Jsonrpc: rework sync request handling (bug#80623)

When the remote endpoint is handling a local request 'LR' it can
sometimes make a remote sync request 'RR' as part of its handling.  Some
endpoints (like Go's gopls) wait for Emacs's reply to 'RR' before
responding to 'LR'.  Others (like Julia's JETLS) respond to 'LR'
immediately, and only then wait for Emacs's reply to 'RR'.  Both
approaches are valid.  However, in the latter case, the handling of 'RR'
(which could well be waiting for user input from the minibuffer to
complete) is vulnerable to 'throw' from process filters handling (which
is just what happens when the endpoints replies to 'LR'), so if that
happens it will unexpectedly be aborted when the reply to 'LR' comes in,
suprising the user and causing a spurious -32603 reply to be sent.

To solve this problem, this commit first refactors the sync
request/anxious queue handling and replace plain integer keys of the
rebaptized "scontrol" alist with structured (:local ID) / (:remote ID)
pairs, using `equal' for comparisons.  This is to introduce some
clarity/sanity into this somewhat hairy code.

Then, the 'RR' situation is fixed: we push a (:remote ID) entry onto the
top of the 'jsonrpc-connection--control' stack and only then call
rdispatcher.  Any 'LR' reply arriving during dispatch is deferred as an
"anxious" continuation rather than firing its `throw' immediately.  When
rdispatcher is done, we call jsonrpc--continue at a safe point, this
will run any "anxious" continuations.

* lisp/jsonrpc.el (jsonrpc-connection): Rename -sync-request-alist to
-scontrol; accessor from jsonrpc--sync-request-alist to
jsonrpc--scontrol; update docstring for new key structure.

(jsonrpc-connection-receive): Update with-slots binding to scontrol.
Tighten anxious check to match (:local ID) keys with `equal'.  In
remote-request branch, push (:remote ID) entry before rdispatcher and
call jsonrpc--continue after jsonrpc--reply.

(jsonrpc-request): Pass (:local ID) to jsonrpc--continue.

(jsonrpc--continue): Use jsonrpc--scontrol; change `=' to `equal' in
sanity check.
p
(jsonrpc--async-request-1): Push (:local ID) entry.

(jsonrpc--log-event): Use jsonrpc--scontrol.

lisp/jsonrpc.el

@@ -86,12 +86,14 @@
     :documentation "Map (DEFERRED BUF) to (FN TIMER ID).  FN is\
 a saved DEFERRED `async-request' from BUF, to be sent not later\
 than TIMER as ID.")
-   (-sync-request-alist ; bug#67945
+   (-scontrol ; bug#67945
     :initform nil
-    :accessor jsonrpc--sync-request-alist
-    :documentation "List of ((ID [ANXIOUS...])) where ID refers  \
-to a sync `jsonrpc-request' and each ANXIOUS to another completed\
-request that is higher up in the stack but couldn't run.")
+    :accessor jsonrpc--scontrol
+    :documentation "List of ((KEY [ANXIOUS...])) where KEY is \
+(:local ID) for an outstanding sync `jsonrpc-request', or \
+(:remote ID) while a remote request with that ID is being \
+dispatched.  Each ANXIOUS is a completed response deferred \
+until KEY's entry is popped.")
    (-next-request-id
     :initform 0
     :accessor jsonrpc--next-request-id
@@ -291,7 +293,7 @@ dispatcher in CONN."
           (with-slots (last-error
                        (rdispatcher -request-dispatcher)
                        (ndispatcher -notification-dispatcher)
-                       (sr-alist -sync-request-alist))
+                       (scontrol -scontrol))
               conn
             (setf last-error error)
             (cond
@@ -311,23 +313,31 @@ dispatcher in CONN."
               )
              (;; A remote response that can't run yet (bug#67945)
               (and response-p
-                   (and sr-alist (not (eq id (caar sr-alist)))))
+                   (and scontrol (not (equal `(:local ,id) (caar scontrol)))))
               (jsonrpc--event
                conn 'internal
                :log-text
                (format "anxious continuation to %s can't run, held up by %s"
                        id
-                       (mapcar #'car sr-alist)))
+                       (mapcar #'car scontrol)))
               (push (cons cont (list result error))
-                    (cdr (car sr-alist))))
+                    (cdr (car scontrol))))
              (;; A remote response that can continue now
               response-p
-              (jsonrpc--continue conn id cont result error))
+              (jsonrpc--continue conn `(:local ,id) cont result error))
              (;; A remote request
               (and method id)
               (let* ((debug-on-error (and debug-on-error
                                           (not jsonrpc-inhibit-debug-on-error)))
                      reply)
+                ;; While the rdispatcher runs, any arriving response to
+                ;; a previous sync request must not fire its
+                ;; continuation immediately: the resulting `throw' would
+                ;; unwind through the dispatcher and trigger a spurious
+                ;; -32603 instead of a reply.  Use `jsonrpc--scontrol'
+                ;; mechanism to defer such continuations until the end
+                ;; of the cleanup below.
+                (push `((:remote ,id)) (jsonrpc--scontrol conn))
                 (unwind-protect
                     (setq
                      reply
@@ -349,7 +359,8 @@ dispatcher in CONN."
                   (unless reply
                     (setq reply
                           `(:error (:code -32603 :message "Internal error"))))
-                  (apply #'jsonrpc--reply conn id method reply))))
+                  (apply #'jsonrpc--reply conn id method reply)
+                  (jsonrpc--continue conn `(:remote ,id)))))
              (;; A remote notification
               method
               (funcall ndispatcher conn (intern method) params))
@@ -513,7 +524,7 @@ to the original request (normal or error) are ignored."
         ;; request, it might have been holding up any outer
         ;; "anxious" continuations.  The following ensures we
         ;; call them.
-        (jsonrpc--continue connection id)))
+        (jsonrpc--continue connection `(:local ,id))))
     (cond ((eq 'error (car retval))
            (signal 'jsonrpc-error
                    (cons
@@ -902,11 +913,11 @@ Return the full continuation (ID SUCCESS-FN ERROR-FN TIMER)"
 (defun jsonrpc--continue (conn id &optional cont result error)
   (pcase-let* ((`(,cont-id ,_method ,success-fn ,error-fn ,_timer)
                 cont)
-               (head (pop (jsonrpc--sync-request-alist conn)))
+               (head (pop (jsonrpc--scontrol conn)))
                (anxious (cdr head)))
     (cond
      (anxious
-      (when (not (= (car head) id)) ; sanity check
+      (when (not (equal (car head) id)) ; sanity check
         (error "Internal error: please report this bug"))
       ;; If there are "anxious" `jsonrpc-request' continuations
       ;; that should already have been run, they should run now.
@@ -998,7 +1009,7 @@ TIMEOUT is nil)."
     ;; Setup some control structures
     ;;
     (when sync-request
-      (push (list id) (jsonrpc--sync-request-alist connection)))
+      (push `((:local ,id)) (jsonrpc--scontrol connection)))
 
     (jsonrpc--schedule
      connection id method
@@ -1108,7 +1119,7 @@ of the API instead.")
       (cl-destructuring-bind (&key method id error &allow-other-keys) message
         (let* ((inhibit-read-only t)
                (depth (length
-                       (jsonrpc--sync-request-alist connection)))
+                       (jsonrpc--scontrol connection)))
                (preamble (format "[jsonrpc] %s[%s]%s "
                                  (pcase type ('error "E") ('debug "D")
                                         (_ (pcase origin