Add support for arguments in emacsclient's ALTERNATE_EDITOR (Bug #25082)

* lib-src/emacsclient.c (fail): Parse ALTERNATE_EDITOR, or
corresponding command-line argument, into quote- or space-separated
tokens.  If a token starts with a quote, then it naturally is expected
to end with a quote; escaping is not supported. This is enough to cope
with the typical case of requiring the initial path to be quoted,
common on Windows where it may contain spaces.
* etc/NEWS: Document.
* doc/emacs/misc.texi: Likewise.
* doc/man/emacsclient.1: Tweak to remove the implication that only an
editor can be specified (the manual already mentions a “command”).
Fix a small error where “EDITOR” is referred to rather than
“ALTERNATE_EDITOR”.
* test/lib-src/emacsclient-tests.el: Add tests.

5 files changed, 129 insertions, 22 deletions

diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index 73a6bae767a..7602fbb7454 100644
--- a/doc/emacs/misc.texi
+++ b/doc/emacs/misc.texi
@@ -1821,8 +1821,10 @@ listed below:
 @table @samp
 @item -a @var{command}
 @itemx --alternate-editor=@var{command}
-Specify a command to run if @code{emacsclient} fails to contact Emacs.
+Specify a shell command to run if @code{emacsclient} fails to contact Emacs.
 This is useful when running @code{emacsclient} in a script.
+The command may include arguments, which may be quoted "like this".
+Currently, escaping of quotes is not supported.
 
 As a special exception, if @var{command} is the empty string, then
 @code{emacsclient} starts Emacs in daemon mode (as @command{emacs
diff --git a/doc/man/emacsclient.1 b/doc/man/emacsclient.1
index 010eeba19c1..daaacab7f3e 100644
--- a/doc/man/emacsclient.1
+++ b/doc/man/emacsclient.1
@@ -62,10 +62,10 @@ A missing
 is treated as column 1.
 This option applies only to the next file specified.
 .TP
-.B \-a, \-\-alternate-editor=EDITOR
-if the Emacs server is not running, run the specified editor instead.
+.B \-a, \-\-alternate-editor=COMMAND
+if the Emacs server is not running, run the specified shell command instead.
 This can also be specified via the ALTERNATE_EDITOR environment variable.
-If the value of EDITOR is the empty string, run "emacs \-\-daemon" to
+If the value of ALTERNATE_EDITOR is the empty string, run "emacs \-\-daemon" to
 start Emacs in daemon mode, and try to connect to it.
 .TP
 .B -c, \-\-create-frame
diff --git a/etc/NEWS b/etc/NEWS
index ef4d8cda397..0889303f82e 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -485,6 +485,13 @@ Linum mode and all similar packages are henceforth becoming obsolete.
 Users and developers are encouraged to switch to this new feature
 instead.
 
++++
+** emacsclient now accepts command-line options in ALTERNATE_EDITOR
+and --alternate-editor. For example, ALTERNATE_EDITOR="emacs -Q -nw".
+Arguments may be quoted "like this", so that for example an absolute
+path containing a space may be specified; quote escaping is not
+supported.
+
 
 * Editing Changes in Emacs 26.1
 
diff --git a/lib-src/emacsclient.c b/lib-src/emacsclient.c
index f1d4e8976da..5e181ccacb1 100644
--- a/lib-src/emacsclient.c
+++ b/lib-src/emacsclient.c
@@ -110,6 +110,9 @@ char *w32_getenv (const char *);
 /* Name used to invoke this program.  */
 const char *progname;
 
+/* The first argument to main.  */
+int main_argc;
+
 /* The second argument to main.  */
 char **main_argv;
 
@@ -201,6 +204,35 @@ xmalloc (size_t size)
   return result;
 }
 
+/* Like realloc but get fatal error if memory is exhausted.  */
+
+static void *
+xrealloc (void *ptr, size_t size)
+{
+  void *result = realloc (ptr, size);
+  if (result == NULL)
+    {
+      perror ("realloc");
+      exit (EXIT_FAILURE);
+    }
+  return result;
+}
+
+/* Like strdup but get a fatal error if memory is exhausted. */
+char *xstrdup (const char *);
+
+char *
+xstrdup (const char *s)
+{
+  char *result = strdup (s);
+  if (result == NULL)
+    {
+      perror ("strdup");
+      exit (EXIT_FAILURE);
+    }
+  return result;
+}
+
 /* From sysdep.c */
 #if !defined (HAVE_GET_CURRENT_DIR_NAME) || defined (BROKEN_GET_CURRENT_DIR_NAME)
 
