org-mode/lisp/org-persist.el

;;; org-persist.el --- Persist cached data across Emacs sessions         -*- lexical-binding: t; -*-

;; Copyright (C) 2021-2022 Free Software Foundation, Inc.

;; Author: Ihor Radchenko <yantar92 at gmail dot com>
;; Keywords: cache, storage

;; This file is part of GNU Emacs.

;; GNU Emacs 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 Emacs 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 Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:
;;
;; This file implements persistent cache storage across Emacs sessions.
;; Both global and buffer-local data can be stored.  This
;; implementation is not meant to be used to store important data -
;; all the caches should be safe to remove at any time.
;;
;; Example usage:
;;
;; 1. Temporarily cache Elisp symbol value to disk.  Remove upon
;;    closing Emacs:
;;    (org-persist-write 'variable-symbol)
;;    (org-persist-read 'variable-symbol) ;; read the data later
;; 2. Temporarily cache a remote URL file to disk.  Remove upon
;;    closing Emacs:
;;    (org-persist-write 'url "https://static.fsf.org/common/img/logo-new.png")
;;    (org-persist-read 'url "https://static.fsf.org/common/img/logo-new.png")
;;    `org-persist-read' will return the cached file location or nil if cached file
;;    has been removed.
;; 3. Temporarily cache a file, including TRAMP path to disk:
;;    (org-persist-write 'file "/path/to/file")
;; 4. Cache file or URL while some other file exists.
;;    (org-persist-register '(url "https://static.fsf.org/common/img/logo-new.png") '(:file "/path to the other file") :expiry 'never :write-immediately t)
;;    or, if the other file is current buffer file
;;    (org-persist-register '(url "https://static.fsf.org/common/img/logo-new.png") (current-buffer) :expiry 'never :write-immediately t)
;; 5. Cache value of a Elisp variable to disk.  The value will be
;;    saved and restored automatically (except buffer-local
;;    variables).
;;    ;; Until `org-persist-default-expiry'
;;    (org-persist-register 'variable-symbol)
;;    ;; Specify expiry explicitly
;;    (org-persist-register 'variable-symbol :expiry 'never)
;;    ;; Save buffer-local variable (buffer-local will not be
;;    ;; autoloaded!)
;;    (org-persist-register 'org-element--cache (current-buffer))
;;    ;; Save buffer-local variable preserving circular links:
;;    (org-persist-register 'org-element--headline-cache (current-buffer)
;;               :inherit 'org-element--cache)
;; 6. Load variable by side effects assigning variable symbol:
;;    (org-persist-load 'variable-symbol (current-buffer))
;; 7. Version variable value:
;;    (org-persist-register '((elisp variable-symbol) (version "2.0")))
;; 8. Cancel variable persistence:
;;    (org-persist-unregister 'variable-symbol 'all) ; in all buffers
;;    (org-persist-unregister 'variable-symbol) ;; global variable
;;    (org-persist-unregister 'variable-symbol (current-buffer)) ;; buffer-local
;;
;; Most common data type is variable data.  However, other data types
;; can also be stored.
;;
;; Persistent data is stored in individual files.  Each of the files
;; can contain a collection of related data, which is particularly
;; useful when, say, several variables cross-reference each-other's
;; data-cells and we want to preserve their circular structure.
;;
;; Each data collection can be associated with a local or remote file,
;; its inode number, or contents hash.  The persistent data collection
;; can later be accessed using either file buffer, file, inode, or
;; contents hash.
;;
;; The data collections can be versioned and removed upon expiry.
;;
;; In the code below I will use the following naming conventions:
;; 1. Container :: a type of data to be stored
;;    Containers can store elisp variables, files, and version
;;    numbers.  Each container can be customized with container
;;    options.  For example, `elisp' container is customized with
;;    variable symbol.  (elisp variable) is a container storing
;;    Lisp variable value.  Similarly, (version "2.0") container
;;    will store version number.
;; 2. Associated :: an object the container is associated with.  The
;;    object can be a buffer, file, inode number, file contents hash,
;;    a generic key, or multiple of them.  Associated can also be nil.
;; 3. Data collection :: a list of containers linked to an associated
;;    object/objects.  Each data collection can also have auxiliary
;;    records.  Their only purpose is readability of the collection
;;    index.
;; 4. Index file :: a file listing all the stored data collections.
;; 5. Persist file :: a file holding data values or references to
;;    actual data values for a single data collection.  This file
;;    contains an alist associating each data container in data
;;    collection with its value or a reference to the actual value.
;;
;; All the persistent data is stored in `org-persist-directory'.  The data
;; collections are listed in `org-persist-index-file' and the actual data is
;; stored in UID-style subfolders.
;;
;; The `org-persist-index-file' stores the value of `org-persist--index'.
;;
;; Each collection is represented as a plist containing the following
;; properties:
;; - `:container'   : list of data continers to be stored in single
;;                    file;
;; - `:persist-file': data file name;
;; - `:associated'  : list of associated objects;
;; - `:last-access' : last date when the container has been accessed;
;; - `:expiry'      : list of expiry conditions.
;; - all other keywords are ignored
;;
;; The available types of data containers are:
;; 1. (file variable-symbol) or just variable-symbol :: Storing
;;    elisp variable data.
;; 2. (file) :: Store a copy of the associated file preserving the
;;    extension.
;;    (file "/path/to/a/file") :: Store a copy of the file in path.
;; 3. (version "version number") :: Version the data collection.
;;     If the stored collection has different version than "version
;;     number", disregard it.
;; 4. (url) :: Store a downloaded copy of URL object.
;;
;; The data collections can expire, in which case they will be removed
;; from the persistent storage at the end of Emacs session.  The
;; expiry condition can be set when saving/registering data
;; containers.  The expirty condition can be `never' - data will never
;; expire; `nil' - data will expire at the end of current Emacs session;
;; a number - data will expire after the number days from last access;
;; a function - data will expire if the function, called with a single
;; argument - collection, returns non-nil.
;;
;;
;; Data collections associated with files will automatically expire
;; when the file is removed.  If the associated file is remote, the
;; expiry is controlled by `org-persist-remote-files' instead.
;;
;; Data loading/writing can be more accurately controlled using
;; `org-persist-before-write-hook', `org-persist-before-read-hook', and `org-persist-after-read-hook'.

