path: root/guix/build/utils.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.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 build utils)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-11)
  #:use-module (ice-9 ftw)
  #:use-module (ice-9 match)
  #:use-module (ice-9 regex)
  #:use-module (ice-9 rdelim)
  #:use-module (ice-9 format)
  #:use-module (rnrs bytevectors)
  #:use-module (rnrs io ports)
  #:re-export (alist-cons
               alist-delete)
  #:export (%store-directory
            parallel-job-count

            directory-exists?
            executable-file?
            call-with-ascii-input-file
            elf-file?
            ar-file?
            with-directory-excursion
            mkdir-p
            copy-recursively
            delete-file-recursively
            find-files

            set-path-environment-variable
            search-path-as-string->list
            list->search-path-as-string
            which

            alist-cons-before
            alist-cons-after
            alist-replace
            with-atomic-file-replacement
            substitute
            substitute*
            dump-port
            set-file-time
            patch-shebang
            patch-makefile-SHELL
            fold-port-matches
            remove-store-references
            wrap-program))


;;;
;;; Directories.
;;;

(define (%store-directory)
  "Return the directory name of the store."
  (or (getenv "NIX_STORE")
      "/gnu/store"))

(define parallel-job-count
  ;; Number of processes to be passed next to GNU Make's `-j' argument.
  (make-parameter
   (match (getenv "NIX_BUILD_CORES")              ;set by the daemon
     (#f  1)
     ("0" (current-processor-count))
     (x   (or (string->number x) 1)))))

(define (directory-exists? dir)
  "Return #t if DIR exists and is a directory."
  (let ((s (stat dir #f)))
    (and s
         (eq? 'directory (stat:type s)))))

(define (executable-file? file)
  "Return #t if FILE exists and is executable."
  (let ((s (stat file #f)))
    (and s
         (not (zero? (logand (stat:mode s) #o100))))))

(define (call-with-ascii-input-file file proc)
  "Open FILE as an ASCII or binary file, and pass the resulting port to
PROC.  FILE is closed when PROC's dynamic extent is left.  Return the
return values of applying PROC to the port."
  (let ((port (with-fluids ((%default-port-encoding #f))
                ;; Use "b" so that `open-file' ignores `coding:' cookies.
                (open-file file "rb"))))
    (dynamic-wind
      (lambda ()
        #t)
      (lambda ()
        (proc port))
      (lambda ()
        (close-input-port port)))))

(define (file-header-match header)
  "Return a procedure that returns true when its argument is a file starting
with the bytes in HEADER, a bytevector."
  (define len
    (bytevector-length header))

  (lambda (file)
    "Return true if FILE starts with the right magic bytes."
    (define (get-header)
      (call-with-input-file file
        (lambda (port)
          (get-bytevector-n port len))
        #:binary #t #:guess-encoding #f))

    (catch 'system-error
      (lambda ()
        (equal? (get-header) header))
      (lambda args
        (if (= EISDIR (system-error-errno args))
            #f                                    ;FILE is a directory
            (apply throw args))))))

(define %elf-magic-bytes
  ;; Magic bytes of ELF files.  See <elf.h>.
  (u8-list->bytevector (map char->integer (string->list "\x7FELF"))))

(define elf-file?
  (file-header-match %elf-magic-bytes))

(define %ar-magic-bytes
  ;; Magic bytes of archives created by 'ar'.  See <ar.h>.
  (u8-list->bytevector (map char->integer (string->list "!<arch>\n"))))

(define ar-file?
  (file-header-match %ar-magic-bytes))

(define-syntax-rule (with-directory-excursion dir body ...)
  "Run BODY with DIR as the process's current directory."
  (let ((init (getcwd)))
   (dynamic-wind
     (lambda ()
       (chdir dir))
     (lambda ()
       body ...)
     (lambda ()
       (chdir init)))))

(define (mkdir-p dir)
  "Create directory DIR and all its ancestors."
  (define absolute?
    (string-prefix? "/" dir))

