image-dired: Check all external programs available

[Tino Calancha <tino.calancha@gmail.com>]

image-dired uses several external programs.  The file
introduces user options with the name of the executables;
for instance, image-dired-cmd-create-thumbnail-program
has default value 'convert'.

If an user don't have installed one of these external programs,
and s?he run one function requiring that missing executable,
then the error is not very clear.

For instance, calling image-dired-rotate-original when
image-dired-cmd-rotate-original-program is not installed throw an error:
'Could not rotate image'.
For this case seems better to report that the required executable
is not available.  Such error would avoid the need to read the code
to understand what was actually wrong.

I propose a patch which set default value nil for the option
if the executable is not found.  Every function using an external
program has a nil check for the option storing the executable name:
when the option is nil, the function throw an error reporting that the
option is nil.

Do you think this proposal is useful?

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From 4cf0cb63adec2d5b6c89eee78645106d2479462f Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Sun, 28 Aug 2016 23:33:09 +0900
Subject: [PATCH] image-dired: Report when a necessary executable is not 
found

* lisp/image-dired.el (image-dired-cmd-rotate-original-program)
(image-dired-cmd-create-thumbnail-program)
(image-dired-cmd-create-temp-image-program)
(image-dired-cmd-rotate-thumbnail-program)
(image-dired-cmd-write-exif-data-program)
(image-dired-cmd-read-exif-data-program):
Use executable-find to set the defaut value for this option.
(image-dired-cmd-rotate-original-program):
Search for program 'convert' if 'jpegtran' is not found.
(image-dired-cmd-rotate-original-options):
Set the default value consistent with the executable in
image-dired-cmd-rotate-original-program.
(image-dired-thumb-name, image-dired-display-image)
(image-dired-image-at-point-p, image-dired-rotate-original)
(image-dired-thumbnail-set-image-description)
(image-dired-set-exif-data):
Throw and error when the executable used in the function is not
found.
---
  lisp/image-dired.el | 110 
++++++++++++++++++++++++++++++----------------------
  1 file changed, 64 insertions(+), 46 deletions(-)

diff --git a/lisp/image-dired.el b/lisp/image-dired.el
index 67b023d..2511e13 100644
--- a/lisp/image-dired.el
+++ b/lisp/image-dired.el
@@ -224,7 +224,7 @@ image-dired-gallery-thumb-image-root-url
    :group 'image-dired)

  (defcustom image-dired-cmd-create-thumbnail-program
-  "convert"
+  (executable-find "convert")
    "Executable used to create thumbnail.
  Used together with `image-dired-cmd-create-thumbnail-options'."
    :type 'string
@@ -242,7 +242,7 @@ image-dired-cmd-create-thumbnail-options
    :group 'image-dired)

  (defcustom image-dired-cmd-create-temp-image-program
-  "convert"
+  (executable-find "convert")
    "Executable used to create temporary image.
  Used together with `image-dired-cmd-create-temp-image-options'."
    :type 'string
@@ -308,7 +308,7 @@ image-dired-cmd-create-standard-thumbnail-command
    :group 'image-dired)

  (defcustom image-dired-cmd-rotate-thumbnail-program
-  "mogrify"
+  (executable-find "mogrify")
    "Executable used to rotate thumbnail.
  Used together with `image-dired-cmd-rotate-thumbnail-options'."
    :type 'string
@@ -326,14 +326,20 @@ image-dired-cmd-rotate-thumbnail-options
    :group 'image-dired)

  (defcustom image-dired-cmd-rotate-original-program
-  "jpegtran"
+  (cond ((executable-find "jpegtran"))
+        ((executable-find "convert")))
    "Executable used to rotate original image.
  Used together with `image-dired-cmd-rotate-original-options'."
    :type 'string
    :group 'image-dired)