;;; Code:

(require 'org-macs)
(org-assert-version)

(require 'org-compat)
(require 'org-id)
(require 'xdg nil t)

(declare-function org-back-to-heading "org" (&optional invisible-ok))
(declare-function org-next-visible-heading "org" (arg))
(declare-function org-at-heading-p "org" (&optional invisible-not-ok))


(defconst org-persist--storage-version "2.5"
  "Persistent storage layout version.")

(defgroup org-persist nil
  "Persistent cache for Org mode."
  :tag "Org persist"
  :group 'org)

(defcustom org-persist-directory (expand-file-name
                                  (org-file-name-concat
                                   (let ((cache-dir (when (fboundp 'xdg-cache-home)
                                                      (xdg-cache-home))))
                                     (if (or (seq-empty-p cache-dir)
                                             (not (file-exists-p cache-dir))
                                             (file-exists-p (org-file-name-concat
                                                             user-emacs-directory
                                                             "org-persist")))
                                         user-emacs-directory
                                       cache-dir))
                                   "org-persist/"))
  "Directory where the data is stored."
  :type 'directory)

(defcustom org-persist-remote-files 100
  "Whether to keep persistent data for remote files.

When this variable is nil, never save persitent data associated with
remote files.  When t, always keep the data.  When
`check-existence', contact remote server containing the file and only
keep the data when the file exists on the server.  When a number, keep
up to that number persistent values for remote files.

Note that the last option `check-existence' may cause Emacs to show
password prompts to log in."
  :group 'org-persist
  :type '(choice (const :tag "Never" nil)
                 (const :tag "Always" t)
                 (number :tag "Keep not more than X files")
                 (const :tag "Check if exist on remote" check-existence)))

(defcustom org-persist-default-expiry 30
  "Default expiry condition for persistent data.

When this variable is nil, all the data vanishes at the end of Emacs
session.  When `never', the data never vanishes.  When a number, the
data is deleted that number days after last access.  When a function,
it should be a function returning non-nil when the data is expired.  The
function will be called with a single argument - collection."
  :group 'org-persist
  :type '(choice (const :tag "Never" never)
                 (const :tag "Always" nil)
                 (number :tag "Keep N days")
                 (function :tag "Function")))

(defconst org-persist-index-file "index"
  "File name used to store the data index.")

(defvar org-persist-disable-when-emacs-Q t
  "Disable persistence when Emacs is called with -Q command line arg.")

(defvar org-persist-before-write-hook nil
  "Abnormal hook ran before saving data.
The hook must accept the same arguments as `org-persist-write'.
The hooks will be evaluated until a hook returns non-nil.
If any of the hooks return non-nil, do not save the data.")

(defvar org-persist-before-read-hook nil
  "Abnormal hook ran before reading data.
The hook must accept the same arguments as `org-persist-read'.
The hooks will be evaluated until a hook returns non-nil.
If any of the hooks return non-nil, do not read the data.")

(defvar org-persist-after-read-hook nil
  "Abnormal hook ran after reading data.
The hook must accept the same arguments as `org-persist-read'.")

(defvar org-persist--index nil
  "Global index.

The index is a list of plists.  Each plist contains information about
persistent data storage.  Each plist contains the following
properties:

  - `:container'  : list of data continers to be stored in single file
  - `:persist-file': data file name
  - `:associated'  : list of associated objects
  - `:last-access' : last date when the container has been read
  - `:expiry'      : list of expiry conditions
  - all other keywords are ignored.")

(defvar org-persist--index-hash nil
  "Hash table storing `org-persist--index'.  Used for quick access.
They keys are conses of (container . associated).")

(defvar org-persist--report-time 0.5
  "Whether to report read/write time.

When the value is a number, it is a threshold number of seconds.  If
the read/write time of a single variable exceeds the threshold, a
message is displayed.

