From 2a755c1061098e73975dfd539eb25cb3fff98533 Mon Sep 17 00:00:00 2001
From: Andrew Tropin <andrew@trop.in>
Date: Thu, 26 Aug 2021 09:39:38 +0300
Subject: [PATCH v2] home-services: Add symlink-manager.

* gnu/home-services/symlink-manager.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add home-services/symlink-manager.scm.
---
 gnu/home-services/symlink-manager.scm | 247 ++++++++++++++++++++++++++
 gnu/local.mk                          |   1 +
 2 files changed, 248 insertions(+)
 create mode 100644 gnu/home-services/symlink-manager.scm

diff --git a/gnu/home-services/symlink-manager.scm b/gnu/home-services/symlink-manager.scm
new file mode 100644
index 0000000000..dc409d2ae2
--- /dev/null
+++ b/gnu/home-services/symlink-manager.scm
@@ -0,0 +1,247 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Andrew Tropin <andrew@trop.in>
+;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
+;;;
+;;; 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 home-services symlink-manager)
+  #:use-module (gnu home-services)
+  #:use-module (guix gexp)
+
+  #:export (home-symlink-manager-service-type))
+
+;;; Comment:
+;;;
+;;; symlink-manager cares about configuration files: it backs up files
+;;; created by user, removes symlinks and directories created by a
+;;; previous generation, and creates new directories and symlinks to
+;;; configuration files according to the content of files/ directory
+;;; (created by home-files-service) of the current home environment
+;;; generation.
+;;;
+;;; Code:
+
+(define (update-symlinks-script)
+  (program-file
+   "update-symlinks"
+   #~(begin
+       (use-modules (ice-9 ftw)
+                    (ice-9 curried-definitions)
+                    (ice-9 match)
+                    (srfi srfi-1))
+       (define ((simplify-file-tree parent) file)
+         "Convert the result produced by `file-system-tree' to less
+verbose and more suitable for further processing format.
+
+Extract dir/file info from stat and compose a relative path to the
+root of the file tree.
+
+Sample output:
+
+((dir . \".\")
+ ((dir . \"config\")
+  ((dir . \"config/fontconfig\")
+   (file . \"config/fontconfig/fonts.conf\"))
+  ((dir . \"config/isync\")
+   (file . \"config/isync/mbsyncrc\"))))
+"
+         (match file
+           ((name stat) `(file . ,(string-append parent name)))
+           ((name stat children ...)
+            (cons `(dir . ,(string-append parent name))
+                  (map (simplify-file-tree
+                        (if (equal? name ".")
+                            ""
+                            (string-append parent name "/")))
+                       children)))))
+
+       (define ((file-tree-traverse preordering) node)
+         "Traverses the file tree in different orders, depending on PREORDERING.
+
+if PREORDERING is @code{#t} resulting list will contain directories
+before files located in those directories, otherwise directory will
+appear only after all nested items already listed."
+         (let ((prepend (lambda (a b) (append b a))))
+           (match node
+             (('file . path) (list node))
+             ((('dir . path) . rest)
+              ((if preordering append prepend)
+               (list (cons 'dir path))
+               (append-map (file-tree-traverse preordering) rest))))))
+
+       (use-modules (guix build utils))
+
+       (let* ((config-home    (or (getenv "XDG_CONFIG_HOME")
+                                  (string-append (getenv "HOME") "/.config")))
+
+              (he-path (string-append (getenv "HOME") "/.guix-home"))
+              (new-he-path (string-append he-path ".new"))
+              (new-home (getenv "GUIX_NEW_HOME"))
+              (old-home (getenv "GUIX_OLD_HOME"))
+
+              (new-files-path (string-append new-home "/files"))
+              ;; Trailing dot is required, because files itself is symlink and
+              ;; to make file-system-tree works it should be a directory.
+              (new-files-dir-path (string-append new-files-path "/."))
+
+              (home-path (getenv "HOME"))
+              (backup-dir (string-append home-path "/"
+                                         (number->string (current-time))
+                                         "-guix-home-legacy-configs-backup"))
+
+              (old-tree (if (file-exists? old-home)
+                          ((simplify-file-tree "")
+                           (file-system-tree
+                            (string-append old-home "/files/.")))
+                          #f))
+              (new-tree ((simplify-file-tree "")
+                         (file-system-tree new-files-dir-path)))
+
+              (get-source-path
+               (lambda (path)
+                 (readlink (string-append new-files-path "/" path))))
+
+              (get-target-path
+               (lambda (path)
+                 (string-append home-path "/." path)))
+
+              (get-backup-path
+               (lambda (path)
+                 (string-append backup-dir "/." path)))
+
+              (directory?
