path: root/guix/derivations.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
;;;
;;; 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 (guix derivations)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-26)
  #:use-module (rnrs io ports)
  #:use-module (rnrs bytevectors)
  #:use-module (ice-9 match)
  #:use-module (ice-9 rdelim)
  #:use-module (guix store)
  #:use-module (guix utils)
  #:use-module (guix hash)
  #:use-module (guix base32)
  #:export (<derivation>
            derivation?
            derivation-outputs
            derivation-inputs
            derivation-sources
            derivation-system
            derivation-builder-arguments
            derivation-builder-environment-vars
            derivation-prerequisites
            derivation-prerequisites-to-build

            <derivation-output>
            derivation-output?
            derivation-output-path
            derivation-output-hash-algo
            derivation-output-hash

            <derivation-input>
            derivation-input?
            derivation-input-path
            derivation-input-sub-derivations
            derivation-input-output-paths

            fixed-output-derivation?
            derivation-hash

            read-derivation
            write-derivation
            derivation-path->output-path
            derivation-path->output-paths
            derivation

            %guile-for-build
            imported-modules
            compiled-modules
            build-expression->derivation
            imported-files))

;;;
;;; Nix derivations, as implemented in Nix's `derivations.cc'.
;;;

(define-record-type <derivation>
  (make-derivation outputs inputs sources system builder args env-vars)
  derivation?
  (outputs  derivation-outputs)      ; list of name/<derivation-output> pairs
  (inputs   derivation-inputs)       ; list of <derivation-input>
  (sources  derivation-sources)      ; list of store paths
  (system   derivation-system)       ; string
  (builder  derivation-builder)      ; store path
  (args     derivation-builder-arguments)         ; list of strings
  (env-vars derivation-builder-environment-vars)) ; list of name/value pairs

(define-record-type <derivation-output>
  (make-derivation-output path hash-algo hash)
  derivation-output?
  (path       derivation-output-path)             ; store path
  (hash-algo  derivation-output-hash-algo)        ; symbol | #f
  (hash       derivation-output-hash))            ; bytevector | #f

(define-record-type <derivation-input>
  (make-derivation-input path sub-derivations)
  derivation-input?
  (path            derivation-input-path)             ; store path
  (sub-derivations derivation-input-sub-derivations)) ; list of strings

(define (fixed-output-derivation? drv)
  "Return #t if DRV is a fixed-output derivation, such as the result of a
download with a fixed hash (aka. `fetchurl')."
  (match drv
    (($ <derivation>
        (($ <derivation-output> _ (? symbol?) (? string?))))
     #t)
    (_ #f)))

(define (derivation-input-output-paths input)
  "Return the list of output paths corresponding to INPUT, a
<derivation-input>."
  (match input
    (($ <derivation-input> path sub-drvs)
     (map (cut derivation-path->output-path path <>)
          sub-drvs))))

(define (derivation-prerequisites drv)
  "Return the list of derivation-inputs required to build DRV, recursively."
  (let loop ((drv    drv)
             (result '()))
    (let ((inputs (remove (cut member <> result)  ; XXX: quadratic
                          (derivation-inputs drv))))
      (fold loop
            (append inputs result)
            (map (lambda (i)
                   (call-with-input-file (derivation-input-path i)
                     read-derivation))
                 inputs)))))

(define* (derivation-prerequisites-to-build store drv
                                            #:key
                                            (outputs
                                             (map
                                              car
                                              (derivation-outputs drv)))
                                            (use-substitutes? #t))
  "Return two values: the list of derivation-inputs required to build the
OUTPUTS of DRV and not already available in STORE, recursively, and the list
of required store paths that can be substituted.  When USE-SUBSTITUTES? is #f,
that second value is the empty list."
  (define (derivation-output-paths drv sub-drvs)
    (match drv
      (($ <derivation> outputs)
       (map (lambda (sub-drv)
              (derivation-output-path (assoc-ref outputs sub-drv)))
            sub-drvs))))

  (define built?
    (cut valid-path? store <>))