When the value is a non-nil non-number, always display the message.
When the value is nil, never display the message.")

;;;; Common functions

(defun org-persist--display-time (duration format &rest args)
  "Report DURATION according to FORMAT + ARGS message.
FORMAT and ARGS are passed to `message'."
  (when (or (and org-persist--report-time
                 (numberp org-persist--report-time)
                 (>= duration org-persist--report-time))
            (and org-persist--report-time
                 (not (numberp org-persist--report-time))))
    (apply #'message
           (format "org-persist: %s took %%.2f sec" format)
           (append args (list duration)))))

(defun org-persist--read-elisp-file (&optional buffer-or-file)
  "Read elisp data from BUFFER-OR-FILE or current buffer."
  (unless buffer-or-file (setq buffer-or-file (current-buffer)))
  (with-temp-buffer
    (if (bufferp buffer-or-file)
        (set-buffer buffer-or-file)
      (insert-file-contents buffer-or-file))
    (condition-case err
        (let ((coding-system-for-read 'utf-8)
              (read-circle t)
              (start-time (float-time)))
          ;; FIXME: Reading sometimes fails to read circular objects.
          ;; I suspect that it happens when we have object reference
          ;; #N# read before object definition #N=.  If it is really
          ;; so, it should be Emacs bug - either in `read' or in
          ;; `prin1'.  Meanwhile, just fail silently when `read'
          ;; fails to parse the saved cache object.
          (prog1
              (read (current-buffer))
            (org-persist--display-time
             (- (float-time) start-time)
             "Reading from %S" buffer-or-file)))
      ;; Recover gracefully if index file is corrupted.
      (error
       ;; Remove problematic file.
       (unless (bufferp buffer-or-file) (delete-file buffer-or-file))
       ;; Do not report the known error to user.
       (if (string-match-p "Invalid read syntax" (error-message-string err))
           (message "Emacs reader failed to read data in %S. The error was: %S"
                    buffer-or-file (error-message-string err))
         (warn "Emacs reader failed to read data in %S. The error was: %S"
               buffer-or-file (error-message-string err)))
       nil))))

(defun org-persist--write-elisp-file (file data &optional no-circular pp)
  "Write elisp DATA to FILE."
  (let ((print-circle (not no-circular))
        print-level
        print-length
        print-quoted
        (print-escape-control-characters t)
        (print-escape-nonascii t)
        (print-continuous-numbering t)
        print-number-table
        (start-time (float-time)))
    (unless (file-exists-p (file-name-directory file))
      (make-directory (file-name-directory file) t))
    (with-temp-file file
      (if pp
          (pp data (current-buffer))
        (prin1 data (current-buffer))))
    (org-persist--display-time
     (- (float-time) start-time)
     "Writing to %S" file)))

(defmacro org-persist-gc:generic (container collection)
  "Garbage collect CONTAINER data from COLLECTION."
  `(let* ((c (org-persist--normalize-container ,container))
          (gc-func-symbol (intern (format "org-persist-gc:%s" (car c)))))
     (unless (fboundp gc-func-symbol)
       (error "org-persist: GC function %s not defined"
              gc-func-symbol))
     (funcall gc-func-symbol c ,collection)))

(defmacro org-persist--gc-expired-p (cnd collection)
  "Check if expiry condition CND triggers for COLLECTION."
  `(pcase ,cnd
     (`nil t)
     (`never nil)
     ((pred numberp)
      (when (plist-get ,collection :last-access)
        (> (float-time) (+ (plist-get ,collection :last-access) (* ,cnd 24 60 60)))))
     ((pred functionp)
      (funcall ,cnd ,collection))
     (_ (error "org-persist: Unsupported expiry type %S" ,cnd))))

;;;; Working with index

(defmacro org-persist-collection-let (collection &rest body)
  "Bind container and associated from COLLECTION and execute BODY."
  (declare (debug (form body)) (indent 1))
  `(with-no-warnings
     (let* ((container (plist-get ,collection :container))
            (associated (plist-get ,collection :associated))
            (path (plist-get associated :file))
            (inode (plist-get associated :inode))
            (hash (plist-get associated :hash))
            (key (plist-get associated :key)))
       ;; Suppress "unused variable" warnings.
       (ignore container associated path inode hash key)
       ,@body)))

(defun org-persist--find-index (collection)
"Find COLLECTION in `org-persist--index'."
(org-persist-collection-let collection
  (and org-persist--index-hash
       (catch :found
         (dolist (cont (cons container container))
           (let (r)
             (setq r (or (gethash (cons cont associated) org-persist--index-hash)
                         (and path (gethash (cons cont (list :file path)) org-persist--index-hash))
                         (and inode (gethash (cons cont (list :inode inode)) org-persist--index-hash))
                         (and hash (gethash (cons cont (list :hash hash)) org-persist--index-hash))
                         (and key (gethash (cons cont (list :key key)) org-persist--index-hash))))
             (when r (throw :found r))))))))