@@ -264,21 +296,6 @@ get_current_dir_name (void)
 
 #ifdef WINDOWSNT
 
-/* Like strdup but get a fatal error if memory is exhausted. */
-char *xstrdup (const char *);
-
-char *
-xstrdup (const char *s)
-{
-  char *result = strdup (s);
-  if (result == NULL)
-    {
-      perror ("strdup");
-      exit (EXIT_FAILURE);
-    }
-  return result;
-}
-
 #define REG_ROOT "SOFTWARE\\GNU\\Emacs"
 
 char *w32_get_resource (HKEY, const char *, LPDWORD);
@@ -673,7 +690,7 @@ Report bugs with M-x report-emacs-bug.\n");
 }
 
 /* Try to run a different command, or --if no alternate editor is
-   defined-- exit with an errorcode.
+   defined-- exit with an error code.
    Uses argv, but gets it from the global variable main_argv.  */
 
 static _Noreturn void
@@ -681,9 +698,38 @@ fail (void)
 {
   if (alternate_editor)
     {
-      int i = optind - 1;
+      size_t extra_args_size = (main_argc - optind + 1) * sizeof (char *);
+      size_t new_argv_size = extra_args_size;
+      char **new_argv = NULL;
+      char *s = xstrdup (alternate_editor);
+      unsigned toks = 0;
+
+      /* Unpack alternate_editor's space-separated tokens into new_argv.  */
+      for (char *tok = s; tok != NULL && *tok != '\0';)
+        {
+          /* Allocate new token.  */
+          ++toks;
+          new_argv = xrealloc (new_argv, new_argv_size + toks * sizeof (char *));
+
+          /* Skip leading delimiters, and set separator, skipping any
+             opening quote.  */
+          size_t skip = strspn (tok, " \"");
+          tok += skip;
+          char sep = (skip > 0 && tok[-1] == '"') ? '"' : ' ';
+
+          /* Record start of token.  */
+          new_argv[toks - 1] = tok;
+
+          /* Find end of token and overwrite it with NUL.  */
+          tok = strchr (tok, sep);
+          if (tok != NULL)
+            *tok++ = '\0';
+        }
+
+      /* Append main_argv arguments to new_argv.  */
+      memcpy (&new_argv[toks], main_argv + optind, extra_args_size);
 
-      execvp (alternate_editor, main_argv + i);
+      execvp (*new_argv, new_argv);
       message (true, "%s: error executing alternate editor \"%s\"\n",
                progname, alternate_editor);
     }
@@ -696,6 +742,7 @@ fail (void)
 int
 main (int argc, char **argv)
 {
+  main_argc = argc;
   main_argv = argv;
   progname = argv[0];
   message (true, "%s: Sorry, the Emacs server is supported only\n"
@@ -1629,6 +1676,7 @@ main (int argc, char **argv)
   int start_daemon_if_needed;
   int exit_status = EXIT_SUCCESS;
 
+  main_argc = argc;
   main_argv = argv;
   progname = argv[0];
 
diff --git a/test/lib-src/emacsclient-tests.el b/test/lib-src/emacsclient-tests.el
new file mode 100644
index 00000000000..ea757f69144
--- /dev/null
+++ b/test/lib-src/emacsclient-tests.el
@@ -0,0 +1,50 @@
+;;; emacsclient-tests.el --- Test emacsclient
+
+;; Copyright (C) 2016 Free Software Foundation, Inc.
+
+;; This program 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.
+
+;; This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;
+
+;;; Code:
+
+(require 'ert)
+
+(defconst emacsclient-test-emacs
+  (expand-file-name "emacsclient" (concat
+                                   (file-name-directory
+                                    (directory-file-name
+                                     (file-name-directory invocation-directory)))
+                                   "lib-src"))
+  "Path to emacsclient binary in build tree.")
+
+(ert-deftest emacsclient-test-alternate-editor-allows-arguments ()
+  (let (process-environment process-environment)
+    (setenv "ALTERNATE_EDITOR" (concat
+                                (expand-file-name invocation-name invocation-directory)
+                                " --batch"))
+    (should (= 0 (call-process emacsclient-test-emacs nil nil nil "foo")))))
+
+(ert-deftest emacsclient-test-alternate-editor-allows-quotes ()
+  (let (process-environment process-environment)
+    (setenv "ALTERNATE_EDITOR" (concat
+                                "\""
+                                (expand-file-name invocation-name invocation-directory)
+                                "\"" " --batch"))
+    (should (= 0 (call-process emacsclient-test-emacs nil nil nil "foo")))))
+
+(provide 'emacsclient-tests)
+;;; emacsclient-tests.el ends here