  (define substitutable?
    ;; Return true if the given path is substitutable.  Call
    ;; `substitutable-paths' upfront, to benefit from parallelism in the
    ;; substituter.
    (if use-substitutes?
        (let ((s (substitutable-paths store
                                      (append
                                       (derivation-output-paths drv outputs)
                                       (append-map
                                        derivation-input-output-paths
                                        (derivation-prerequisites drv))))))
          (cut member <> s))
        (const #f)))

  (define input-built?
    (compose (cut any built? <>) derivation-input-output-paths))

  (define input-substitutable?
    ;; Return true if and only if all of SUB-DRVS are subsitutable.  If at
    ;; least one is missing, then everything must be rebuilt.
    (compose (cut every substitutable? <>) derivation-input-output-paths))

  (define (derivation-built? drv sub-drvs)
    (every built? (derivation-output-paths drv sub-drvs)))

  (define (derivation-substitutable? drv sub-drvs)
    (every substitutable? (derivation-output-paths drv sub-drvs)))

  (let loop ((drv        drv)
             (sub-drvs   outputs)
             (build      '())
             (substitute '()))
    (cond ((derivation-built? drv sub-drvs)
           (values build substitute))
          ((derivation-substitutable? drv sub-drvs)
           (values build
                   (append (derivation-output-paths drv sub-drvs)
                           substitute)))
          (else
           (let ((inputs (remove (lambda (i)
                                   (or (member i build) ; XXX: quadratic
                                       (input-built? i)
                                       (input-substitutable? i)))
                                 (derivation-inputs drv))))
             (fold2 loop
                    (append inputs build)
                    (append (append-map (lambda (input)
                                          (if (and (not (input-built? input))
                                                   (input-substitutable? input))
                                              (derivation-input-output-paths
                                               input)
                                              '()))
                                        (derivation-inputs drv))
                            substitute)
                    (map (lambda (i)
                           (call-with-input-file (derivation-input-path i)
                             read-derivation))
                         inputs)
                    (map derivation-input-sub-derivations inputs)))))))

(define (%read-derivation drv-port)
  ;; Actually read derivation from DRV-PORT.

  (define comma (string->symbol ","))

  (define (ununquote x)
    (match x
      (('unquote x) (ununquote x))
      ((x ...)      (map ununquote x))
      (_            x)))

  (define (outputs->alist x)
    (fold-right (lambda (output result)
                  (match output
                    ((name path "" "")
                     (alist-cons name
                                 (make-derivation-output path #f #f)
                                 result))
                    ((name path hash-algo hash)
                     ;; fixed-output
                     (let ((algo (string->symbol hash-algo))
                           (hash (base16-string->bytevector hash)))
                       (alist-cons name
                                   (make-derivation-output path algo hash)
                                   result)))))
                '()
                x))

  (define (make-input-drvs x)
    (fold-right (lambda (input result)
                  (match input
                    ((path (sub-drvs ...))
                     (cons (make-derivation-input path sub-drvs)
                           result))))
                '()
                x))

  ;; The contents of a derivation are typically ASCII, but choosing
  ;; UTF-8 allows us to take the fast path for Guile's `scm_getc'.
  (set-port-encoding! drv-port "UTF-8")

  (let loop ((exp    (read drv-port))
             (result '()))
    (match exp
      ((? eof-object?)
       (let ((result (reverse result)))
         (match result
           (('Derive ((outputs ...) (input-drvs ...)
                      (input-srcs ...)
                      (? string? system)
                      (? string? builder)
                      ((? string? args) ...)
                      ((var value) ...)))
            (make-derivation (outputs->alist outputs)
                             (make-input-drvs input-drvs)
                             input-srcs
                             system builder args
                             (fold-right alist-cons '() var value)))
           (_
            (error "failed to parse derivation" drv-port result)))))
      ((? (cut eq? <> comma))
       (loop (read drv-port) result))
      (_
       (loop (read drv-port)
             (cons (ununquote exp) result))))))

(define read-derivation
  (let ((cache (make-weak-value-hash-table 200)))
    (lambda (drv-port)
      "Read the derivation from DRV-PORT and return the corresponding
<derivation> object."
      ;; Memoize that operation because `%read-derivation' is quite expensive,
      ;; and because the same argument is read more than 15 times on average
      ;; during something like (package-derivation s gdb).
      (let ((file (and=> (port-filename drv-port) basename)))
        (or (and file (hash-ref cache file))
            (let ((drv (%read-derivation drv-port)))
              (hash-set! cache file drv)
              drv))))))