  (define not-slash
    (char-set-complement (char-set #\/)))

  (let loop ((components (string-tokenize dir not-slash))
             (root       (if absolute?
                             ""
                             ".")))
    (match components
      ((head tail ...)
       (let ((path (string-append root "/" head)))
         (catch 'system-error
           (lambda ()
             (mkdir path)
             (loop tail path))
           (lambda args
             (if (= EEXIST (system-error-errno args))
                 (loop tail path)
                 (apply throw args))))))
      (() #t))))

(define* (copy-recursively source destination
                           #:key
                           (log (current-output-port))
                           (follow-symlinks? #f)
                           keep-mtime?)
  "Copy SOURCE directory to DESTINATION.  Follow symlinks if FOLLOW-SYMLINKS?
is true; otherwise, just preserve them.  When KEEP-MTIME? is true, keep the
modification time of the files in SOURCE on those of DESTINATION.  Write
verbose output to the LOG port."
  (define strip-source
    (let ((len (string-length source)))
      (lambda (file)
        (substring file len))))

  (file-system-fold (const #t)                    ; enter?
                    (lambda (file stat result)    ; leaf
                      (let ((dest (string-append destination
                                                 (strip-source file))))
                        (format log "`~a' -> `~a'~%" file dest)
                        (case (stat:type stat)
                          ((symlink)
                           (let ((target (readlink file)))
                             (symlink target dest)))
                          (else
                           (copy-file file dest)
                           (when keep-mtime?
                             (set-file-time dest stat))))))
                    (lambda (dir stat result)     ; down
                      (let ((target (string-append destination
                                                   (strip-source dir))))
                        (mkdir-p target)
                        (when keep-mtime?
                          (set-file-time target stat))))
                    (lambda (dir stat result)     ; up
                      result)
                    (const #t)                    ; skip
                    (lambda (file stat errno result)
                      (format (current-error-port) "i/o error: ~a: ~a~%"
                              file (strerror errno))
                      #f)
                    #t
                    source

                    (if follow-symlinks?
                        stat
                        lstat)))

(define* (delete-file-recursively dir
                                  #:key follow-mounts?)
  "Delete DIR recursively, like `rm -rf', without following symlinks.  Don't
follow mount points either, unless FOLLOW-MOUNTS? is true.  Report but ignore
errors."
  (let ((dev (stat:dev (lstat dir))))
    (file-system-fold (lambda (dir stat result)    ; enter?
                        (or follow-mounts?
                            (= dev (stat:dev stat))))
                      (lambda (file stat result)   ; leaf
                        (delete-file file))
                      (const #t)                   ; down
                      (lambda (dir stat result)    ; up
                        (rmdir dir))
                      (const #t)                   ; skip
                      (lambda (file stat errno result)
                        (format (current-error-port)
                                "warning: failed to delete ~a: ~a~%"
                                file (strerror errno)))
                      #t
                      dir

                      ;; Don't follow symlinks.
                      lstat)))

(define (find-files dir regexp)
  "Return the lexicographically sorted list of files under DIR whose basename
matches REGEXP."
  (define file-rx
    (if (regexp? regexp)
        regexp
        (make-regexp regexp)))

  ;; Sort the result to get deterministic results.
  (sort (file-system-fold (const #t)
                          (lambda (file stat result)   ; leaf
                            (if (regexp-exec file-rx (basename file))
                                (cons file result)
                                result))
                          (lambda (dir stat result)    ; down
                            result)
                          (lambda (dir stat result)    ; up
                            result)
                          (lambda (file stat result)   ; skip
                            result)
                          (lambda (file stat errno result)
                            (format (current-error-port) "find-files: ~a: ~a~%"
                                    file (strerror errno))
                            result)
                          '()
                          dir)
        string<?))