  (defcustom image-dired-cmd-rotate-original-options
-  "%p -rotate %d -copy all -outfile %t \"%o\""
+  (when image-dired-cmd-rotate-original-program
+    (pcase image-dired-cmd-rotate-original-program
+      ((pred (lambda (x) (string-match "jpegtran" x)))
+       "%p -rotate %d -copy all -outfile %t \"%o\"")
+      ((pred (lambda (x) (string-match "convert" x)))
+       "%p -rotate %d \"%o\" %t")))
    "Format of command used to rotate original image.
  Available options are %p which is replaced by
  `image-dired-cmd-rotate-original-program', %d which is replaced by the
@@ -358,7 +364,7 @@ image-dired-rotate-original-ask-before-overwrite
    :group 'image-dired)

  (defcustom image-dired-cmd-write-exif-data-program
-  "exiftool"
+  (executable-find "exiftool")
    "Program used to write EXIF data to image.
  Used together with `image-dired-cmd-write-exif-data-options'."
    :type 'string
@@ -375,7 +381,7 @@ image-dired-cmd-write-exif-data-options
    :group 'image-dired)

  (defcustom image-dired-cmd-read-exif-data-program
-  "exiftool"
+  (executable-find "exiftool")
    "Program used to read EXIF data to image.
  Used together with `image-dired-cmd-read-exif-data-program-options'."
    :type 'string
@@ -615,6 +621,8 @@ image-dired-thumb-name

  (defun image-dired-create-thumb (original-file thumbnail-file)
    "For ORIGINAL-FILE, create thumbnail image named THUMBNAIL-FILE."
+  (unless image-dired-cmd-create-thumbnail-program
+    (error "image-dired-cmd-create-thumbnail-program is nil"))
    (let* ((width (int-to-string image-dired-thumb-width))
           (height (int-to-string image-dired-thumb-height))
           (modif-time (format "%.0f" (float-time (nth 5 (file-attributes
@@ -1810,6 +1818,8 @@ image-dired-display-image
          (progn
            (setq width (image-dired-display-window-width))
            (setq height (image-dired-display-window-height))
+          (unless image-dired-cmd-create-temp-image-program
+            (error "image-dired-cmd-create-temp-image-program is nil"))
            (setq command
                  (format-spec
                   image-dired-cmd-create-temp-image-options
@@ -1866,20 +1876,22 @@ image-dired-image-at-point-p

  (defun image-dired-rotate-thumbnail (degrees)
    "Rotate thumbnail DEGREES degrees."
-  (if (not (image-dired-image-at-point-p))
-      (message "No thumbnail at point")
-    (let ((file (image-dired-thumb-name 
(image-dired-original-file-name)))
-          command)
-      (setq command (format-spec
-                     image-dired-cmd-rotate-thumbnail-options
-                     (list
-                      (cons ?p image-dired-cmd-rotate-thumbnail-program)
-                      (cons ?d degrees)
-                      (cons ?t (expand-file-name file)))))
-      (call-process shell-file-name nil nil nil shell-command-switch 
command)
-      ;; Clear the cache to refresh image. I wish I could just refresh
-      ;; the current file but I do not know how to do that. Yet...
-      (clear-image-cache))))
+  (unless (image-dired-image-at-point-p)
+    (message "No thumbnail at point"))
+  (unless image-dired-cmd-rotate-thumbnail-program
+    (error "image-dired-cmd-rotate-thumbnail-program is nil"))
+  (let ((file (image-dired-thumb-name (image-dired-original-file-name)))
+        command)
+    (setq command (format-spec
+                   image-dired-cmd-rotate-thumbnail-options
+                   (list
+                    (cons ?p image-dired-cmd-rotate-thumbnail-program)
+                    (cons ?d degrees)
+                    (cons ?t (expand-file-name file)))))
+    (call-process shell-file-name nil nil nil shell-command-switch 
command)
+    ;; Clear the cache to refresh image. I wish I could just refresh
+    ;; the current file but I do not know how to do that. Yet...
+    (clear-image-cache)))

  (defun image-dired-rotate-thumbnail-left ()
    "Rotate thumbnail left (counter clockwise) 90 degrees.
@@ -1908,31 +1920,33 @@ image-dired-refresh-thumb