(define-inlinable (write-sequence lst write-item port)
  ;; Write each element of LST with WRITE-ITEM to PORT, separating them with a
  ;; comma.
  (match lst
    (()
     #t)
    ((prefix (... ...) last)
     (for-each (lambda (item)
                 (write-item item port)
                 (display "," port))
               prefix)
     (write-item last port))))

(define-inlinable (write-list lst write-item port)
  ;; Write LST as a derivation list to PORT, using WRITE-ITEM to write each
  ;; element.
  (display "[" port)
  (write-sequence lst write-item port)
  (display "]" port))

(define-inlinable (write-tuple lst write-item port)
  ;; Same, but write LST as a tuple.
  (display "(" port)
  (write-sequence lst write-item port)
  (display ")" port))

(define (write-derivation drv port)
  "Write the ATerm-like serialization of DRV to PORT.  See Section 2.4 of
Eelco Dolstra's PhD dissertation for an overview of a previous version of
that form."

  ;; Make sure we're using the faster implementation.
  (define format simple-format)

  (define (write-string-list lst)
    (write-list lst write port))

  (define (coalesce-duplicate-inputs inputs)
    ;; Return a list of inputs, such that when INPUTS contains the same DRV
    ;; twice, they are coalesced, with their sub-derivations merged.  This is
    ;; needed because Nix itself keeps only one of them.
    (fold (lambda (input result)
            (match input
              (($ <derivation-input> path sub-drvs)
               ;; XXX: quadratic
               (match (find (match-lambda
                             (($ <derivation-input> p s)
                              (string=? p path)))
                            result)
                 (#f
                  (cons input result))
                 ((and dup ($ <derivation-input> _ sub-drvs2))
                  ;; Merge DUP with INPUT.
                  (let ((sub-drvs (delete-duplicates
                                   (append sub-drvs sub-drvs2))))
                    (cons (make-derivation-input path sub-drvs)
                          (delq dup result))))))))
          '()
          inputs))

  (define (write-output output port)
    (match output
     ((name . ($ <derivation-output> path hash-algo hash))
      (write-tuple (list name path
                         (or (and=> hash-algo symbol->string) "")
                         (or (and=> hash bytevector->base16-string)
                             ""))
                   write
                   port))))

  (define (write-input input port)
    (match input
      (($ <derivation-input> path sub-drvs)
       (display "(" port)
       (write path port)
       (display "," port)
       (write-string-list (sort sub-drvs string<?))
       (display ")" port))))

  (define (write-env-var env-var port)
    (match env-var
      ((name . value)
       (display "(" port)
       (write name port)
       (display "," port)
       (write value port)
       (display ")" port))))

  ;; Note: lists are sorted alphabetically, to conform with the behavior of
  ;; C++ `std::map' in Nix itself.

  (match drv
    (($ <derivation> outputs inputs sources
        system builder args env-vars)
     (display "Derive(" port)
     (write-list (sort outputs
                       (lambda (o1 o2)
                         (string<? (car o1) (car o2))))
                 write-output
                 port)
     (display "," port)
     (write-list (sort (coalesce-duplicate-inputs inputs)
                       (lambda (i1 i2)
                         (string<? (derivation-input-path i1)
                                   (derivation-input-path i2))))
                 write-input
                 port)
     (display "," port)
     (write-string-list (sort sources string<?))
     (format port ",~s,~s," system builder)
     (write-string-list args)
     (display "," port)
     (write-list (sort env-vars
                       (lambda (e1 e2)
                         (string<? (car e1) (car e2))))
                 write-env-var
                 port)
     (display ")" port))))

(define derivation-path->output-path
  ;; This procedure is called frequently, so memoize it.
  (memoize
   (lambda* (path #:optional (output "out"))
     "Read the derivation from PATH (`/nix/store/xxx.drv'), and return the store
path of its output OUTPUT."
     (let* ((drv     (call-with-input-file path read-derivation))
            (outputs (derivation-outputs drv)))
       (and=> (assoc-ref outputs output) derivation-output-path)))))

