Fix nrepl-client not connecting to ssh-remote after cider-jack-in

Calling cider-jack-in from a tramp buffer on a ssh-remote would throw:
"error in process filter: Wrong type argument: stringp, nil"

This happened because nrepl--ssh-tunnel-connect is trying to
string-match on (buffer-file-name) which returns nil, because the
execution context is the nrepl buffer.

To work around this, we pass down the buffer from which cider-jack-in
was called.

Considerations:

- This should be reverted if the cause of this feature breaking is
  found and fixable. It seemed to have worked before,
  see: clojure-emacs#3264

- In nrepl-client.el the param name "jack-in-buffer" seems a bit too
  specific. But, as long as this workaround is its only usecase, I
  consider it reasonable for discoverability.

- This should be refactored as soon as there's a more general way to convey
  contextual information to lower level functions.

Fixes:
clojure-emacs#3541
clojure-emacs#3303 (partially)

cider-connection.el

@@ -126,7 +126,8 @@ PARAMS is a plist containing :host, :port, :server and other parameters for
     (plist-get params :server)
     (lambda (_)
       (cider-repl-create params))
-    (plist-get params :socket-file))))
+    (plist-get params :socket-file)
+    (plist-get params :jack-in-buffer))))
 
 (defun cider-sessions ()
   "Return a list of all active CIDER sessions."

cider.el

@@ -1335,6 +1335,7 @@ double prefix prompt for all these parameters."
   (interactive "P")
   (let ((params (thread-first
                   params
+                  (plist-put :jack-in-buffer (current-buffer))
                   (cider--update-project-dir)
                   (cider--check-existing-session)
                   (cider--update-jack-in-cmd))))

nrepl-client.el

@@ -533,15 +533,16 @@ If NO-ERROR is non-nil, show messages instead of throwing an error."
                  (error msg))
                nil)))))
 
-(defun nrepl-connect (host port)
+(defun nrepl-connect (host port &optional jack-in-buffer)
   "Connect to the nREPL server identified by HOST and PORT.
 For local hosts use a direct connection.  For remote hosts, if
 `nrepl-force-ssh-for-remote-hosts' is nil, attempt a direct connection
 first.  If `nrepl-force-ssh-for-remote-hosts' is non-nil or the direct
 connection failed (and `nrepl-use-ssh-fallback-for-remote-hosts' is
 non-nil), try to start a SSH tunneled connection.  Return a plist of the
 form (:proc PROC :host \"HOST\" :port PORT) that might contain additional
-key-values depending on the connection type."
+key-values depending on the connection type. If JACK-IN-BUFFER is given
+and a tramp buffer, reuse its ssh parameters."
   (let ((localp (if host
                     (nrepl-local-host-p host)
                   (not (file-remote-p default-directory)))))
@@ -554,15 +555,15 @@ key-values depending on the connection type."
               ;; fallback to ssh tunneling if enabled
               (and nrepl-use-ssh-fallback-for-remote-hosts
                    (message "[nREPL] Falling back to SSH tunneled connection ...")
-                   (nrepl--ssh-tunnel-connect host port))
+                   (nrepl--ssh-tunnel-connect host port jack-in-buffer))
               ;; fallback is either not enabled or it failed as well
               (if (and (null nrepl-use-ssh-fallback-for-remote-hosts)
                        (not localp))
                   (error "[nREPL] Direct connection to %s:%s failed; try setting `nrepl-use-ssh-fallback-for-remote-hosts' to t"
                          host port)
                 (error "[nREPL] Cannot connect to %s:%s" host port)))
         ;; `nrepl-force-ssh-for-remote-hosts' is non-nil
-        (nrepl--ssh-tunnel-connect host port)))))
+        (nrepl--ssh-tunnel-connect host port jack-in-buffer)))))
 
 (defun nrepl--direct-connect (host port &optional no-error)
   "If HOST and PORT are given, try to `open-network-stream'.
@@ -584,10 +585,11 @@ If NO-ERROR is non-nil, show messages instead of throwing an error."
                  (error msg))
                nil)))))
 
-(defun nrepl--ssh-tunnel-connect (host port)
-  "Connect to a remote machine identified by HOST and PORT through SSH tunnel."
+(defun nrepl--ssh-tunnel-connect (host port &optional jack-in-buffer)
+  "Connect to a remote machine identified by HOST and PORT through SSH tunnel.
+If JACK-IN-BUFFER is given and a tramp buffer, reuse its connection parameters. Defaults to the current buffer."
   (message "[nREPL] Establishing SSH tunneled connection to %s:%s ..." host port)
-  (let* ((current-buf (buffer-file-name))
+  (let* ((current-buf (buffer-file-name (or jack-in-buffer (current-buffer))))
          (tramp-file-regexp "/ssh:\\(.+@\\)?\\(.+?\\)\\(:\\|#\\).+")
          (remote-dir (cond
                       ;; If current buffer is a TRAMP buffer and its host is
@@ -704,7 +706,7 @@ Do not kill the server if there is a REPL connected to that server."
                            (buffer-list)))
         (nrepl-kill-server-buffer server-buf)))))
 
-(defun nrepl-start-client-process (&optional host port server-proc buffer-builder socket-file)
+(defun nrepl-start-client-process (&optional host port server-proc buffer-builder socket-file jack-in-buffer)
   "Create new client process identified by either HOST and PORT or SOCKET-FILE.
 If SOCKET-FILE is non-nil, it takes precedence.  In remote buffers, HOST
 and PORT are taken from the current tramp connection.  SERVER-PROC must be
@@ -713,7 +715,7 @@ of one argument (endpoint returned by `nrepl-connect') which returns a
 client buffer.  Return the newly created client process."
   (let* ((endpoint (if socket-file
                        (nrepl--unix-connect (expand-file-name socket-file))
-                     (nrepl-connect host port)))
+                     (nrepl-connect host port jack-in-buffer)))
          (client-proc (plist-get endpoint :proc))
          (builder (or buffer-builder (error "`buffer-builder' must be provided")))
          (client-buf (funcall builder endpoint)))