fix: drop stale async weather responses that overwrite a newer query

wttrin-query reuses the single *wttr.in* buffer, so requesting Paris then Berlin
and having Paris return last overwrote the Berlin buffer. Each query now stamps
a monotonic request id on the buffer; the callback displays only when its
captured id still matches, so a superseded response is ignored.

1 files changed, 56 insertions, 0 deletions

diff --git a/tests/test-wttrin-query-request-id.el b/tests/test-wttrin-query-request-id.el
new file mode 100644
index 0000000..cf3c371
--- /dev/null
+++ b/tests/test-wttrin-query-request-id.el
@@ -0,0 +1,56 @@
+;;; test-wttrin-query-request-id.el --- Stale async response guard -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2024-2026 Craig Jennings
+
+;;; Commentary:
+;; wttrin-query reuses the single *wttr.in* buffer, so an async response for an
+;; earlier query that completes after a later one must not overwrite the newer
+;; buffer.  A per-buffer request id, captured in the callback closure, drops the
+;; stale response.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+(ert-deftest test-wttrin-query-normal-current-callback-displays ()
+  "Normal: a query's own callback (matching request id) displays its result."
+  (let ((cb nil) (displayed nil))
+    (unwind-protect
+        (cl-letf (((symbol-function 'wttrin--get-cached-or-fetch)
+                   (lambda (_loc c) (setq cb c)))
+                  ((symbol-function 'switch-to-buffer)
+                   (lambda (buf &rest _) (set-buffer buf)))
+                  ((symbol-function 'wttrin--display-weather)
+                   (lambda (query &rest _) (setq displayed query))))
+          (wttrin-query "Paris")
+          (funcall cb "paris-data")
+          (should (equal displayed "Paris")))
+      (when (get-buffer "*wttr.in*") (kill-buffer "*wttr.in*")))))
+
+(ert-deftest test-wttrin-query-error-stale-callback-does-not-overwrite-newer ()
+  "Error: a slow response for an earlier query does not overwrite a newer query.
+Paris then Berlin share the *wttr.in* buffer; the Paris callback arriving after
+Berlin must be ignored."
+  (let ((callbacks nil) (displayed nil))
+    (unwind-protect
+        (cl-letf (((symbol-function 'wttrin--get-cached-or-fetch)
+                   (lambda (_loc cb) (push cb callbacks)))
+                  ((symbol-function 'switch-to-buffer)
+                   (lambda (buf &rest _) (set-buffer buf)))
+                  ((symbol-function 'wttrin--display-weather)
+                   (lambda (query &rest _) (setq displayed query))))
+          (wttrin-query "Paris")
+          (wttrin-query "Berlin")
+          (let ((paris-cb (cadr callbacks))   ; first pushed
+                (berlin-cb (car callbacks)))  ; last pushed
+            (funcall berlin-cb "berlin-data")
+            (should (equal displayed "Berlin"))
+            (funcall paris-cb "paris-data")
+            (should (equal displayed "Berlin"))))
+      (when (get-buffer "*wttr.in*") (kill-buffer "*wttr.in*")))))
+
+(provide 'test-wttrin-query-request-id)
+;;; test-wttrin-query-request-id.el ends here