;;;
;;; Search paths.
;;;

(define (search-path-as-list sub-directories input-dirs)
  "Return the list of directories among SUB-DIRECTORIES that exist in
INPUT-DIRS.  Example:

  (search-path-as-list '(\"share/emacs/site-lisp\" \"share/emacs/24.1\")
                       (list \"/package1\" \"/package2\" \"/package3\"))
  => (\"/package1/share/emacs/site-lisp\"
      \"/package3/share/emacs/site-lisp\")

"
  (append-map (lambda (input)
                (filter-map (lambda (dir)
                              (let ((dir (string-append input "/"
                                                        dir)))
                                (and (directory-exists? dir)
                                     dir)))
                            sub-directories))
              input-dirs))

(define (list->search-path-as-string lst separator)
  (string-join lst separator))

(define* (search-path-as-string->list path #:optional (separator #\:))
  (string-tokenize path (char-set-complement (char-set separator))))

(define* (set-path-environment-variable env-var sub-directories input-dirs
                                        #:key (separator ":"))
  "Look for each of SUB-DIRECTORIES in INPUT-DIRS.  Set ENV-VAR to a
SEPARATOR-separated path accordingly.  Example:

  (set-path-environment-variable \"PKG_CONFIG\"
                                 '(\"lib/pkgconfig\")
                                 (list package1 package2))
"
  (let* ((path  (search-path-as-list sub-directories input-dirs))
         (value (list->search-path-as-string path separator)))
    (if (string-null? value)
        (begin
          ;; Never set ENV-VAR to an empty string because often, the empty
          ;; string is equivalent to ".".  This is the case for
          ;; GUILE_LOAD_PATH in Guile 2.0, for instance.
          (unsetenv env-var)
          (format #t "environment variable `~a' unset~%" env-var))
        (begin
          (setenv env-var value)
          (format #t "environment variable `~a' set to `~a'~%"
                  env-var value)))))

(define (which program)
  "Return the complete file name for PROGRAM as found in $PATH, or #f if
PROGRAM could not be found."
  (search-path (search-path-as-string->list (getenv "PATH"))
               program))


;;;
;;; Phases.
;;;
;;; In (guix build gnu-build-system), there are separate phases (configure,
;;; build, test, install).  They are represented as a list of name/procedure
;;; pairs.  The following procedures make it easy to change the list of
;;; phases.
;;;

(define* (alist-cons-before reference key value alist
                            #:optional (key=? equal?))
  "Insert the KEY/VALUE pair before the first occurrence of a pair whose key
is REFERENCE in ALIST.  Use KEY=? to compare keys."
  (let-values (((before after)
                (break (match-lambda
                        ((k . _)
                         (key=? k reference)))
                       alist)))
    (append before (alist-cons key value after))))

(define* (alist-cons-after reference key value alist
                           #:optional (key=? equal?))
  "Insert the KEY/VALUE pair after the first occurrence of a pair whose key
is REFERENCE in ALIST.  Use KEY=? to compare keys."
  (let-values (((before after)
                (break (match-lambda
                        ((k . _)
                         (key=? k reference)))
                       alist)))
    (match after
      ((reference after ...)
       (append before (cons* reference `(,key . ,value) after)))
      (()
       (append before `((,key . ,value)))))))

(define* (alist-replace key value alist #:optional (key=? equal?))
  "Replace the first pair in ALIST whose car is KEY with the KEY/VALUE pair.
An error is raised when no such pair exists."
  (let-values (((before after)
                (break (match-lambda
                        ((k . _)
                         (key=? k key)))
                       alist)))
    (match after
      ((_ after ...)
       (append before (alist-cons key value after))))))


;;;
;;; Text substitution (aka. sed).
;;;

