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+;;; test-wttrin-error-propagation.el --- Tests for error info through callbacks -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025-2026 Craig Jennings
+
+;;; Commentary:
+
+;; Tests that error information flows from the fetch layer through callbacks
+;; to the display layer, so users see specific error messages instead of
+;; "Perhaps the location was misspelled?" for every failure.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Setup and Teardown
+
+(defun test-wttrin-error-propagation-setup ()
+  "Setup for error propagation tests."
+  (testutil-wttrin-setup)
+  (when (get-buffer "*wttr.in*")
+    (kill-buffer "*wttr.in*")))
+
+(defun test-wttrin-error-propagation-teardown ()
+  "Teardown for error propagation tests."
+  (testutil-wttrin-teardown)
+  (when (get-buffer "*wttr.in*")
+    (kill-buffer "*wttr.in*")))
+
+;;; --------------------------------------------------------------------------
+;;; handle-fetch-callback passes error info to callback
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin-error-propagation-network-error-reaches-callback ()
+  "Network error description should be passed as second callback argument."
+  (let ((received-error nil))
+    (cl-letf (((symbol-function 'wttrin--extract-response-body)
+               (lambda () nil))
+              ((symbol-function 'message) #'ignore))
+      (wttrin--handle-fetch-callback
+       '(:error (error "Connection refused"))
+       (lambda (_data &optional error-msg)
+         (setq received-error error-msg)))
+      (should received-error)
+      (should (string-match-p "network" (downcase received-error))))))
+
+(ert-deftest test-wttrin-error-propagation-http-error-reaches-callback ()
+  "HTTP error description should be passed as second callback argument."
+  (let ((received-error nil))
+    (cl-letf (((symbol-function 'wttrin--extract-response-body)
+               (lambda () nil))
+              ((symbol-function 'wttrin--extract-http-status)
+               (lambda () 404))
+              ((symbol-function 'message) #'ignore))
+      (wttrin--handle-fetch-callback
+       nil
+       (lambda (_data &optional error-msg)
+         (setq received-error error-msg)))
+      (should received-error)
+      (should (string-match-p "not found\\|404" (downcase received-error))))))
+
+(ert-deftest test-wttrin-error-propagation-success-no-error ()
+  "Successful fetch should pass nil as the error argument."
+  (let ((received-error 'not-called))
+    (cl-letf (((symbol-function 'wttrin--extract-response-body)
+               (lambda () "weather data"))
+              ((symbol-function 'wttrin--extract-http-status)
+               (lambda () 200)))
+      (wttrin--handle-fetch-callback
+       nil
+       (lambda (_data &optional error-msg)
+         (setq received-error error-msg)))
+      (should-not received-error))))
+
+;;; --------------------------------------------------------------------------
+;;; get-cached-or-fetch forwards error to caller's callback
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin-error-propagation-cache-miss-forwards-error ()
+  "When fetch fails with no cache, the error message should reach the caller."
+  (test-wttrin-error-propagation-setup)
+  (unwind-protect
+      (let ((received-error nil))
+        (cl-letf (((symbol-function 'wttrin-fetch-raw-string)
+                   (lambda (_query callback)
+                     (funcall callback nil "Network error — check your connection"))))
+          (let ((wttrin--force-refresh t))
+            (wttrin--get-cached-or-fetch
+             "Paris"
+             (lambda (_data &optional error-msg)
+               (setq received-error error-msg)))
+            (should received-error)
+            (should (string-match-p "network" (downcase received-error))))))
+    (test-wttrin-error-propagation-teardown)))
+
+(ert-deftest test-wttrin-error-propagation-stale-cache-no-error ()
+  "When fetch fails but stale cache exists, serve the data without an error."
+  (test-wttrin-error-propagation-setup)
+  (unwind-protect
+      (let ((received-data nil)
+            (received-error 'not-called))
+        (testutil-wttrin-add-to-cache "Paris" "old weather" 9999)
+        (cl-letf (((symbol-function 'wttrin-fetch-raw-string)
+                   (lambda (_query callback)
+                     (funcall callback nil "Network error")))
+                  ((symbol-function 'message) #'ignore))
+          (let ((wttrin--force-refresh t))
+            (wttrin--get-cached-or-fetch
+             "Paris"
+             (lambda (data &optional error-msg)
+               (setq received-data data)
+               (setq received-error error-msg)))
+            ;; Should get stale data, not an error
+            (should received-data)
+            (should-not received-error))))
+    (test-wttrin-error-propagation-teardown)))
+
+;;; --------------------------------------------------------------------------
+;;; display-weather shows specific error message
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin-error-propagation-display-shows-specific-error ()
+  "When an error message is provided, display-weather should show it
+instead of the generic 'Perhaps the location was misspelled?' text."