(defun org-persist--add-to-index (collection &optional hash-only)
  "Add or update COLLECTION in `org-persist--index'.
When optional HASH-ONLY is non-nil, only modify the hash table.
Return PLIST."
  (org-persist-collection-let collection
    (let ((existing (org-persist--find-index collection)))
      (if existing
          (progn
            (plist-put existing :container container)
            (plist-put (plist-get existing :associated) :file path)
            (plist-put (plist-get existing :associated) :inode inode)
            (plist-put (plist-get existing :associated) :hash hash)
            (plist-put (plist-get existing :associated) :key key)
            existing)
        (unless hash-only (push collection org-persist--index))
        (unless org-persist--index-hash (setq org-persist--index-hash (make-hash-table :test 'equal)))
        (dolist (cont (cons container container))
          (puthash (cons cont associated) collection org-persist--index-hash)
          (when path (puthash (cons cont (list :file path)) collection org-persist--index-hash))
          (when inode (puthash (cons cont (list :inode inode)) collection org-persist--index-hash))
          (when hash (puthash (cons cont (list :hash inode)) collection org-persist--index-hash))
          (when key (puthash (cons cont (list :key inode)) collection org-persist--index-hash)))
        collection))))

(defun org-persist--remove-from-index (collection)
  "Remove COLLECTION from `org-persist--index'."
  (let ((existing (org-persist--find-index collection)))
    (when existing
      (org-persist-collection-let collection
        (dolist (cont (cons container container))
          (unless (listp (car container))
            (org-persist-gc:generic cont collection))
          (remhash (cons cont associated) org-persist--index-hash)
          (when path (remhash (cons cont (list :file path)) org-persist--index-hash))
          (when inode (remhash (cons cont (list :inode inode)) org-persist--index-hash))
          (when hash (remhash (cons cont (list :hash hash)) org-persist--index-hash))
          (when key (remhash (cons cont (list :key key)) org-persist--index-hash))))
      (setq org-persist--index (delq existing org-persist--index)))))

(defun org-persist--get-collection (container &optional associated &rest misc)
  "Return or create collection used to store CONTAINER for ASSOCIATED.
When ASSOCIATED is nil, it is a global CONTAINER.
ASSOCIATED can also be a (:buffer buffer) or buffer, (:file file-path)
or file-path, (:inode inode), (:hash hash), or or (:key key).
MISC, if non-nil will be appended to the collection."
  (unless (and (listp container) (listp (car container)))
    (setq container (list container)))
  (setq associated (org-persist--normalize-associated associated))
  (unless (equal misc '(nil))
    (setq associated (append associated misc)))
  (or (org-persist--find-index
       `( :container ,(org-persist--normalize-container container)
          :associated ,associated))
      (org-persist--add-to-index
       (list :container (org-persist--normalize-container container)
             :persist-file
             (replace-regexp-in-string "^.." "\\&/" (org-id-uuid))
             :associated associated))))

;;;; Reading container data.

(defun org-persist--normalize-container (container)
  "Normalize CONTAINER representation into (type . settings)."
  (if (and (listp container) (listp (car container)))
      (mapcar #'org-persist--normalize-container container)
    (pcase container
      ((or `elisp `version `file `index `url)
       (list container nil))
      ((pred symbolp)
       (list `elisp container))
      (`(,(or `elisp `version `file `index `url) . ,_)
       container)
      (_ (error "org-persist: Unknown container type: %S" container)))))

(defvar org-persist--associated-buffer-cache (make-hash-table :weakness 'key)
  "Buffer hash cache.")

(defun org-persist--normalize-associated (associated)
  "Normalize ASSOCIATED representation into (:type value)."
  (pcase associated
    ((or (pred stringp) `(:file ,_))
     (unless (stringp associated)
       (setq associated (cadr associated)))
     (let* ((rtn `(:file ,associated))
            (inode (and (fboundp 'file-attribute-inode-number)
                        (file-attribute-inode-number
                         (file-attributes associated)))))
       (when inode (plist-put rtn :inode inode))
       rtn))
    ((or (pred bufferp) `(:buffer ,_))
     (unless (bufferp associated)
       (setq associated (cadr associated)))
     (let ((cached (gethash associated org-persist--associated-buffer-cache))
           file inode hash)
       (if (and cached (eq (buffer-modified-tick associated)
                           (car cached)))
           (progn
             (setq file (nth 1 cached)
                   inode (nth 2 cached)
                   hash (nth 3 cached)))
         (setq file (buffer-file-name
                     (or (buffer-base-buffer associated)
                         associated)))
         (setq inode (when (and file
                                (fboundp 'file-attribute-inode-number))
                       (file-attribute-inode-number
                        (file-attributes file))))
         (setq hash (secure-hash 'md5 associated))
         (puthash associated
                  (list (buffer-modified-tick associated)
                        file inode hash)
                  org-persist--associated-buffer-cache))
       (let ((rtn `(:hash ,hash)))
         (when file (setq rtn (plist-put rtn :file file)))
         (when inode (setq rtn (plist-put rtn :inode inode)))
         rtn)))
    ((pred listp)
     associated)
    (_ (error "Unknown associated object %S" associated))))

