perf(host-env): try cheap timezone methods before the zoneinfo scan

The content-comparison scan reads hundreds of zoneinfo files and ran first on every call. TZ, /etc/timezone, and the symlink target are O(1) and answer on almost every system; the scan is now the last resort for a copied /etc/localtime. Verified the symlink and scan agree on this machine.

2 files changed, 25 insertions, 13 deletions

diff --git a/modules/host-environment.el b/modules/host-environment.el
index 0afb39cb..1c33e342 100644
--- a/modules/host-environment.el
+++ b/modules/host-environment.el
@@ -138,15 +138,13 @@ find /usr/share/zoneinfo -type f ! -name `posixrules' \\
 
 (defun cj/detect-system-timezone ()
   "Detect the system timezone in IANA format (e.g., `America/Los_Angeles').
-Tries multiple methods in order of reliability:
-1. File comparison of /etc/localtime with zoneinfo database
-2. Environment variable TZ
-3. /etc/timezone file contents
-4. /etc/localtime symlink target"
+Tries the cheap methods first and the exhaustive scan last:
+1. Environment variable TZ (most explicit if set)
+2. /etc/timezone file contents (Debian/Ubuntu)
+3. /etc/localtime symlink target (O(1) on symlinked systems)
+4. File comparison of /etc/localtime against the zoneinfo database
+   (reads hundreds of files; only needed when localtime is a copy)"
   (or
-   ;; Compare file contents (reliable on Arch/modern systems)
-   (cj/match-localtime-to-zoneinfo)
-
    ;; Environment variable (most explicit if set)
    (getenv "TZ")
 
@@ -156,12 +154,15 @@ Tries multiple methods in order of reliability:
 	   (insert-file-contents "/etc/timezone")
 	   (string-trim (buffer-string))))
 
-   ;; Method 4: Parse symlink (fallback for older systems)
+   ;; Parse the symlink -- O(1), answers on any symlinked /etc/localtime
    (when (file-symlink-p "/etc/localtime")
 	 (let ((target (file-truename "/etc/localtime")))
 	   (when (string-match ".*/zoneinfo/\\(.+\\)" target)
 		 (match-string 1 target))))
 
+   ;; Compare file contents -- the last resort for a copied /etc/localtime
+   (cj/match-localtime-to-zoneinfo)
+
    ;; Default to nil if detection fails
    nil))
 
diff --git a/tests/test-host-environment--detect-system-timezone.el b/tests/test-host-environment--detect-system-timezone.el
index 209283d1..0d76c206 100644
--- a/tests/test-host-environment--detect-system-timezone.el
+++ b/tests/test-host-environment--detect-system-timezone.el
@@ -17,12 +17,23 @@
 (add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
 (require 'host-environment)
 
-(ert-deftest test-host-environment-detect-tz-match-localtime-wins ()
-  "Normal: when match-localtime-to-zoneinfo returns a value, that wins."
+(ert-deftest test-host-environment-detect-tz-env-wins-without-content-scan ()
+  "Normal: an explicit TZ wins and the exhaustive zoneinfo scan never runs.
+The scan reads hundreds of files; it used to run first on every call even
+when a cheap O(1) method would answer."
   (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
-             (lambda () "America/Los_Angeles"))
+             (lambda () (error "content scan should not have run")))
             ((symbol-function 'getenv)
-             (lambda (_ &rest _) (error "TZ should not have been consulted"))))
+             (lambda (name &rest _) (when (string= name "TZ") "America/Chicago"))))
+    (should (equal (cj/detect-system-timezone) "America/Chicago"))))
+
+(ert-deftest test-host-environment-detect-tz-content-scan-is-last-resort ()
+  "Boundary: with every cheap method empty, the content scan still answers."
+  (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
+             (lambda () "America/Los_Angeles"))
+            ((symbol-function 'getenv) (lambda (&rest _) nil))
+            ((symbol-function 'file-exists-p) (lambda (&rest _) nil))
+            ((symbol-function 'file-symlink-p) (lambda (&rest _) nil)))
     (should (equal (cj/detect-system-timezone) "America/Los_Angeles"))))
 
 (ert-deftest test-host-environment-detect-tz-env-var-wins-when-match-nil ()