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.

2 files changed, 6 insertions, 4 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