(defmacro org-persist-read:generic (container reference-data collection)
  "Read and return the data stored in CONTAINER.
REFERENCE-DATA is associated with CONTAINER in the persist file.
COLLECTION is the plist holding data collection."
  `(let* ((c (org-persist--normalize-container ,container))
          (read-func-symbol (intern (format "org-persist-read:%s" (car c)))))
     (setf ,collection (plist-put ,collection :last-access (float-time)))
     (setf ,collection (plist-put ,collection :last-access-hr (format-time-string "%FT%T%z" (float-time))))
     (unless (fboundp read-func-symbol)
       (error "org-persist: Read function %s not defined"
              read-func-symbol))
     (funcall read-func-symbol c ,reference-data ,collection)))

(defun org-persist-read:elisp (_ lisp-value __)
  "Read elisp container and return LISP-VALUE."
  lisp-value)

(defun org-persist-read:version (container _ __)
  "Read version CONTAINER."
  (cadr container))

(defun org-persist-read:file (_ path __)
  "Read file container from PATH."
  (when (and path (file-exists-p (concat org-persist-directory path)))
    (concat org-persist-directory path)))

(defun org-persist-read:url (_ path __)
  "Read file container from PATH."
  (when (and path (file-exists-p (concat org-persist-directory path)))
    (concat org-persist-directory path)))

(defun org-persist-read:index (cont index-file _)
  "Read index container CONT from INDEX-FILE."
  (when (file-exists-p index-file)
    (let ((index (org-persist--read-elisp-file index-file)))
      (when index
        (catch :found
          (dolist (collection index)
            (org-persist-collection-let collection
              (when (and (not associated)
                         (pcase container
                           (`((index ,version))
                            (equal version (cadr cont)))
                           (_ nil)))
                (throw :found index)))))))))

;;;; Applying container data for side effects.

(defmacro org-persist-load:generic (container reference-data collection)
  "Load the data stored in CONTAINER for side effects.
REFERENCE-DATA is associated with CONTAINER in the persist file.
COLLECTION is the plist holding data collection."
  `(let* ((container (org-persist--normalize-container ,container))
          (load-func-symbol (intern (format "org-persist-load:%s" (car container)))))
     (setf ,collection (plist-put ,collection :last-access (float-time)))
     (setf ,collection (plist-put ,collection :last-access-hr (format-time-string "%FT%T%z" (float-time))))
     (unless (fboundp load-func-symbol)
       (error "org-persist: Load function %s not defined"
              load-func-symbol))
     (funcall load-func-symbol container ,reference-data ,collection)))

(defun org-persist-load:elisp (container lisp-value collection)
  "Assign elisp CONTAINER in COLLECTION LISP-VALUE."
  (let ((lisp-symbol (cadr container))
        (buffer (when (plist-get (plist-get collection :associated) :file)
                  (get-file-buffer (plist-get (plist-get collection :associated) :file)))))
    (if buffer
        (with-current-buffer buffer
          (make-variable-buffer-local lisp-symbol)
          (set lisp-symbol lisp-value))
      (set lisp-symbol lisp-value))))

(defalias 'org-persist-load:version #'org-persist-read:version)
(defalias 'org-persist-load:file #'org-persist-read:file)

(defun org-persist-load:index (container index-file _)
  "Load `org-persist--index' from INDEX-FILE according to CONTAINER."
  (unless org-persist--index
    (setq org-persist--index (org-persist-read:index container index-file nil))
    (setq org-persist--index-hash nil)
    (if org-persist--index
        (mapc (lambda (collection) (org-persist--add-to-index collection 'hash)) org-persist--index)
      (setq org-persist--index nil)
      (when (file-exists-p org-persist-directory)
        (dolist (file (directory-files org-persist-directory 'absolute "^[^.][^.]"))
          (if (file-directory-p file)
              (delete-directory file t)
            (delete-file file))))
      (plist-put (org-persist--get-collection container) :expiry 'never))))

(defun org-persist--load-index ()
  "Load `org-persist--index."
  (org-persist-load:index
   `(index ,org-persist--storage-version)
   (org-file-name-concat org-persist-directory org-persist-index-file)
   nil))

;;;; Writing container data

(defmacro org-persist-write:generic (container collection)
  "Write CONTAINER in COLLECTION."
  `(let* ((c (org-persist--normalize-container ,container))
          (write-func-symbol (intern (format "org-persist-write:%s" (car c)))))
     (setf ,collection (plist-put ,collection :last-access (float-time)))
     (setf ,collection (plist-put ,collection :last-access-hr (format-time-string "%FT%T%z" (float-time))))
     (unless (fboundp write-func-symbol)
       (error "org-persist: Write function %s not defined"
              write-func-symbol))
     (funcall write-func-symbol c ,collection)))

