;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2012, 2013, 2014, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2014 Mark H Weaver <mhw@netris.org>
;;; Copyright © 2017 Huang Ying <huang.ying.caritas@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 (guix build union)
  #:use-module (ice-9 match)
  #:use-module (ice-9 format)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-26)
  #:use-module (rnrs bytevectors)
  #:use-module (rnrs io ports)
  #:export (union-build

            warn-about-collision

            relative-file-name
            symlink-relative))

;;; Commentary:
;;;
;;; Build a directory that is the union of a set of directories, using
;;; symbolic links.
;;;
;;; Code:

(define (files-in-directory dirname)
  (let ((dir (opendir dirname)))
    (let loop ((files '()))
      (match (readdir dir)
        ((or "." "..")
         (loop files))
        ((? eof-object?)
         (closedir dir)
         (sort files string<?))
        (file
         (loop (cons file files)))))))

(define (file-is-directory? file)
  (match (stat file #f)
    (#f #f)                                       ;maybe a dangling symlink
    (st (eq? 'directory (stat:type st)))))

(define (file=? file1 file2)
  "Return #t if FILE1 and FILE2 are regular files and their contents are
identical, #f otherwise."
  (let ((st1 (stat file1 #f))
        (st2 (stat file2 #f)))
    ;; When deduplication is enabled, identical files share the same inode.
    (and st1 st2
         (or (= (stat:ino st1) (stat:ino st2))
             (and (eq? (stat:type st1) 'regular)
                  (eq? (stat:type st2) 'regular)
                  (= (stat:size st1) (stat:size st2))
                  (call-with-input-file file1
                    (lambda (port1)
                      (call-with-input-file file2
                        (lambda (port2)
                          (define len 8192)
                          (define buf1 (make-bytevector len))
                          (define buf2 (make-bytevector len))
                          (let loop ()
                            (let ((n1 (get-bytevector-n! port1 buf1 0 len))
                                  (n2 (get-bytevector-n! port2 buf2 0 len)))
                              (and (equal? n1 n2)
                                   (or (eof-object? n1)
                                       (loop))))))))))))))

(define %harmless-collisions
  ;; This is a list of files that are known to collide, but for which emitting
  ;; a warning doesn't make sense.  For example, "icon-theme.cache" is
  ;; regenerated by a profile hook which shadows the file provided by
  ;; individual packages, and "gschemas.compiled" is made available to
  ;; applications via 'glib-or-gtk-build-system'.
  '("icon-theme.cache" "gschemas.compiled"))

(define (warn-about-collision files)
  "Handle the collision among FILES by emitting a warning and choosing the
first one of THEM."
  (let ((file (first files)))
    (unless (member (basename file) %harmless-collisions)
      (format (current-error-port)
              "~%warning: collision encountered:~%~{  ~a~%~}"
              files)
      (format (current-error-port) "warning: choosing ~a~%" file))
    file))

(define* (union-build output inputs
                      #:key (log-port (current-error-port))
                      (create-all-directories? #f)
                      (symlink symlink)
                      (resolve-collision warn-about-collision))
  "Build in the OUTPUT directory a symlink tree that is the union of all the
INPUTS, using SYMLINK to create symlinks.  As a special case, if
CREATE-ALL-DIRECTORIES?, creates the subdirectories in the output directory to
make sure the caller can modify them later.

When two or more regular files collide, call RESOLVE-COLLISION with the list
of colliding files and use the one that it returns; or, if RESOLVE-COLLISION
returns #f, skip the faulty file altogether."

  (define (symlink* input output)
    (format log-port "`~a' ~~> `~a'~%" input output)
    (symlink input output))

  (define (resolve-collisions output dirs files)
    (cond ((null? dirs)
           ;; The inputs are all files.
           (match (resolve-collision files)
             (#f #f)
             ((? string? file)
              (symlink* file output))))

          (else
           ;; The inputs are a mixture of files and directories
           (error "union-build: collision between file and directories"
                  `((files ,files) (dirs ,dirs))))))

  (define (union output inputs)
    (match inputs
      ((input)
       ;; There's only one input, so just make a link unless
       ;; create-all-directories?.
       (if (and create-all-directories? (file-is-directory? input))
           (union-of-directories output inputs)
           (symlink* input output)))
      (_
       (call-with-values (lambda () (partition file-is-directory? inputs))
         (match-lambda*
           ((dirs ())
            ;; All inputs are directories.
            (union-of-directories output dirs))

           ((() (file (? (cut file=? <> file)) ...))
            ;; There are no directories, and all files have the same contents,
            ;; so there's no conflict.
            (symlink* file output))

           ((dirs files)
            (resolve-collisions output dirs files)))))))

  (define (union-of-directories output dirs)
    ;; Create a new directory where we will merge the input directories.
    (mkdir output)

    ;; Build a hash table mapping each file to a list of input
    ;; directories containing that file.
    (let ((table (make-hash-table)))

      (define (add-to-table! file dir)
        (hash-set! table file (cons dir (hash-ref table file '()))))

      ;; Populate the table.
      (for-each (lambda (dir)
                  (for-each (cut add-to-table! <> dir)
                            (files-in-directory dir)))
                dirs)

      ;; Now iterate over the table and recursively
      ;; perform a union for each entry.
      (hash-for-each (lambda (file dirs-with-file)
                       (union (string-append output "/" file)
                              (map (cut string-append <> "/" file)
                                   (reverse dirs-with-file))))
                     table)))

  (setvbuf (current-output-port) _IOLBF)
  (setvbuf (current-error-port) _IOLBF)
  (when (file-port? log-port)
    (setvbuf log-port _IOLBF))

  (union-of-directories output (delete-duplicates inputs)))


;;;
;;; Relative symlinks.
;;;

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

(define (relative-file-name reference file)
  "Given REFERENCE and FILE, both of which are absolute file names, return the
file name of FILE relative to REFERENCE.

  (relative-file-name \"/gnu/store/foo\" \"/gnu/store/bin/bar\")
  => \"../bin/bar\"

Note that this is from a purely lexical standpoint; conversely, \"..\" is
*not* resolved lexically on POSIX in the presence of symlinks."
  (if (and (string-prefix? "/" file) (string-prefix? "/" reference))
      (let loop ((reference (string-tokenize reference %not-slash))
                 (file      (string-tokenize file %not-slash)))
        (define (finish)
          (string-join (append (make-list (length reference) "..") file)
                       "/"))

        (match reference
          (()
           (finish))
          ((head . tail)
           (match file
             (()
              (finish))
             ((head* . tail*)
              (if (string=? head head*)
                  (loop tail tail*)
                  (finish)))))))
      file))

(define (symlink-relative old new)
  "Assuming both OLD and NEW are absolute file names, make NEW a symlink to
OLD, but using a relative file name."
  (symlink (relative-file-name (dirname new) old)
           new))

;;; union.scm ends here
