bug#33684: DocView bombs out upon password protected PDFs

[Federico Tedin <federicotedin@gmail.com>]

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積丹尼 Dan Jacobson <jidanni@jidanni.org> writes:

> When trying to view a password protected PDF:
> DocView: process pdf/ps->png changed status to exited abnormally with code 1.
> Maybe it should ask for the password.
> emacs-version "25.2.2"

Would something like this make sense? A short patch that adds
`doc-view-pdf-password-protected-p', and an extra option to pass to
Ghostscript (-sPDFPassword=) when the command is invoked (only when the
PDF file is password protected). I tried it out by creating a
password-protected PDF using LibreOffice Writer, and then opening it
with find-file.


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diff --git a/lisp/doc-view.el b/lisp/doc-view.el
index df8a9fc70f..8691d2a3a6 100644
--- a/lisp/doc-view.el
+++ b/lisp/doc-view.el
@@ -183,11 +183,16 @@ doc-view-pdf->png-converter-function
 (defcustom doc-view-ghostscript-options
   '("-dSAFER" ;; Avoid security problems when rendering files from untrusted
     ;; sources.
-    "-dNOPAUSE" "-sDEVICE=png16m" "-dTextAlphaBits=4"
-    "-dBATCH" "-dGraphicsAlphaBits=4" "-dQUIET")
+    "-dNOPAUSE" "-dTextAlphaBits=4" "-dBATCH"
+    "-dGraphicsAlphaBits=4" "-dQUIET")
   "A list of options to give to ghostscript."
   :type '(repeat string))
 
+(defcustom doc-view-ghostscript-device "png16m"
+  "Output device to give to ghostscript."
+  :type 'string
+  :version "27.1")
+
 (defcustom doc-view-resolution 100
   "Dots per inch resolution used to render the documents.
 Higher values result in larger images."
@@ -950,16 +955,30 @@ doc-view-dvi->pdf
 			    (list "-o" pdf dvi)
 			    callback)))
 
+(defun doc-view-pdf-password-protected-p (pdf)
+  "Using Ghostscript, check if a PDF file is password-protected."
+  (with-temp-buffer
+    (apply #'call-process doc-view-ghostscript-program nil (current-buffer)
+           nil `(,@doc-view-ghostscript-options
+                 "-sNODISPLAY"
+                 ,pdf))
+    (goto-char (point-min))
+    (search-forward "This file requires a password for access." nil t)))
+
 (defun doc-view-pdf->png-converter-ghostscript (pdf png page callback)
-  (doc-view-start-process
-   "pdf/ps->png" doc-view-ghostscript-program
-   `(,@doc-view-ghostscript-options
-     ,(format "-r%d" (round doc-view-resolution))
-     ,@(if page `(,(format "-dFirstPage=%d" page)))
-     ,@(if page `(,(format "-dLastPage=%d" page)))
-     ,(concat "-sOutputFile=" png)
-     ,pdf)
-   callback))
+  (let ((pdf-passwd (if (doc-view-pdf-password-protected-p pdf)
+                        (read-passwd "Enter password for PDF file: "))))
+    (doc-view-start-process
+     "pdf/ps->png" doc-view-ghostscript-program
+     `(,@doc-view-ghostscript-options
+       ,(concat "-sDEVICE=" doc-view-ghostscript-device)
+       ,(format "-r%d" (round doc-view-resolution))
+       ,@(if page `(,(format "-dFirstPage=%d" page)))
+       ,@(if page `(,(format "-dLastPage=%d" page)))
+       ,@(if pdf-passwd `(,(format "-sPDFPassword=%s" pdf-passwd)))
+       ,(concat "-sOutputFile=" png)
+       ,pdf)
+     callback)))
 
 (defalias 'doc-view-ps->png-converter-ghostscript
   'doc-view-pdf->png-converter-ghostscript)