DRY pass #2, batch 7, F21: update.rs reads installer markers one way (#5372)

update.rs read_marker(): read the installer's marker file, parse it, and
log one that doesn't parse. adopt_installer_channel and
adopt_installer_server each wrote that out; they already shared adopt().

normalize_server's doc now says why it stays apart from
compat::normalize_base_url: one tidies what a person types, the other
refuses anything odd in what a script wrote. The tauri.conf updater
endpoint stays: read_source already documents that it is never consulted,
and removing it is a config change CI would be the first to try.

rustfmt --check is clean in the CI image.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

desktop/src-tauri/src/update.rs

@@ -171,6 +171,11 @@ impl Source {
 /// reason: this value is spliced into every update URL, and an address carrying a
 /// query, a fragment or whitespace is a paste gone wrong rather than a place to
 /// fetch from.
+///
+/// Deliberately stricter than `sync::compat::normalize_base_url`, which tidies what a
+/// person types into Sync (it supplies a missing `https://`, and lets a path or
+/// query through for the probe to judge). This one checks a value a script wrote,
+/// where anything unusual is a fault to refuse rather than a typo to forgive.
 fn normalize_server(raw: &str) -> Option<String> {
     let trimmed = raw.trim().trim_end_matches('/');
     let rest = trimmed
@@ -267,15 +272,8 @@ fn self_update_blocker() -> Option<String> {
 /// Windows installer that has no channel concept — and none of that should keep the
 /// app from opening.
 pub fn adopt_installer_channel(db: &Db, data_dir: &Path) {
-    let path = data_dir.join(INSTALL_MARKER);
-    let Ok(raw) = std::fs::read_to_string(&path) else {
-        return;
-    };
-    let Some(channel) = Channel::from_marker(&raw) else {
-        log::warn!(
-            "ignoring an unreadable install channel marker at {}",
-            path.display()
-        );
+    let Some(channel) = read_marker(data_dir, INSTALL_MARKER, "channel", Channel::from_marker)
+    else {
         return;
     };
     match adopt(db, CHANNEL_PREF, CHANNEL_SEED_PREF, channel.as_str()) {
@@ -292,15 +290,7 @@ pub fn adopt_installer_channel(db: &Db, data_dir: &Path) {
 /// and for the same reasons: once per new marker value, so a choice made in the app
 /// afterwards sticks, and reinstalling from a different server is honoured.
 pub fn adopt_installer_server(db: &Db, data_dir: &Path) {
-    let path = data_dir.join(SERVER_MARKER);
-    let Ok(raw) = std::fs::read_to_string(&path) else {
-        return;
-    };
-    let Some(server) = normalize_server(&raw) else {
-        log::warn!(
-            "ignoring an unreadable install server marker at {}",
-            path.display()
-        );
+    let Some(server) = read_marker(data_dir, SERVER_MARKER, "server", normalize_server) else {
         return;
     };
     match adopt(db, SERVER_PREF, SERVER_SEED_PREF, &server) {
@@ -310,6 +300,26 @@ pub fn adopt_installer_server(db: &Db, data_dir: &Path) {
     }
 }
 
+/// The installer's marker `name` in `data_dir`, parsed. `None` when there is none,
+/// which is ordinary, or when it doesn't parse, which is logged and stepped over.
+fn read_marker<T>(
+    data_dir: &Path,
+    name: &str,
+    what: &str,
+    parse: impl Fn(&str) -> Option<T>,
+) -> Option<T> {
+    let path = data_dir.join(name);
+    let raw = std::fs::read_to_string(&path).ok()?;
+    let parsed = parse(&raw);
+    if parsed.is_none() {
+        log::warn!(
+            "ignoring an unreadable install {what} marker at {}",
+            path.display()
+        );
+    }
+    parsed
+}
+
 /// Write `value` to `pref` unless this same marker value was already applied.
 /// Returns whether anything changed.
 fn adopt(db: &Db, pref: &str, seed_pref: &str, value: &str) -> Result<bool, String> {