+               (lambda (path)
+                 (equal? (stat:type (stat path)) 'directory)))
+
+              (empty-directory?
+               (lambda (dir)
+                 (equal? (scandir dir) '("." ".."))))
+
+              (symlink-to-store?
+               (lambda (path)
+                 (and
+                  (equal? (stat:type (lstat path)) 'symlink)
+                  (store-file-name? (readlink path)))))
+
+              (backup-file
+               (lambda (path)
+                 (mkdir-p backup-dir)
+                 (format #t "Backing up ~a..." (get-target-path path))
+                 (mkdir-p (dirname (get-backup-path path)))
+                 (rename-file (get-target-path path) (get-backup-path path))
+                 (display " done\n")))
+
+              (cleanup-symlinks
+               (lambda ()
+                 (let ((to-delete ((file-tree-traverse #f) old-tree)))
+                   (display
+                    "Cleaning up symlinks from previous home-environment.\n\n")
+                   (map
+                    (match-lambda
+                      (('dir . ".")
+                       (display "Cleanup finished.\n\n"))
+
+                      (('dir . path)
+                       (if (and
+                            (file-exists? (get-target-path path))
+                            (directory? (get-target-path path))
+                            (empty-directory? (get-target-path path)))
+                           (begin
+                             (format #t "Removing ~a..."
+                                     (get-target-path path))
+                             (rmdir (get-target-path path))
+                             (display " done\n"))
+                           (format
+                            #t "Skipping ~a (not an empty directory)... done\n"
+                            (get-target-path path))))
+
+                      (('file . path)
+                       (when (file-exists? (get-target-path path))
+                         ;; DO NOT remove the file if it is no longer
+                         ;; a symlink to the store, it will be backed
+                         ;; up later during create-symlinks phase.
+                         (if (symlink-to-store? (get-target-path path))
+                             (begin
+                               (format #t "Removing ~a..." (get-target-path path))
+                               (delete-file (get-target-path path))
+                               (display " done\n"))
+                             (format
+                              #t
+                              "Skipping ~a (not a symlink to store)... done\n"
+                              (get-target-path path))))))
+                    to-delete))))
+
+              (create-symlinks
+               (lambda ()
+                 (let ((to-create ((file-tree-traverse #t) new-tree)))
+                   (map
+                    (match-lambda
+                      (('dir . ".")
+                       (display
+                        "New symlinks to home-environment will be created soon.\n")
+                       (format
+                        #t "All conflicting files will go to ~a.\n\n" backup-dir))
+
+                      (('dir . path)
+                       (let ((target-path (get-target-path path)))
+                         (when (and (file-exists? target-path)
+                                    (not (directory? target-path)))
+                           (backup-file path))
+
+                         (if (file-exists? target-path)
+                             (format
+                              #t "Skipping   ~a (directory already exists)... done\n"
+                              target-path)
+                             (begin
+                               (format #t "Creating   ~a..." target-path)
+                               (mkdir target-path)
+                               (display " done\n")))))
+
+                      (('file . path)
+                       (when (file-exists? (get-target-path path))
+                         (backup-file path))
+                       (format #t "Symlinking ~a -> ~a..."
+                               (get-target-path path) (get-source-path path))
+                       (symlink (get-source-path path) (get-target-path path))
+                       (display " done\n")))
+                    to-create)))))
+
+         (when old-tree
+           (cleanup-symlinks))
+
+         (create-symlinks)
+
+         (symlink new-home new-he-path)
+         (rename-file new-he-path he-path)
+
+         (display " done\nFinished updating symlinks.\n\n")))))
+
+
+(define (update-symlinks-gexp _)
+  #~(primitive-load #$(update-symlinks-script)))
+
+(define home-symlink-manager-service-type
+  (service-type (name 'home-symlink-manager)
+                (extensions
+                 (list
+                  (service-extension
+                   home-activation-service-type
+                   update-symlinks-gexp)))
+                (default-value #f)
+                (description "Provide an @code{update-symlinks}
+script, which creates symlinks to configuration files and directories
+on every activation.  If an existing file would be overwritten by a
+symlink, backs up that file first.")))
diff --git a/gnu/local.mk b/gnu/local.mk
index bd3aed77e8..91c3b0da3d 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -73,6 +73,7 @@ GNU_SYSTEM_MODULES =				\
   %D%/bootloader/depthcharge.scm                \
   %D%/ci.scm					\
   %D%/home-services.scm                         \
+  %D%/home-services/symlink-manager.scm         \
   %D%/image.scm					\
   %D%/packages.scm				\
   %D%/packages/abduco.scm			\
-- 
2.33.0

