fix(db): centralize DB_AHEAD marker as shared const (Sourcery on PR #64)

src-tauri/src/db/mod.rs

@@ -64,12 +64,12 @@ pub async fn init_pool() -> Result<SqlitePool, String> {
     // Catch the "DB ahead of binary" case BEFORE letting sqlx fire its
     // cryptic `migration N was previously applied but is missing in the
     // resolved migrations` error. The setup hook in `lib.rs` matches the
-    // `DB_AHEAD` prefix to render a friendly recovery notice — typical
-    // user landing here ran a newer dev build against this DB and is now
-    // trying to launch an older installed binary.
+    // `DB_AHEAD_MARKER` token to render a friendly recovery notice — the
+    // typical user landing here ran a newer dev build against this DB
+    // and is now trying to launch an older installed binary.
     check_db_is_not_ahead_of_binary(&pool)
         .await
-        .map_err(|e| format!("DB_AHEAD::{e}"))?;
+        .map_err(|e| format!("{DB_AHEAD_MARKER}{e}"))?;
 
     // Run migrations
     sqlx::migrate!("src/db/migrations")
@@ -80,6 +80,17 @@ pub async fn init_pool() -> Result<SqlitePool, String> {
     Ok(pool)
 }
 
+/// Token prepended to the error message when `check_db_is_not_ahead_of_binary`
+/// detects the failure mode. The setup hook in `lib.rs` matches this exact
+/// token via `find()` (not `strip_prefix`) so the consumer is robust to any
+/// future wrapping the producer adds. Single source of truth on both sides:
+/// changing the token here must compile-error if the consumer drifts.
+///
+/// Boundary string `::` chosen because it's syntactically unusual in plain
+/// French error messages, so a false positive on `find()` is effectively
+/// impossible.
+pub const DB_AHEAD_MARKER: &str = "DB_AHEAD::";
+
 /// Detect the "DB has been migrated by a newer version than this binary
 /// knows about" failure mode BEFORE `sqlx::migrate!().run()` does. The
 /// stock sqlx error message — `migration 18 was previously applied but
@@ -679,6 +690,34 @@ mod migration_tests {
             .expect("matching-version DB must pass the pre-flight");
     }
 
+    #[test]
+    fn db_ahead_marker_is_findable_even_when_wrapped() {
+        // Regression guard against the Sourcery review on PR #64: the
+        // consumer in lib.rs uses `find(db::DB_AHEAD_MARKER)` rather
+        // than `strip_prefix("Failed to init SQLite: DB_AHEAD::")`.
+        // This test simulates the marker wrapped under arbitrary
+        // prefixes and confirms the detection still works — so a
+        // future refactor of the error wrapping in lib.rs cannot
+        // silently break the STARTUP_BLOCKED.txt path.
+        let marker = super::DB_AHEAD_MARKER;
+        let wrappings = [
+            format!("Failed to init SQLite: {marker}payload"),
+            format!("Some new wrapping: {marker}payload"),
+            format!("{marker}payload"), // no wrapping at all
+            format!("Layer A → Layer B: {marker}payload"),
+        ];
+        for wrapped in &wrappings {
+            let idx = wrapped
+                .find(marker)
+                .unwrap_or_else(|| panic!("marker must be findable in wrapping: {wrapped}"));
+            let extracted = &wrapped[idx + marker.len()..];
+            assert_eq!(
+                extracted, "payload",
+                "payload must be cleanly extractable after the marker"
+            );
+        }
+    }
+
     #[tokio::test]
     async fn db_ahead_check_blocks_when_db_has_future_migration() {
         // Reproduces the v0.3.8 ← v0.3.9 dev workflow crash: the user

src-tauri/src/lib.rs

@@ -115,11 +115,21 @@ pub fn run() {
                 // Special handler for the "DB ahead of binary" pre-flight
                 // failure surfaced by db::check_db_is_not_ahead_of_binary.
                 // The user can't read the log file from a crashed app; we
-                // write a self-explanatory text file to the data dir so
-                // double-clicking app.db's folder reveals the recovery
-                // path. Pure best-effort — if the write fails too we just
+                // write a self-explanatory text file to the data dir
+                // (same dir as `app.db`, NOT the log dir which lives
+                // under the bundle identifier on Windows) so double-
+                // clicking that folder reveals the recovery path.
+                // Pure best-effort — if the write fails too we just
                 // fall through to the normal log-and-exit.
-                if let Some(payload) = e.strip_prefix("Failed to init SQLite: DB_AHEAD::") {
+                //
+                // We `find()` the marker rather than `strip_prefix()`
+                // so the detection survives any future change to the
+                // upstream wrapping in `setup_result` (e.g. a different
+                // prefix than "Failed to init SQLite: "). The marker
+                // itself is the single source of truth, defined in
+                // `db::DB_AHEAD_MARKER` and prepended in `init_pool`.
+                if let Some(idx) = e.find(db::DB_AHEAD_MARKER) {
+                    let payload = &e[idx + db::DB_AHEAD_MARKER.len()..];
                     write_startup_blocked_notice(payload);
                 }
                 return Err(e.into());