Eglot: introduce eglot-advertise-cancellation

Setting this variable to true causes Eglot to send special
cancellation notification for certain stale client request.

This may help some LSP servers avoid doing costly but ultimately useless
work on behalf of the client, improving overall performance.

Request cancellation is described in

   https://microsoft.github.io/language-server-protocol/
      specifications/lsp/3.17/specification/#cancelRequest

* lisp/jsonrpc.el (jsonrpc-request): Accept function as value for
CANCEL-ON-INPUT.

* lisp/progmodes/eglot.el (eglot--request): Rework.

* doc/misc/eglot.texi (Customizing Eglot): Mention
eglot-advertise-cancellation.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/misc/eglot.texi b/doc/misc/eglot.texi
index df24cf2a1dc..47649455cec 100644
--- a/doc/misc/eglot.texi
+++ b/doc/misc/eglot.texi
@@ -919,6 +919,13 @@ Set this variable to non-nil if you'd like progress notifications coming
 from the language server to be handled by Emacs's progress reporting
 facilities.  If the value is the symbol @code{messages} the message
 buffer is used, else the progress is reported in the mode line.
+
+@cindex request cancellation
+@item eglot-advertise-cancellation
+Setting this variable to true causes Eglot to send special cancellation
+notification for certain stale client request.  This may help some LSP
+servers avoid doing costly but ultimately useless work on behalf of the
+client, improving overall performance.
 @end vtable
 
 @node Other Variables