  (defun image-dired-rotate-original (degrees)
    "Rotate original image DEGREES degrees."
-  (if (not (image-dired-image-at-point-p))
-      (message "No image at point")
-    (let ((file (image-dired-original-file-name))
-          command)
-      (if (not (string-match "\\.[jJ][pP[eE]?[gG]$" file))
-          (error "Only JPEG images can be rotated!"))
-      (setq command (format-spec
-                     image-dired-cmd-rotate-original-options
-                     (list
-                      (cons ?p image-dired-cmd-rotate-original-program)
-                      (cons ?d degrees)
-                      (cons ?o (expand-file-name file))
-                      (cons ?t image-dired-temp-rotate-image-file))))
-      (if (not (= 0 (call-process shell-file-name nil nil nil
-				  shell-command-switch command)))
-          (error "Could not rotate image")
-        (image-dired-display-image image-dired-temp-rotate-image-file)
-        (if (or (and image-dired-rotate-original-ask-before-overwrite
-                     (y-or-n-p
-		      "Rotate to temp file OK.  Overwrite original image? 
"))
-                (not image-dired-rotate-original-ask-before-overwrite))
-            (progn
-              (copy-file image-dired-temp-rotate-image-file file t)
-              (image-dired-refresh-thumb))
-          (image-dired-display-image file))))))
+  (unless (image-dired-image-at-point-p)
+    (message "No image at point"))
+  (unless image-dired-cmd-rotate-original-program
+    (error "image-dired-cmd-rotate-original-program is nil"))
+  (let ((file (image-dired-original-file-name))
+        command)
+    (if (not (string-match "\\.[jJ][pP[eE]?[gG]$" file))
+        (error "Only JPEG images can be rotated!"))
+    (setq command (format-spec
+                   image-dired-cmd-rotate-original-options
+                   (list
+                    (cons ?p image-dired-cmd-rotate-original-program)
+                    (cons ?d degrees)
+                    (cons ?o (expand-file-name file))
+                    (cons ?t image-dired-temp-rotate-image-file))))
+    (if (not (= 0 (call-process shell-file-name nil nil nil
+                                shell-command-switch command)))
+        (error "Could not rotate image")
+      (image-dired-display-image image-dired-temp-rotate-image-file)
+      (if (or (and image-dired-rotate-original-ask-before-overwrite
+                   (y-or-n-p
+                    "Rotate to temp file OK.  Overwrite original image? 
"))
+              (not image-dired-rotate-original-ask-before-overwrite))
+          (progn
+            (copy-file image-dired-temp-rotate-image-file file t)
+            (image-dired-refresh-thumb))
+        (image-dired-display-image file)))))

  (defun image-dired-rotate-original-left ()
    "Rotate original image left (counter clockwise) 90 degrees."
@@ -1987,6 +2001,8 @@ image-dired-thumbnail-set-image-description

  (defun image-dired-set-exif-data (file tag-name tag-value)
    "In FILE, set EXIF tag TAG-NAME to value TAG-VALUE."
+  (unless image-dired-cmd-write-exif-data-program
+    (error "image-dired-cmd-write-exif-data-program is nil"))
    (let (command)
      (setq command (format-spec
                     image-dired-cmd-write-exif-data-options
@@ -1999,6 +2015,8 @@ image-dired-set-exif-data

  (defun image-dired-get-exif-data (file tag-name)
    "From FILE, return EXIF tag TAG-NAME."
+  (unless image-dired-cmd-read-exif-data-program
+    (error "image-dired-cmd-read-exif-data-program is nil"))
    (let ((buf (get-buffer-create "*image-dired-get-exif-data*"))
          command tag-value)
      (setq command (format-spec
-- 
2.9.3

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In GNU Emacs 25.1.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.20.9)
  of 2016-08-28
Repository revision: 7fcce24e75b8281621a0b8816dc58cbdc05fdc91