(defun org-persist-write:elisp (container collection)
  "Write elisp CONTAINER according to COLLECTION."
  (if (and (plist-get (plist-get collection :associated) :file)
           (get-file-buffer (plist-get (plist-get collection :associated) :file)))
      (let ((buf (get-file-buffer (plist-get (plist-get collection :associated) :file))))
        ;; FIXME: There is `buffer-local-boundp' introduced in Emacs 28.
        ;; Not using it yet to keep backward compatibility.
        (condition-case nil
            (buffer-local-value (cadr container) buf)
          (void-variable nil)))
    (when (boundp (cadr container))
      (symbol-value (cadr container)))))

(defalias 'org-persist-write:version #'ignore)

(defun org-persist-write:file (c collection)
  "Write file container C according to COLLECTION."
  (org-persist-collection-let collection
    (when (or (and path (file-exists-p path))
              (and (stringp (cadr c)) (file-exists-p (cadr c))))
      (when (and (stringp (cadr c)) (file-exists-p (cadr c)))
        (setq path (cadr c)))
      (let* ((persist-file (plist-get collection :persist-file))
             (ext (file-name-extension path))
             (file-copy (org-file-name-concat
                         org-persist-directory
                         (format "%s-%s.%s" persist-file (md5 path) ext))))
        (unless (file-exists-p (file-name-directory file-copy))
          (make-directory (file-name-directory file-copy) t))
        (copy-file path file-copy 'overwrite)
        (format "%s-%s.%s" persist-file (md5 path) ext)))))

(defun org-persist-write:url (c collection)
  "Write url container C according to COLLECTION."
  (org-persist-collection-let collection
    (when (or path (cadr c))
      (when (cadr c) (setq path (cadr c)))
      (let* ((persist-file (plist-get collection :persist-file))
             (ext (file-name-extension path))
             (file-copy (org-file-name-concat
                         org-persist-directory
                         (format "%s-%s.%s" persist-file (md5 path) ext))))
        (unless (file-exists-p (file-name-directory file-copy))
          (make-directory (file-name-directory file-copy) t))
        (if (org--should-fetch-remote-resource-p path)
            (url-copy-file path file-copy 'overwrite)
          (error "The remote resource %S is considered unsafe, and will not be downloaded."
                 path))
        (format "%s-%s.%s" persist-file (md5 path) ext)))))

(defun org-persist-write:index (container _)
  "Write index CONTAINER."
  (org-persist--get-collection container)
  (unless (file-exists-p org-persist-directory)
    (make-directory org-persist-directory))
  (unless (file-exists-p org-persist-directory)
    (warn "Failed to create org-persist storage in %s."
          org-persist-directory)
    (let ((dir (directory-file-name
                (file-name-as-directory org-persist-directory))))
      (while (and (not (file-exists-p dir))
                  (not (equal dir (setq dir (directory-file-name
                                           (file-name-directory dir)))))))
      (unless (file-writable-p dir)
        (message "Missing write access rights to org-persist-directory: %S"
                 org-persist-directory))))
  (when (file-exists-p org-persist-directory)
    (org-persist--write-elisp-file
     (org-file-name-concat org-persist-directory org-persist-index-file)
     org-persist--index
     t t)
    (org-file-name-concat org-persist-directory org-persist-index-file)))

(defun org-persist--save-index ()
  "Save `org-persist--index."
  (org-persist-write:index
   `(index ,org-persist--storage-version) nil))

;;;; Public API

(cl-defun org-persist-register (container &optional associated &rest misc
                               &key inherit
                               &key (expiry org-persist-default-expiry)
                               &key (write-immediately nil)
                               &allow-other-keys)
  "Register CONTAINER in ASSOCIATED to be persistent across Emacs sessions.
Optional key INHERIT makes CONTAINER dependent on another container.
Such dependency means that data shared between variables will be
preserved (see elisp#Circular Objects).
Optional key EXPIRY will set the expiry condition of the container.
It can be `never', nil - until end of session, a number of days since
last access, or a function accepting a single argument - collection.
EXPIRY key has no effect when INHERIT is non-nil.
Optional key WRITE-IMMEDIATELY controls whether to save the container
data immediately.
MISC will be appended to CONTAINER.
When WRITE-IMMEDIATELY is non-nil, the return value will be the same
with `org-persist-write'."
  (unless org-persist--index (org-persist--load-index))
  (setq container (org-persist--normalize-container container))
  (when inherit
    (setq inherit (org-persist--normalize-container inherit))
    (let ((inherited-collection (org-persist--get-collection inherit associated))
          new-collection)
      (unless (member container (plist-get inherited-collection :container))
        (setq new-collection
              (plist-put (copy-sequence inherited-collection) :container
                         (cons container (plist-get inherited-collection :container))))
        (org-persist--remove-from-index inherited-collection)
        (org-persist--add-to-index new-collection))))
  (let ((collection (org-persist--get-collection container associated misc)))
    (when (and expiry (not inherit))
      (when expiry (plist-put collection :expiry expiry))))
  (when (or (bufferp associated) (bufferp (plist-get associated :buffer)))
    (with-current-buffer (if (bufferp associated)
                             associated
                           (plist-get associated :buffer))
      (add-hook 'kill-buffer-hook #'org-persist-write-all-buffer nil 'local)))
  (when write-immediately (org-persist-write container associated)))

