Revise REPL and command server startup

People continue to report symptoms like issue #344. In some cases it
turned out people needed to update to the latest racket-mode. However
at least a couple users on very recent versions reported still seeing
the problem intermittently.

So:

- Simplify code paths by relying on sentinels to do cleanup when the
  process is deleted or closed.

- Rename racket--repl-ensure-buffer-and-process to racket--repl-start,
  and, it is an error to call it when (racket--repl-live-p) is true.

- Rather than setting racket--cmd-proc back to nil, leave it set to
  the last value, and use things like `process-status` determine if it
  is 'open. Provide a little helper for this: racket--cmd-open-p.

- Wait to start command connect; eliminate racket--cmd-connecting-p
  state variable.

  Until we believe the Racket backend has finished starting up and the
  TCP server would be listening, there is really no point in starting
  to attempt to connect. Ideally we could know this from a process
  sentinel, but there is no appropriate event for a comint buffer
  process. Instead use a comint output filter function to detect first
  output, e.g. the Welcome to Racket banner. This uses run-at-time to
  schedule racket--cmd-connect-attempt (should not do directly from a
  filter function, IIUC). As a result, there is no longer any
  racket--cmd-connect-start function. Now, we only try to connect to
  the command server when we first start the REPL process. If the
  command connection dies thereafter, but the REPL is still alive?
  Tough beans, users will need to restart the REPL process.

  Also as a result, I convinced myself that the
  racket--cmd-connecting-p flag/guard is no longer necessary. Which is
  good, because it is tricky to make all the code paths manage that
  properly. I _think_ I finally got that right in a WIP commit. But
  it's fragile. Some other change, someday, could break this again.
  Better users get a "can't talk to command server" error, and need to
  restart the REPL, than have heisenbugs and reports thereof.

  Note: As before, racket--cmd-connect-attempt will run-at-time itself
        to retry: We wait until the TCP command server could
        _possibly_ be ready -- but it's not necessarily ready yet.
        Although it will probably succeed on the first try, in fact
        I've seen it occasionally take two attempts while testing this
        on a laptop/OS that does the startup very quickly.

- racket-repl-exit: Use comint-kill-subjob when command server dead

- Add defcustom racket-command-startup.

  - When nil (default): We do not try to start the REPL or the
    command server automatically; instead we give the user an error
    message explaining they need to (re)start racket-repl-mode.

  - When a positive number, we do try to start things automatically
    and we wait that number of seconds for the command server to
    connect. While waiting, we remind the user they can C-g to quit
    waiting.

Author: Greg Hendershott

Reference.md

@@ -867,8 +867,18 @@ Pathname of the racket executable.
 ### racket-command-port
 Port number for Racket REPL command server.
 
+### racket-command-startup
+What to do when the REPL and command server aren't available to send a command.
+
+- nil: Show an error message explaining that you might need to
+  start or restart the Racket REPL.
+
+- positive number: Automatically try to start the REPL and wait
+  that number of seconds for command server to become
+  available.
+
 ### racket-command-timeout
-Timeout for Racket REPL command server.
+How many seconds to wait for Racket REPL command server responses.
 
 ### racket-memory-limit
 Terminate the Racket process if memory use exceeds this value in MB.

racket-bug-report.el

@@ -47,6 +47,7 @@
                             racket--rkt-source-dir
                             racket-program
                             racket-command-port
+                            racket-command-startup
                             racket-command-timeout
                             racket-memory-limit
                             racket-error-context

racket-custom.el

@@ -62,8 +62,23 @@
   :risky t
   :group 'racket)
 
+(defcustom racket-command-startup nil
+  "What to do when the REPL and command server aren't available to send a command.
+
+- nil: Show an error message explaining that you might need to
+  start or restart the Racket REPL.
+
+- positive number: Automatically try to start the REPL and wait
+  that number of seconds for command server to become
+  available."
+  :tag "Command Startup"
+  :type '(choice (const   :tag "Safe" nil)
+                 (integer :tag "Auto-start wait seconds" 15))
+  :risky t
+  :group 'racket)
+
 (defcustom racket-command-timeout 10
-  "Timeout for Racket REPL command server."
+  "How many seconds to wait for Racket REPL command server responses."
   :tag "Command Timeout"
   :type 'integer
   :risky t

racket-edit.el

@@ -75,8 +75,7 @@ commands."
                     (pcase prefix
                       (`(4)  'high)
                       (`(16) 'debug)
-                      (_     racket-error-context))
-                    nil))
+                      (_     racket-error-context))))
 
 (defun racket-run-with-errortrace ()
   "Run with `racket-error-context' temporarily set to 'high.
@@ -133,7 +132,7 @@ See also:
       (racket--repl-run
        mod-path
        'coverage
-       (lambda (_what)
+       (lambda (_n/a)
          (message "Getting coverage results...")
          (racket--cmd/async
           `(get-uncovered)
@@ -206,7 +205,7 @@ Please keep in mind the following limitations:
                               (cons (racket--buffer-file-name t) (md5 (current-buffer)))))
                   (y-or-n-p "Run current buffer first? "))
              (racket--repl-run nil nil
-                               (lambda (_what)
+                               (lambda (_n/a)
                                  (racket--do-visit-def-or-mod 'def str)))
            (racket--do-visit-def-or-mod 'def str)))))
 
@@ -238,10 +237,8 @@ See also: `racket-find-collection'."
 
 (defun racket--do-visit-def-or-mod (cmd str)
   "CMD must be 'def or 'mod. STR must be `stringp`."
-  (cl-case major-mode
-    (racket-mode t)
-    ((racket-repl-mode racket-describe-mode) (racket--repl-ensure-buffer-and-process))
-    (otherwise (user-error "Requires racket-mode or racket-repl-mode")))
+  (unless (memq major-mode '(racket-mode racket-repl-mode racket-describe-mode))
+    (user-error "That doesn't work in %s" major-mode))
   (pcase (racket--cmd/await (list cmd str))
     (`(,path ,line ,col)
      (racket--push-loc)

racket-make-doc.el

@@ -146,6 +146,7 @@
   '("General"
     racket-program
     racket-command-port
+    racket-command-startup
     racket-command-timeout
     racket-memory-limit
     racket-error-context

racket-profile.el

@@ -47,7 +47,7 @@ delete compiled/*.zo files."
   (racket--repl-run
    nil
    'profile
-   (lambda (_what)
+   (lambda (_n/a)
      (message "Getting profile results...")
      (racket--cmd/async
       `(get-profile)