(define (with-atomic-file-replacement file proc)
  "Call PROC with two arguments: an input port for FILE, and an output
port for the file that is going to replace FILE.  Upon success, FILE is
atomically replaced by what has been written to the output port, and
PROC's result is returned."
  (let* ((template (string-append file ".XXXXXX"))
         (out      (mkstemp! template))
         (mode     (stat:mode (stat file))))
    (with-throw-handler #t
      (lambda ()
        (call-with-input-file file
          (lambda (in)
            (let ((result (proc in out)))
              (close out)
              (chmod template mode)
              (rename-file template file)
              result))))
      (lambda (key . args)
        (false-if-exception (delete-file template))))))

(define (substitute file pattern+procs)
  "PATTERN+PROCS is a list of regexp/two-argument procedure.  For each line
of FILE, and for each PATTERN that it matches, call the corresponding PROC
as (PROC LINE MATCHES); PROC must return the line that will be written as a
substitution of the original line."
  (let ((rx+proc  (map (match-lambda
                        (((? regexp? pattern) . proc)
                         (cons pattern proc))
                        ((pattern . proc)
                         (cons (make-regexp pattern regexp/extended)
                               proc)))
                       pattern+procs)))
    (with-atomic-file-replacement file
      (lambda (in out)
        (let loop ((line (read-line in 'concat)))
          (if (eof-object? line)
              #t
              (let ((line (fold (lambda (r+p line)
                                  (match r+p
                                    ((regexp . proc)
                                     (match (list-matches regexp line)
                                       ((and m+ (_ _ ...))
                                        (proc line m+))
                                       (_ line)))))
                                line
                                rx+proc)))
                (display line out)
                (loop (read-line in 'concat)))))))))


(define-syntax let-matches
  ;; Helper macro for `substitute*'.
  (syntax-rules (_)
    ((let-matches index match (_ vars ...) body ...)
     (let-matches (+ 1 index) match (vars ...)
                  body ...))
    ((let-matches index match (var vars ...) body ...)
     (let ((var (match:substring match index)))
       (let-matches (+ 1 index) match (vars ...)
                    body ...)))
    ((let-matches index match () body ...)
     (begin body ...))))

(define-syntax substitute*
  (syntax-rules ()
    "Substitute REGEXP in FILE by the string returned by BODY.  BODY is
evaluated with each MATCH-VAR bound to the corresponding positional regexp
sub-expression.  For example:

  (substitute* file
     ((\"hello\")
      \"good morning\\n\")
     ((\"foo([a-z]+)bar(.*)$\" all letters end)
      (string-append \"baz\" letter end)))

Here, anytime a line of FILE contains \"hello\", it is replaced by \"good
morning\".  Anytime a line of FILE matches the second regexp, ALL is bound to
the complete match, LETTERS is bound to the first sub-expression, and END is
bound to the last one.

When one of the MATCH-VAR is `_', no variable is bound to the corresponding
match substring.

Alternatively, FILE may be a list of file names, in which case they are
all subject to the substitutions."
    ((substitute* file ((regexp match-var ...) body ...) ...)
     (let ()
       (define (substitute-one-file file-name)
         (substitute
          file-name
          (list (cons regexp
                      (lambda (l m+)
                        ;; Iterate over matches M+ and return the
                        ;; modified line based on L.
                        (let loop ((m* m+)  ; matches
                                   (o  0)   ; offset in L
                                   (r  '())) ; result
                          (match m*
                            (()
                             (let ((r (cons (substring l o) r)))
                               (string-concatenate-reverse r)))
                            ((m . rest)
                             (let-matches 0 m (match-var ...)
                               (loop rest
                                     (match:end m)
                                     (cons*
                                      (begin body ...)
                                      (substring l o (match:start m))
                                      r))))))))
                ...)))

