type files))

(define setuid-program-service-type
  (service-type (name 'setuid-program)
                (extensions
                 (list (service-extension activation-service-type
                                          (lambda (programs)
                                            #~(activate-setuid-programs
                                               (list #$@programs))))))
                (compose concatenate)
                (extend append)))

(define (packages->profile-entry packages)
  "Return a system entry for the profile containing PACKAGES."
  (mlet %store-monad ((profile (profile-derivation
                                (packages->manifest
                                 (delete-duplicates packages eq?)))))
    (return `(("profile" ,profile)))))

(define profile-service-type
  ;; The service that populates the system's profile---i.e.,
  ;; /run/current-system/profile.  It is extended by package lists.
  (service-type (name 'profile)
                (extensions
                 (list (service-extension system-service-type
                                          packages->profile-entry)))
                (compose concatenate)
                (extend append)))

(define (firmware->activation-gexp firmware)
  "Return a gexp to make the packages listed in FIRMWARE loadable by the
kernel."
  (let ((directory (directory-union "firmware" firmware)))
    ;; Tell the kernel where firmware is.
    #~(activate-firmware (string-append #$directory "/lib/firmware"))))

(define firmware-service-type
  ;; The service that collects firmware.
  (service-type (name 'firmware)
                (extensions
                 (list (service-extension activation-service-type
                                          firmware->activation-gexp)))
                (compose concatenate)
                (extend append)))

(define (gc-roots->system-entry roots)
  "Return an entry in the system's output containing symlinks to ROOTS."
  (mlet %store-monad ((entry (gexp->derivation
                              "gc-roots"
                              #~(let ((roots '#$roots))
                                  (mkdir #$output)
                                  (chdir #$output)
                                  (for-each symlink
                                            roots
                                            (map number->string
                                                 (iota (length roots))))))))
    (return (if (null? roots)
                '()
                `(("gc-roots" ,entry))))))

(define gc-root-service-type
  ;; A service to associate extra garbage-collector roots to the system.  This
  ;; is a simple hack that guarantees that the system retains references to
  ;; the given list of roots.  Roots must be "lowerable" objects like
  ;; packages, or derivations.
  (service-type (name 'gc-roots)
                (extensions
                 (list (service-extension system-service-type
                                          gc-roots->system-entry)))
                (compose concatenate)
                (extend append)))


;;;
;;; Service folding.
;;;

(define-condition-type &missing-target-service-error &service-error
  missing-target-service-error?
  (service      missing-target-service-error-service)
  (target-type  missing-target-service-error-target-type))

(define-condition-type &ambiguous-target-service-error &service-error
  ambiguous-target-service-error?
  (service      ambiguous-target-service-error-service)
  (target-type  ambiguous-target-service-error-target-type))

(define (service-back-edges services)
  "Return a procedure that, when passed a <service>, returns the list of
<service> objects that depend on it."
  (define (add-edges service edges)
    (define (add-edge extension edges)
      (let ((target-type (service-extension-target extension)))
        (match (filter (lambda (service)
                         (eq? (service-kind service) target-type))
                       services)
          ((target)
           (vhash-consq target service edges))
          (()
           (raise
            (condition (&missing-target-service-error
                        (service service)
                        (target-type target-type))
                       (&message
                        (message
                         (format #f (G_ "no target of type '~a' for service '~a'")
                                 (service-type-name target-type)
                                 (service-type-name
                                  (service-kind service))))))))
          (x
           (raise
            (condition (&ambiguous-target-service-error
                        (service service)
                        (target-type target-type))
                       (&message
                        (message
                         (format #f
                                 (G_ "more than one target service of type '~a'")
                                 (service-type-name target-type))))))))))

    (fold add-edge edges (service-type-extensions (service-kind service))))

  (let ((edges (fold add-edges vlist-null services)))
    (lambda (node)
      (reverse (vhash-foldq* cons '() node edges)))))

(define* (fold-services services
                        #:key (target-type system-service-type))
  "Fold SERVICES by propagating their extensions down to the root of type
TARGET-TYPE; return the root service adjusted accordingly."
  (define dependents
    (service-back-edges services))

  (define (matching-extension target)
    (let ((target (service-kind target)))
      (match-lambda
        (($ <service-extension> type)
         (eq? type target)))))

  (define (apply-extension target)
    (lambda (service)
      (match (find (matching-extension target)
                   (service-type-extensions (service-kind service)))
        (($ <service-extension> _ compute)
         (compute (service-value service))))))

  (match (filter (lambda (service)
                   (eq? (service-kind service) target-type))
                 services)
    ((sink)
     (let loop ((sink sink))
       (let* ((dependents (map loop (dependents sink)))
              (extensions (map (apply-extension sink) dependents))
              (extend     (service-type-extend (service-kind sink)))
              (compose    (service-type-compose (service-kind sink)))
              (params     (service-value sink)))
         ;; We distinguish COMPOSE and EXTEND because PARAMS typically has a
         ;; different type than the elements of EXTENSIONS.
         (if extend
             (service (service-kind sink)
                      (extend params (compose extensions)))
             sink))))
    (()
     (raise
      (condition (&missing-target-service-error
                  (service #f)
                  (target-type target-type))
                 (&message
                  (message (format #f (G_ "service of type '~a' not found")
                                   (service-type-name target-type)))))))
    (x
     (raise
      (condition (&ambiguous-target-service-error
                  (service #f)
                  (target-type target-type))
                 (&message
                  (message
                   (format #f
                           (G_ "more than one target service of type '~a'")
                           (service-type-name target-type)))))))))

;;; services.scm ends here.