(defun org-persist-unregister (container &optional associated)
  "Unregister CONTAINER in ASSOCIATED to be persistent.
When ASSOCIATED is `all', unregister CONTAINER everywhere."
  (unless org-persist--index (org-persist--load-index))
  (setq container (org-persist--normalize-container container))
  (setq associated (org-persist--normalize-associated associated))
  (if (eq associated 'all)
      (mapc (lambda (collection)
              (when (member container (plist-get collection :container))
                (org-persist-unregister container (plist-get collection :associated))))
            org-persist--index)
    (let ((collection (org-persist--find-index `(:container ,container :associated ,associated))))
      (when collection
        (if (= (length (plist-get collection :container)) 1)
            (org-persist--remove-from-index collection)
          (plist-put collection :container
                     (remove container (plist-get collection :container)))
          (org-persist--add-to-index collection))))))

(defvar org-persist--write-cache (make-hash-table :test #'equal)
  "Hash table storing as-written data objects.

This data is used to avoid reading the data multiple times.")
(defun org-persist-read (container &optional associated hash-must-match load?)
  "Restore CONTAINER data for ASSOCIATED.
When HASH-MUST-MATCH is non-nil, do not restore data if hash for
ASSOCIATED file or buffer does not match.
ASSOCIATED can be a plist, a buffer, or a string.
A buffer is treated as (:buffer ASSOCIATED).
A string is treated as (:file ASSOCIATED).
When LOAD? is non-nil, load the data instead of reading."
  (setq associated (org-persist--normalize-associated associated))
  (setq container (org-persist--normalize-container container))
  (unless (and org-persist-disable-when-emacs-Q
               ;; FIXME: This is relying on undocumented fact that
               ;; Emacs sets `user-init-file' to nil when loaded with
               ;; "-Q" argument.
               (not user-init-file))
    (let* ((collection (org-persist--find-index `(:container ,container :associated ,associated)))
           (persist-file
            (when collection
              (org-file-name-concat
               org-persist-directory
               (plist-get collection :persist-file))))
           (data nil))
      (when (and collection
                 (file-exists-p persist-file)
                 (or (not (plist-get collection :expiry)) ; current session
                     (not (org-persist--gc-expired-p
                         (plist-get collection :expiry) collection)))
                 (or (not hash-must-match)
                     (and (plist-get associated :hash)
                          (equal (plist-get associated :hash)
                                 (plist-get (plist-get collection :associated) :hash)))))
        (unless (seq-find (lambda (v)
                            (run-hook-with-args-until-success 'org-persist-before-read-hook v associated))
                          (plist-get collection :container))
          (setq data (or (gethash persist-file org-persist--write-cache)
                         (org-persist--read-elisp-file persist-file)))
          (when data
            (cl-loop for container in (plist-get collection :container)
                     with result = nil
                     do
                     (if load?
                         (push (org-persist-load:generic container (alist-get container data nil nil #'equal) collection) result)
                       (push (org-persist-read:generic container (alist-get container data nil nil #'equal) collection) result))
                     (run-hook-with-args 'org-persist-after-read-hook container associated)
                     finally return (if (= 1 (length result)) (car result) result))))))))

(defun org-persist-load (container &optional associated hash-must-match)
  "Load CONTAINER data for ASSOCIATED.
The arguments have the same meaning as in `org-persist-read'."
  (org-persist-read container associated hash-must-match t))

(defun org-persist-load-all (&optional associated)
  "Restore all the persistent data associated with ASSOCIATED."
  (unless org-persist--index (org-persist--load-index))
  (setq associated (org-persist--normalize-associated associated))
  (let (all-containers)
    (dolist (collection org-persist--index)
      (when collection
        (cl-pushnew (plist-get collection :container) all-containers :test #'equal)))
    (dolist (container all-containers)
      (condition-case err
          (org-persist-load container associated t)
        (error
         (message "%s. Deleting bad index entry." err)
         (org-persist--remove-from-index (org-persist--find-index `(:container ,container :associated ,associated)))
         nil)))))

(defun org-persist-load-all-buffer ()
  "Call `org-persist-load-all' in current buffer."
  (org-persist-load-all (current-buffer)))

(defun org-persist-write (container &optional associated ignore-return)
  "Save CONTAINER according to ASSOCIATED.
ASSOCIATED can be a plist, a buffer, or a string.
A buffer is treated as (:buffer ASSOCIATED).
A string is treated as (:file ASSOCIATED).
The return value is nil when writing fails and the written value (as
returned by `org-persist-read') on success.
When IGNORE-RETURN is non-nil, just return t on success without calling
`org-persist-read'."
  (unless (and org-persist-disable-when-emacs-Q
               ;; FIXME: This is relying on undocumented fact that
               ;; Emacs sets `user-init-file' to nil when loaded with
               ;; "-Q" argument.
               (not user-init-file))
    (setq associated (org-persist--normalize-associated associated))
    ;; Update hash
    (when (and (plist-get associated :file)
               (plist-get associated :hash)
               (get-file-buffer (plist-get associated :file)))
      (setq associated (org-persist--normalize-associated (get-file-buffer (plist-get associated :file)))))
    (let ((collection (org-persist--get-collection container associated)))
      (setf collection (plist-put collection :associated associated))
      (unless (seq-find (lambda (v)
                          (run-hook-with-args-until-success 'org-persist-before-write-hook v associated))
                        (plist-get collection :container))
        (when (or (file-exists-p org-persist-directory) (org-persist--save-index))
          (let ((file (org-file-name-concat org-persist-directory (plist-get collection :persist-file)))
                (data (mapcar (lambda (c) (cons c (org-persist-write:generic c collection)))
                              (plist-get collection :container))))
            (puthash file data org-persist--write-cache)
            (org-persist--write-elisp-file file data)
            (or ignore-return (org-persist-read container associated))))))))

