bug#51377: Automatically exit server when it has no remaining clients

[Jim Porter]

On 10/24/2021 11:43 AM, Gregory Heytings wrote:
> > If `save-buffers-kill-emacs' were called after that, the Emacs daemon 
> > would be killed, losing the edits to bar.txt.
>
> Indeed, you are correct.  Updated patch attached, which also takes care 
> of running processes.

Yeah, that looks like it should avoid any data loss. However, it's not 
the behavior I'd personally expect. As I understand it, this will just 
keep the Emacs daemon alive if there are any unsaved files. I'd find it 
easier to use if Emacs warned the user about unsaved files before 
killing the last client. Then, Emacs can safely kill the daemon once 
I've confirmed that that's what I want.

I've attached a lightly-tested patch series that implements things in 
the way that I'd be happiest with. There are a few points I should 
explain though.

First, I chose to add a new --lazy-daemon option (I'm not sure about the 
name; feel free to suggest a better one). This makes it possible to 
distinguish whether the daemon was started on demand via `emacsclient'. 
I chose a command-line option rather than a command accepted by the 
server protocol because all the other server protocol commands can be 
used any time a new `emacsclient' is started; this option only makes 
sense when starting the daemon (since it describes how the daemon was 
started). I also chose to add this as a "third" daemon type rather than 
an entirely separate flag since I don't think there's any reason to 
support a lazily-created foreground daemon.

In the second patch, I added a minor performance optimization to 
`server-kill-emacs-query-function'. Now it stops once it finds the first 
live client. This patch isn't strictly required though, and I could 
rework things to exclude it.

Next, I updated `server-save-buffers-kill-terminal' to check if the 
server is a lazy daemon and if the current client is the last one: if 
so, it calls `save-buffers-kill-emacs', which then calls an improved 
`server-kill-emacs-query-function' that knows to avoid prompting 
unnecessarily when this is the last client.

Notably, the change to `server-save-buffers-kill-terminal' alters the 
behavior of killing a frame created by `emacsclient -n'. Looking at the 
code before, I don't see how it could possibly have been the right 
behavior: if there were multiple frames open, it only closed *one* 
frame, even if all of those frames were created after a single call of 
`emacsclient -n' (e.g. by calling `make-frame-command'). If it was the 
*last* frame, it killed Emacs, even if the user explicitly called `emacs 
--daemon' on system boot and wants the daemon to live forever. I've 
changed the behavior so that when you call this from a `nowait' frame, 
it kills Emacs if a) it's a lazy daemon and b) there are no other 
clients. That's a lot closer to the behavior when calling this from a 
"normal" client frame.

Finally, these patches would all require documentation improvements to 
merge. However, it didn't seem like a good use of my time to do that 
until people agree that this is the right strategy overall.

0001-Add-lazy-daemon-option-used-to-start-a-daemon-on-dem.patch
Description: Text document

0002-Stop-searching-for-live-clients-once-we-find-one.patch
Description: Text document

0003-When-killing-the-last-client-attached-to-a-lazy-daem.patch
Description: Text document