+  (test-wttrin-error-propagation-setup)
+  (unwind-protect
+      (let ((displayed-message nil))
+        (cl-letf (((symbol-function 'message)
+                   (lambda (fmt &rest args)
+                     (setq displayed-message (apply #'format fmt args)))))
+          (wttrin--display-weather "Paris" nil "Network error — check your connection")
+          (should displayed-message)
+          (should (string-match-p "network" (downcase displayed-message)))
+          ;; Should NOT show the generic message
+          (should-not (string-match-p "misspelled" (downcase displayed-message)))))
+    (test-wttrin-error-propagation-teardown)))
+
+(ert-deftest test-wttrin-error-propagation-display-falls-back-to-generic ()
+  "When no error message is provided, display-weather should show the
+generic message for backward compatibility."
+  (test-wttrin-error-propagation-setup)
+  (unwind-protect
+      (let ((displayed-message nil))
+        (cl-letf (((symbol-function 'message)
+                   (lambda (fmt &rest args)
+                     (setq displayed-message (apply #'format fmt args)))))
+          (wttrin--display-weather "Paris" nil)
+          (should displayed-message)
+          (should (string-match-p "Cannot retrieve" displayed-message))))
+    (test-wttrin-error-propagation-teardown)))
+
+;;; --------------------------------------------------------------------------
+;;; End-to-end: full query chain
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin-error-propagation-e2e-404-shows-not-found ()
+  "Full chain: user queries a bad location, API returns 404, user sees
+'not found' — not the generic 'misspelled' message."
+  (test-wttrin-error-propagation-setup)
+  (unwind-protect
+      (let ((displayed-message nil))
+        (cl-letf (((symbol-function 'url-retrieve)
+                   (lambda (_url callback)
+                     (with-current-buffer (generate-new-buffer " *test-404*")
+                       (insert "HTTP/1.1 404 Not Found\r\n\r\nUnknown location")
+                       (funcall callback nil))))
+                  ((symbol-function 'message)
+                   (lambda (fmt &rest args)
+                     (setq displayed-message (apply #'format fmt args)))))
+          (wttrin-query "Nonexistent Place")
+          (should displayed-message)
+          (should (string-match-p "not found\\|404" (downcase displayed-message)))))
+    (test-wttrin-error-propagation-teardown)))
+
+(ert-deftest test-wttrin-error-propagation-e2e-network-error-shows-network ()
+  "Full chain: network is down, user sees 'network error'."
+  (test-wttrin-error-propagation-setup)
+  (unwind-protect
+      (let ((displayed-message nil))
+        (cl-letf (((symbol-function 'url-retrieve)
+                   (lambda (_url callback)
+                     (with-temp-buffer
+                       (funcall callback '(:error (error "Connection refused"))))))
+                  ((symbol-function 'message)
+                   (lambda (fmt &rest args)
+                     (setq displayed-message (apply #'format fmt args)))))
+          (let ((wttrin--force-refresh t))
+            (wttrin-query "Paris"))
+          (should displayed-message)
+          (should (string-match-p "network" (downcase displayed-message)))))
+    (test-wttrin-error-propagation-teardown)))
+
+;;; --------------------------------------------------------------------------
+;;; Passthrough layers
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin-error-propagation-fetch-raw-string-forwards-error ()
+  "wttrin-fetch-raw-string is a passthrough — error-msg from the fetch
+layer must not be dropped."
+  (let ((received-error nil))
+    (cl-letf (((symbol-function 'wttrin--fetch-url)
+               (lambda (_url callback)
+                 (funcall callback nil "test error message"))))
+      (wttrin-fetch-raw-string
+       "Paris"
+       (lambda (_data &optional error-msg)
+         (setq received-error error-msg)))
+      ;; fetch-raw-string calls fetch-url which calls handle-fetch-callback
+      ;; which calls our callback — but we mocked fetch-url to call directly,
+      ;; so the error-msg should pass through
+      (should (equal received-error "test error message")))))
+
+(ert-deftest test-wttrin-error-propagation-http-3xx-shows-unexpected ()
+  "HTTP 3xx that reaches us (unusual — url-retrieve follows redirects)
+should still produce an error message, not silently return nil."
+  (let ((received-error nil))
+    (cl-letf (((symbol-function 'wttrin--extract-response-body)
+               (lambda () nil))
+              ((symbol-function 'wttrin--extract-http-status)
+               (lambda () 301))
+              ((symbol-function 'message) #'ignore))
+      (wttrin--handle-fetch-callback
+       nil
+       (lambda (_data &optional error-msg)
+         (setq received-error error-msg)))
+      (should received-error)
+      (should (string-match-p "301\\|unexpected" (downcase received-error))))))
+
+(provide 'test-wttrin-error-propagation)
+;;; test-wttrin-error-propagation.el ends here