(defun org-persist-write-all (&optional associated)
  "Save all the persistent data.
When ASSOCIATED is non-nil, only save the matching data."
  (unless org-persist--index (org-persist--load-index))
  (setq associated (org-persist--normalize-associated associated))
  (let (all-containers)
    (dolist (collection org-persist--index)
      (if associated
          (when collection
            (cl-pushnew (plist-get collection :container) all-containers :test #'equal))
        (condition-case err
            (org-persist-write (plist-get collection :container) (plist-get collection :associated) t)
          (error
           (message "%s. Deleting bad index entry." err)
           (org-persist--remove-from-index collection)
           nil))))
    (dolist (container all-containers)
      (let ((collection (org-persist--find-index `(:container ,container :associated ,associated))))
        (when collection
          (condition-case err
              (org-persist-write container associated t)
            (error
             (message "%s. Deleting bad index entry." err)
             (org-persist--remove-from-index collection)
             nil)))))))

(defun org-persist-write-all-buffer ()
  "Call `org-persist-write-all' in current buffer.
Do nothing in an indirect buffer."
  (unless (buffer-base-buffer (current-buffer))
    (org-persist-write-all (current-buffer))))

(defalias 'org-persist-gc:elisp #'ignore)
(defalias 'org-persist-gc:index #'ignore)

(defun org-persist-gc:file (container collection)
  "Garbage collect file CONTAINER in COLLECTION."
  (let ((file (org-persist-read container (plist-get collection :associated))))
    (when (file-exists-p file)
      (delete-file file))))

(defun org-persist-gc:url (container collection)
  "Garbage collect url CONTAINER in COLLECTION."
  (let ((file (org-persist-read container (plist-get collection :associated))))
    (when (file-exists-p file)
      (delete-file file))))

(defmacro org-persist--gc-persist-file (persist-file)
  "Garbage collect PERSIST-FILE."
  `(when (file-exists-p ,persist-file)
     (delete-file ,persist-file)
     (when (org-directory-empty-p (file-name-directory ,persist-file))
       (delete-directory (file-name-directory ,persist-file)))))

(defun org-persist-gc ()
  "Remove expired or unregistered containers.
Also, remove containers associated with non-existing files."
  (unless (and org-persist-disable-when-emacs-Q
               ;; FIXME: This is relying on undocumented fact that
               ;; Emacs sets `user-init-file' to nil when loaded with
               ;; "-Q" argument.
               (not user-init-file))
    (let (new-index (remote-files-num 0))
      (dolist (collection org-persist--index)
        (let* ((file (plist-get (plist-get collection :associated) :file))
               (file-remote (when file (file-remote-p file)))
               (persist-file (when (plist-get collection :persist-file)
                               (org-file-name-concat
                                org-persist-directory
                                (plist-get collection :persist-file))))
               (expired? (org-persist--gc-expired-p
                          (plist-get collection :expiry) collection)))
          (when persist-file
            (when file
              (when file-remote (cl-incf remote-files-num))
              (unless (if (not file-remote)
                          (file-exists-p file)
                        (pcase org-persist-remote-files
                          ('t t)
                          ('check-existence
                           (file-exists-p file))
                          ((pred numberp)
                           (<= org-persist-remote-files remote-files-num))
                          (_ nil)))
                (setq expired? t)))
            (if expired?
                (org-persist--gc-persist-file persist-file)
              (push collection new-index)))))
      (setq org-persist--index (nreverse new-index)))))

;; Automatically write the data, but only when we have write access.
(let ((dir (directory-file-name
            (file-name-as-directory org-persist-directory))))
  (while (and (not (file-exists-p dir))
              (not (equal dir (setq dir (directory-file-name
                                       (file-name-directory dir)))))))
  (if (not (file-writable-p dir))
      (message "Missing write access rights to org-persist-directory: %S"
               org-persist-directory)
    (add-hook 'kill-emacs-hook #'org-persist-write-all)
    ;; `org-persist-gc' should run before `org-persist-write-all'.
    ;; So we are adding the hook after `org-persist-write-all'.
    (add-hook 'kill-emacs-hook #'org-persist-gc)))

(add-hook 'after-init-hook #'org-persist-load-all)

(provide 'org-persist)

;;; org-persist.el ends here