       (match file
         ((files (... ...))
          (for-each substitute-one-file files))
         ((? string? f)
          (substitute-one-file f)))))))


;;;
;;; Patching shebangs---e.g., /bin/sh -> /gnu/store/xyz...-bash/bin/sh.
;;;

(define* (dump-port in out
                    #:key (buffer-size 16384)
                    (progress (lambda (t k) (k))))
  "Read as much data as possible from IN and write it to OUT, using
chunks of BUFFER-SIZE bytes.  Call PROGRESS after each successful
transfer of BUFFER-SIZE bytes or less, passing it the total number of
bytes transferred and the continuation of the transfer as a thunk."
  (define buffer
    (make-bytevector buffer-size))

  (let loop ((total 0)
             (bytes (get-bytevector-n! in buffer 0 buffer-size)))
    (or (eof-object? bytes)
        (let ((total (+ total bytes)))
          (put-bytevector out buffer 0 bytes)
          (progress total
                    (lambda ()
                      (loop total
                            (get-bytevector-n! in buffer 0 buffer-size))))))))

(define (set-file-time file stat)
  "Set the atime/mtime of FILE to that specified by STAT."
  (utime file
         (stat:atime stat)
         (stat:mtime stat)
         (stat:atimensec stat)
         (stat:mtimensec stat)))

(define patch-shebang
  (let ((shebang-rx (make-regexp "^[[:blank:]]*([[:graph:]]+)[[:blank:]]*([[:graph:]]*)(.*)$")))
    (lambda* (file
              #:optional
              (path (search-path-as-string->list (getenv "PATH")))
              #:key (keep-mtime? #t))
      "Replace the #! interpreter file name in FILE by a valid one found in
PATH, when FILE actually starts with a shebang.  Return #t when FILE was
patched, #f otherwise.  When KEEP-MTIME? is true, the atime/mtime of
FILE are kept unchanged."
      (define (patch p interpreter rest-of-line)
        (let* ((template (string-append file ".XXXXXX"))
               (out      (mkstemp! template))
               (st       (stat file))
               (mode     (stat:mode st)))
          (with-throw-handler #t
            (lambda ()
              (format out "#!~a~a~%"
                      interpreter rest-of-line)
              (dump-port p out)
              (close out)
              (chmod template mode)
              (rename-file template file)
              (when keep-mtime?
                (set-file-time file st))
              #t)
            (lambda (key . args)
              (format (current-error-port)
                      "patch-shebang: ~a: error: ~a ~s~%"
                      file key args)
              (false-if-exception (delete-file template))
              #f))))

      (call-with-ascii-input-file file
        (lambda (p)
          (and (eq? #\# (read-char p))
               (eq? #\! (read-char p))
               (let ((line (false-if-exception (read-line p))))
                 (and=> (and line (regexp-exec shebang-rx line))
                        (lambda (m)
                          (let* ((interp (match:substring m 1))
                                 (arg1 (match:substring m 2))
                                 (rest (match:substring m 3))
                                 (has-env (string-suffix? "/env" interp))
                                 (cmd (if has-env arg1 (basename interp)))
                                 (bin (search-path path cmd)))
                            (if bin
                                (if (string=? bin interp)
                                    #f            ; nothing to do
                                    (if has-env
                                        (begin
                                          (format (current-error-port)
                                                  "patch-shebang: ~a: changing `~a' to `~a'~%"
                                                  file (string-append interp " " arg1) bin)
                                          (patch p bin rest))
                                      (begin 
                                        (format (current-error-port)
                                                "patch-shebang: ~a: changing `~a' to `~a'~%"
                                                file interp bin)
                                        (patch p bin
                                               (if (string-null? arg1)
                                                   ""
                                                   (string-append " " arg1 rest))))))
                                (begin
                                  (format (current-error-port)
                                          "patch-shebang: ~a: warning: no binary for interpreter `~a' found in $PATH~%"
                                          file (basename cmd))
                                  #f))))))))))))

(define* (patch-makefile-SHELL file #:key (keep-mtime? #t))
  "Patch the `SHELL' variable in FILE, which is supposedly a makefile.
When KEEP-MTIME? is true, the atime/mtime of FILE are kept unchanged."

  ;; For instance, Gettext-generated po/Makefile.in.in do not honor $SHELL.

  ;; XXX: Unlike with `patch-shebang', FILE is always touched.