(define (derivation-path->output-paths path)
  "Read the derivation from PATH (`/nix/store/xxx.drv'), and return the
list of name/path pairs of its outputs."
  (let* ((drv     (call-with-input-file path read-derivation))
         (outputs (derivation-outputs drv)))
    (map (match-lambda
          ((name . output)
           (cons name (derivation-output-path output))))
         outputs)))


;;;
;;; Derivation primitive.
;;;

(define (compressed-hash bv size)                 ; `compressHash'
  "Given the hash stored in BV, return a compressed version thereof that fits
in SIZE bytes."
  (define new (make-bytevector size 0))
  (define old-size (bytevector-length bv))
  (let loop ((i 0))
    (if (= i old-size)
        new
        (let* ((j (modulo i size))
               (o (bytevector-u8-ref new j)))
          (bytevector-u8-set! new j
                              (logxor o (bytevector-u8-ref bv i)))
          (loop (+ 1 i))))))

(define derivation-hash            ; `hashDerivationModulo' in derivations.cc
  (memoize
   (lambda (drv)
    "Return the hash of DRV, modulo its fixed-output inputs, as a bytevector."
    (match drv
      (($ <derivation> ((_ . ($ <derivation-output> path
                                (? symbol? hash-algo) (? bytevector? hash)))))
       ;; A fixed-output derivation.
       (sha256
        (string->utf8
         (string-append "fixed:out:" (symbol->string hash-algo)
                        ":" (bytevector->base16-string hash)
                        ":" path))))
      (($ <derivation> outputs inputs sources
          system builder args env-vars)
       ;; A regular derivation: replace the path of each input with that
       ;; input's hash; return the hash of serialization of the resulting
       ;; derivation.
       (let* ((inputs (map (match-lambda
                            (($ <derivation-input> path sub-drvs)
                             (let ((hash (call-with-input-file path
                                           (compose bytevector->base16-string
                                                    derivation-hash
                                                    read-derivation))))
                               (make-derivation-input hash sub-drvs))))
                           inputs))
              (drv    (make-derivation outputs inputs sources
                                       system builder args env-vars)))

         ;; XXX: At this point this remains faster than `port-sha256', because
         ;; the SHA256 port's `write' method gets called for every single
         ;; character.
         (sha256
          (with-fluids ((%default-port-encoding "UTF-8"))
            (string->utf8 (call-with-output-string
                           (cut write-derivation drv <>)))))))))))

(define (store-path type hash name)               ; makeStorePath
  "Return the store path for NAME/HASH/TYPE."
  (let* ((s (string-append type ":sha256:"
                           (bytevector->base16-string hash) ":"
                           (%store-prefix) ":" name))
         (h (sha256 (string->utf8 s)))
         (c (compressed-hash h 20)))
    (string-append (%store-prefix) "/"
                   (bytevector->nix-base32-string c) "-"
                   name)))

(define (output-path output hash name)            ; makeOutputPath
  "Return an output path for OUTPUT (the name of the output as a string) of
the derivation called NAME with hash HASH."
  (store-path (string-append "output:" output) hash
              (if (string=? output "out")
                  name
                  (string-append name "-" output))))

(define* (derivation store name builder args
                     #:key
                     (system (%current-system)) (env-vars '())
                     (inputs '()) (outputs '("out"))
                     hash hash-algo hash-mode
                     references-graphs)
  "Build a derivation with the given arguments.  Return the resulting
store path and <derivation> object.  When HASH, HASH-ALGO, and HASH-MODE
are given, a fixed-output derivation is created---i.e., one whose result is
known in advance, such as a file download.

When REFERENCES-GRAPHS is true, it must be a list of file name/store path
pairs.  In that case, the reference graph of each store path is exported in
the build environment in the corresponding file, in a simple text format."
  (define direct-store-path?
    (let ((len (+ 1 (string-length (%store-prefix)))))
      (lambda (p)
        ;; Return #t if P is a store path, and not a sub-directory of a
        ;; store path.  This predicate is needed because files *under* a
        ;; store path are not valid inputs.
        (and (store-path? p)
             (not (string-index (substring p len) #\/))))))

  (define (add-output-paths drv)
    ;; Return DRV with an actual store path for each of its output and the
    ;; corresponding environment variable.
    (match drv
      (($ <derivation> outputs inputs sources
          system builder args env-vars)
       (let* ((drv-hash (derivation-hash drv))
              (outputs  (map (match-lambda
                              ((output-name . ($ <derivation-output>
                                                 _ algo hash))
                               (let ((path (output-path output-name
                                                        drv-hash name)))
                                 (cons output-name
                                       (make-derivation-output path algo
                                                               hash)))))
                             outputs)))
         (make-derivation outputs inputs sources system builder args
                          (map (match-lambda
                                ((name . value)
                                 (cons name
                                       (or (and=> (assoc-ref outputs name)
                                                  derivation-output-path)
                                           value))))
                               env-vars))))))

  (define (user+system-env-vars)
    ;; Some options are passed to the build daemon via the env. vars of
    ;; derivations (urgh!).  We hide that from our API, but here is the place
    ;; where we kludgify those options.
    (match references-graphs
      (((file . path) ...)
       (let ((value (map (cut string-append <> " " <>)
                         file path)))
         ;; XXX: This all breaks down if an element of FILE or PATH contains
         ;; white space.
         `(("exportReferencesGraph" . ,(string-join value " "))
           ,@env-vars)))
      (#f
       env-vars)))

  (define (env-vars-with-empty-outputs env-vars)
    ;; Return a variant of ENV-VARS where each OUTPUTS is associated with an
    ;; empty string, even outputs that do not appear in ENV-VARS.
    (let ((e (map (match-lambda
                   ((name . val)
                    (if (member name outputs)
                        (cons name "")
                        (cons name val))))
                  env-vars)))
      (fold (lambda (output-name env-vars)
              (if (assoc output-name env-vars)
                  env-vars
                  (append env-vars `((,output-name . "")))))
            e
            outputs)))

  (let* ((outputs    (map (lambda (name)
                            ;; Return outputs with an empty path.
                            (cons name
                                  (make-derivation-output "" hash-algo hash)))
                          outputs))
         (inputs     (map (match-lambda
                           (((? direct-store-path? input))
                            (make-derivation-input input '("out")))
                           (((? direct-store-path? input) sub-drvs ...)
                            (make-derivation-input input sub-drvs))
                           ((input . _)
                            (let ((path (add-to-store store
                                                      (basename input)
                                                      #t "sha256" input)))
                              (make-derivation-input path '()))))
                          (delete-duplicates inputs)))
         (env-vars   (env-vars-with-empty-outputs (user+system-env-vars)))
         (drv-masked (make-derivation outputs
                                      (filter (compose derivation-path?
                                                       derivation-input-path)
                                              inputs)
                                      (filter-map (lambda (i)
                                                    (let ((p (derivation-input-path i)))
                                                      (and (not (derivation-path? p))
                                                           p)))
                                                  inputs)
                                      system builder args env-vars))
         (drv        (add-output-paths drv-masked)))

    ;; (write-derivation drv-masked (current-error-port))
    ;; (newline (current-error-port))
    (values (add-text-to-store store (string-append name ".drv")
                               (call-with-output-string
                                (cut write-derivation drv <>))
                               (map derivation-input-path
                                    inputs))
            drv)))


;;;
;;; Guile-based builders.
;;;

(define %guile-for-build
  ;; The derivation of the Guile to be used within the build environment,
  ;; when using `build-expression->derivation'.
  (make-parameter #f))

(define (parent-directories file-name)
  "Return the list of parent dirs of FILE-NAME, in the order in which an
`mkdir -p' implementation would make them."
  (let ((not-slash (char-set-complement (char-set #\/))))
    (reverse
     (fold (lambda (dir result)
             (match result
               (()
                (list dir))
               ((prev _ ...)
                (cons (string-append prev "/" dir)
                      result))))
           '()
           (remove (cut string=? <> ".")
                   (string-tokenize (dirname file-name) not-slash))))))

(define* (imported-files store files
                         #:key (name "file-import")
                         (system (%current-system))
                         (guile (%guile-for-build)))
  "Return a derivation that imports FILES into STORE.  FILES must be a list
of (FINAL-PATH . FILE-NAME) pairs; each FILE-NAME is read from the file
system, imported, and appears under FINAL-PATH in the resulting store path."
  (let* ((files   (map (match-lambda
                        ((final-path . file-name)
                         (list final-path
                               (add-to-store store (basename final-path) #f
                                             "sha256" file-name))))
                       files))
         (builder
          `(begin
             (mkdir %output) (chdir %output)
             ,@(append-map (match-lambda
                            ((final-path store-path)
                             (append (match (parent-directories final-path)
                                       (() '())
                                       ((head ... tail)
                                        (append (map (lambda (d)
                                                       `(false-if-exception
                                                         (mkdir ,d)))
                                                     head)
                                                `((or (file-exists? ,tail)
                                                      (mkdir ,tail))))))
                                     `((symlink ,store-path ,final-path)))))
                           files))))
    (build-expression->derivation store name system
                                  builder files
                                  #:guile-for-build guile)))

(define* (imported-modules store modules
                           #:key (name "module-import")
                           (system (%current-system))
                           (guile (%guile-for-build))
                           (module-path %load-path))
  "Return a derivation that contains the source files of MODULES, a list of
module names such as `(ice-9 q)'.  All of MODULES must be in the MODULE-PATH
search path."
  ;; TODO: Determine the closure of MODULES, build the `.go' files,
  ;; canonicalize the source files through read/write, etc.
  (let ((files (map (lambda (m)
                      (let ((f (string-append
                                (string-join (map symbol->string m) "/")
                                ".scm")))
                        (cons f (search-path module-path f))))
                    modules)))
    (imported-files store files #:name name #:system system
                    #:guile guile)))

(define* (compiled-modules store modules
                           #:key (name "module-import-compiled")
                           (system (%current-system))
                           (guile (%guile-for-build))
                           (module-path %load-path))
  "Return a derivation that builds a tree containing the `.go' files
corresponding to MODULES.  All the MODULES are built in a context where
they can refer to each other."
  (let* ((module-drv (imported-modules store modules
                                       #:system system
                                       #:guile guile
                                       #:module-path module-path))
         (module-dir (derivation-path->output-path module-drv))
         (files      (map (lambda (m)
                            (let ((f (string-join (map symbol->string m)
                                                  "/")))
                              (cons (string-append f ".go")
                                    (string-append module-dir "/" f ".scm"))))
                      modules)))
    (define builder
      `(begin
         (use-modules (system base compile))
         (let ((out (assoc-ref %outputs "out")))
           (mkdir out)
           (chdir out))

         (set! %load-path
               (cons ,module-dir %load-path))

         ,@(map (match-lambda
                 ((output . input)
                  (let ((make-parent-dirs (map (lambda (dir)
                                                 `(unless (file-exists? ,dir)
                                                    (mkdir ,dir)))
                                               (parent-directories output))))
                   `(begin
                      ,@make-parent-dirs
                      (compile-file ,input
                                    #:output-file ,output
                                    #:opts %auto-compilation-options)))))
                files)))

    (build-expression->derivation store name system builder
                                  `(("modules" ,module-drv))
                                  #:guile-for-build guile)))

(define* (build-expression->derivation store name system exp inputs
                                       #:key (outputs '("out"))
                                       hash hash-algo
                                       (env-vars '())
                                       (modules '())
                                       guile-for-build
                                       references-graphs)
  "Return a derivation that executes Scheme expression EXP as a builder
for derivation NAME.  INPUTS must be a list of (NAME DRV-PATH SUB-DRV)
tuples; when SUB-DRV is omitted, \"out\" is assumed.  MODULES is a list
of names of Guile modules from the current search path to be copied in
the store, compiled, and made available in the load path during the
execution of EXP.

EXP is evaluated in an environment where %OUTPUT is bound to the main
output path, %OUTPUTS is bound to a list of output/path pairs, and where
%BUILD-INPUTS is bound to an alist of string/output-path pairs made from
INPUTS.  Optionally, ENV-VARS is a list of string pairs specifying the
name and value of environment variables visible to the builder.  The
builder terminates by passing the result of EXP to `exit'; thus, when
EXP returns #f, the build is considered to have failed.

EXP is built using GUILE-FOR-BUILD (a derivation).  When GUILE-FOR-BUILD is
omitted or is #f, the value of the `%guile-for-build' fluid is used instead.

See the `derivation' procedure for the meaning of REFERENCES-GRAPHS."
  (define guile-drv
    (or guile-for-build (%guile-for-build)))

  (define guile
    (string-append (derivation-path->output-path guile-drv)
                   "/bin/guile"))

  (define module-form?
    (match-lambda
     (((or 'define-module 'use-modules) _ ...) #t)
     (_ #f)))

  (define source-path
    ;; When passed an input that is a source, return its path; otherwise
    ;; return #f.
    (match-lambda
     ((_ path _ ...)
      (and (not (derivation-path? path))
           path))))

  (let* ((prologue `(begin
                      ,@(match exp
                          ((_ ...)
                           ;; Module forms must appear at the top-level so
                           ;; that any macros they export can be expanded.
                           (filter module-form? exp))
                          (_ `(,exp)))

                      (define %output (getenv "out"))
                      (define %outputs
                        (map (lambda (o)
                               (cons o (getenv o)))
                             ',outputs))
                      (define %build-inputs
                        ',(map (match-lambda
                                ((name drv . rest)
                                 (let ((sub (match rest
                                              (() "out")
                                              ((x) x))))
                                   (cons name
                                         (if (derivation-path? drv)
                                             (derivation-path->output-path drv
                                                                           sub)
                                             drv)))))
                               inputs))

                      ,@(if (null? modules)
                            '()
                            ;; Remove our own settings.
                            '((unsetenv "GUILE_LOAD_COMPILED_PATH")))

                      ;; Guile sets it, but remove it to avoid conflicts when
                      ;; building Guile-using packages.
                      (unsetenv "LD_LIBRARY_PATH")))
         (builder  (add-text-to-store store
                                      (string-append name "-guile-builder")

                                      ;; Explicitly use UTF-8 for determinism,
                                      ;; and also because UTF-8 output is faster.
                                      (with-fluids ((%default-port-encoding
                                                     "UTF-8"))
                                        (call-with-output-string
                                         (lambda (port)
                                           (write prologue port)
                                           (write
                                            `(exit
                                              ,(match exp
                                                 ((_ ...)
                                                  (remove module-form? exp))
                                                 (_ `(,exp))))
                                            port))))

                                      ;; The references don't really matter
                                      ;; since the builder is always used in
                                      ;; conjunction with the drv that needs
                                      ;; it.  For clarity, we add references
                                      ;; to the subset of INPUTS that are
                                      ;; sources, avoiding references to other
                                      ;; .drv; otherwise, BUILDER's hash would
                                      ;; depend on those, even if they are
                                      ;; fixed-output.
                                      (filter-map source-path inputs)))

         (mod-drv  (and (pair? modules)
                        (imported-modules store modules
                                          #:guile guile-drv
                                          #:system system)))
         (mod-dir  (and mod-drv
                        (derivation-path->output-path mod-drv)))
         (go-drv   (and (pair? modules)
                        (compiled-modules store modules
                                          #:guile guile-drv
                                          #:system system)))
         (go-dir   (and go-drv
                        (derivation-path->output-path go-drv))))
    (derivation store name guile
                `("--no-auto-compile"
                  ,@(if mod-dir `("-L" ,mod-dir) '())
                  ,builder)

                #:system system

                #:inputs `((,(or guile-for-build (%guile-for-build)))
                           (,builder)
                           ,@(map cdr inputs)
                           ,@(if mod-drv `((,mod-drv) (,go-drv)) '()))

                ;; When MODULES is non-empty, shamelessly clobber
                ;; $GUILE_LOAD_COMPILED_PATH.
                #:env-vars (if go-dir
                               `(("GUILE_LOAD_COMPILED_PATH" . ,go-dir)
                                 ,@(alist-delete "GUILE_LOAD_COMPILED_PATH"
                                                 env-vars))
                               env-vars)

                #:hash hash #:hash-algo hash-algo
                #:outputs outputs
                #:references-graphs references-graphs)))