  (define (find-shell name)
    (let ((shell
           (search-path (search-path-as-string->list (getenv "PATH"))
                        name)))
      (unless shell
        (format (current-error-port)
                "patch-makefile-SHELL: warning: no binary for shell `~a' found in $PATH~%"
                name))
      shell))

  (let ((st (stat file)))
   (substitute* file
     (("^ *SHELL[[:blank:]]*=[[:blank:]]*([[:graph:]]*/)([[:graph:]]+)(.*)$"
       _ dir shell args)
      (let* ((old (string-append dir shell))
             (new (or (find-shell shell) old)))
        (unless (string=? new old)
          (format (current-error-port)
                  "patch-makefile-SHELL: ~a: changing `SHELL' from `~a' to `~a'~%"
                  file old new))
        (string-append "SHELL = " new args))))

   (when keep-mtime?
     (set-file-time file st))))

(define* (fold-port-matches proc init pattern port
                            #:optional (unmatched (lambda (_ r) r)))
  "Read from PORT character-by-character; for each match against
PATTERN, call (PROC MATCH RESULT), where RESULT is seeded with INIT.
PATTERN is a list of SRFI-14 char-sets.  Call (UNMATCHED CHAR RESULT)
for each unmatched character."
  (define initial-pattern
    ;; The poor developer's regexp.
    (if (string? pattern)
        (map char-set (string->list pattern))
        pattern))

  (define (get-char p)
    ;; We call it `get-char', but that's really a binary version
    ;; thereof.  (The real `get-char' cannot be used here because our
    ;; bootstrap Guile is hacked to always use UTF-8.)
    (match (get-u8 p)
      ((? integer? x) (integer->char x))
      (x x)))

  ;; Note: we're not really striving for performance here...
  (let loop ((chars   '())
             (pattern initial-pattern)
             (matched '())
             (result  init))
    (cond ((null? chars)
           (loop (list (get-char port))
                 pattern
                 matched
                 result))
          ((null? pattern)
           (loop chars
                 initial-pattern
                 '()
                 (proc (list->string (reverse matched)) result)))
          ((eof-object? (car chars))
           (fold-right unmatched result matched))
          ((char-set-contains? (car pattern) (car chars))
           (loop (cdr chars)
                 (cdr pattern)
                 (cons (car chars) matched)
                 result))
          ((null? matched)                        ; common case
           (loop (cdr chars)
                 pattern
                 matched
                 (unmatched (car chars) result)))
          (else
           (let ((matched (reverse matched)))
             (loop (append (cdr matched) chars)
                   initial-pattern
                   '()
                   (unmatched (car matched) result)))))))

(define* (remove-store-references file
                                  #:optional (store (%store-directory)))
  "Remove from FILE occurrences of file names in STORE; return #t when
store paths were encountered in FILE, #f otherwise.  This procedure is
known as `nuke-refs' in Nixpkgs."
  (define pattern
    (let ((nix-base32-chars
           '(#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9
             #\a #\b #\c #\d #\f #\g #\h #\i #\j #\k #\l #\m #\n
             #\p #\q #\r #\s #\v #\w #\x #\y #\z)))
      `(,@(map char-set (string->list store))
        ,(char-set #\/)
        ,@(make-list 32 (list->char-set nix-base32-chars))
        ,(char-set #\-))))

  (with-fluids ((%default-port-encoding #f))
    (with-atomic-file-replacement file
      (lambda (in out)
        ;; We cannot use `regexp-exec' here because it cannot deal with
        ;; strings containing NUL characters.
        (format #t "removing store references from `~a'...~%" file)
        (setvbuf in _IOFBF 65536)
        (setvbuf out _IOFBF 65536)
        (fold-port-matches (lambda (match result)
                             (put-bytevector out (string->utf8 store))
                             (put-u8 out (char->integer #\/))
                             (put-bytevector out
                                             (string->utf8
                                              "eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-"))
                             #t)
                           #f
                           pattern
                           in
                           (lambda (char result)
                             (put-u8 out (char->integer char))
                             result))))))

(define* (wrap-program prog #:rest vars)
  "Make a wrapper for PROG.  VARS should look like this:

  '(VARIABLE DELIMITER POSITION LIST-OF-DIRECTORIES)

where DELIMITER is optional.  ':' will be used if DELIMITER is not given.

For example, this command:

  (wrap-program \"foo\"
                '(\"PATH\" \":\" = (\"/gnu/.../bar/bin\"))
                '(\"CERT_PATH\" suffix (\"/gnu/.../baz/certs\"
                                        \"/qux/certs\")))

will copy 'foo' to '.foo-real' and create the file 'foo' with the following
contents:

  #!location/of/bin/bash
  export PATH=\"/gnu/.../bar/bin\"
  export CERT_PATH=\"$CERT_PATH${CERT_PATH:+:}/gnu/.../baz/certs:/qux/certs\"
  exec location/of/.foo-real

This is useful for scripts that expect particular programs to be in $PATH, for
programs that expect particular shared libraries to be in $LD_LIBRARY_PATH, or
modules in $GUILE_LOAD_PATH, etc.

If PROG has previously been wrapped by wrap-program the wrapper will point to
the previous wrapper."
  (define (wrapper-file-name number)
    (format #f "~a/.~a-wrap-~2'0d" (dirname prog) (basename prog) number))
  (define (next-wrapper-number)
    (let ((wrappers
           (find-files (dirname prog)
                       (string-append "\\." (basename prog) "-wrap-.*"))))
      (if (null? wrappers)
          0
          (string->number (string-take-right (last wrappers) 2)))))
  (define (wrapper-target number)
    (if (zero? number)
        (let ((prog-real (string-append (dirname prog) "/."
                                        (basename prog) "-real")))
          (copy-file prog prog-real)
          prog-real)
        (wrapper-file-name number)))
  (let* ((number   (next-wrapper-number))
         (target   (wrapper-target number))
         (wrapper  (wrapper-file-name (1+ number)))
         (prog-tmp (string-append target "-tmp")))
    (define (export-variable lst)
      ;; Return a string that exports an environment variable.
      (match lst
        ((var sep '= rest)
         (format #f "export ~a=\"~a\""
                 var (string-join rest sep)))
        ((var sep 'prefix rest)
         (format #f "export ~a=\"~a${~a~a+~a}$~a\""
                 var (string-join rest sep) var sep sep var))
        ((var sep 'suffix rest)
         (format #f "export ~a=\"$~a${~a~a+~a}~a\""
                 var var var sep sep (string-join rest sep)))
        ((var '= rest)
         (format #f "export ~a=\"~a\""
                 var (string-join rest ":")))
        ((var 'prefix rest)
         (format #f "export ~a=\"~a${~a:+:}$~a\""
                 var (string-join rest ":") var var))
        ((var 'suffix rest)
         (format #f "export ~a=\"$~a${~a:+:}~a\""
                 var var var (string-join rest ":")))))

    (with-output-to-file prog-tmp
      (lambda ()
        (format #t
                "#!~a~%~a~%exec \"~a\" \"$@\"~%"
                (which "bash")
                (string-join (map export-variable vars)
                             "\n")
                (canonicalize-path target))))

    (chmod prog-tmp #o755)
    (rename-file prog-tmp wrapper)
    (symlink wrapper prog-tmp)
    (rename-file prog-tmp prog)))

;;; Local Variables:
;;; eval: (put 'call-with-output-file/atomic 'scheme-indent-function 1)
;;; eval: (put 'call-with-ascii-input-file 'scheme-indent-function 1)
;;; eval: (put 'with-throw-handler 'scheme-indent-function 1)
;;; eval: (put 'let-matches 'scheme-indent-function 3)
;;; eval: (put 'with-atomic-file-replacement 'scheme-indent-function 1)
;;; End: