fix(rules): readers no longer wait on a cache refresh

crates/devkit-ports/src/run.rs

@@ -1173,7 +1173,11 @@ mod tests {
             }
             std::thread::sleep(std::time::Duration::from_millis(100));
         }
-        assert!(ready, "server never started listening");
+        assert!(
+            ready,
+            "server never started listening on {port}; its log:\n{}",
+            std::fs::read_to_string(&tmp).unwrap_or_else(|e| format!("unreadable: {e}"))
+        );
 
         // Cleanup: stop the spawned pid and release the reservation.
         if let Some(pid) = out[0].pid {

crates/devkit-rules/src/cache.rs

@@ -4,10 +4,10 @@
 //! Every reader loads the whole [`RuleIndex`] and filters it in memory, so
 //! the file keeps each file and rule as JSON in index order, beside a `meta`
 //! row naming the source kind, repository, remote and revision it holds. A
-//! refresh replaces everything in one transaction, so a reader sees the
-//! whole old index or the whole new one. A file whose format, kind,
-//! repository or remote is not this cache's reads as no cache, and the next
-//! refresh replaces it.
+//! refresh replaces everything in one WAL-mode transaction, so a reader sees
+//! the whole old index or the whole new one and never waits on a refresh.
+//! A file whose format, kind, repository or remote is not this cache's reads
+//! as no cache, and the next refresh replaces it.
 
 use std::{
     path::{Path, PathBuf},
@@ -22,8 +22,8 @@ use crate::model::{Rule, RuleFile, RuleIndex};
 /// The layout this version of devkit reads and writes.
 const FORMAT: i64 = 1;
 
-/// How long a connection waits on another's lock: a reader on a refresh,
-/// or one refresh on another.
+/// How long a connection waits on another's lock, chiefly one refresh on
+/// another.
 const BUSY: Duration = Duration::from_secs(2);
 
 const LAYOUT: &str = "
@@ -130,7 +130,7 @@ impl RuleCache {
         }
         let mut conn = Connection::open(&self.path).with_context(writing)?;
         conn.busy_timeout(BUSY).with_context(writing)?;
-        conn.pragma_update(None, "journal_mode", "DELETE")
+        conn.pragma_update(None, "journal_mode", "WAL")
             .with_context(writing)?;
         let tx = conn
             .transaction_with_behavior(TransactionBehavior::Immediate)

crates/devkit-rules/tests/cache.rs

@@ -142,6 +142,21 @@ fn at_state_dir_names_the_file_by_kind_and_repository() {
     );
 }
 
+/// A reader reads the whole old index straight through a refresh that has
+/// not committed, rather than waiting on it.
+#[test]
+fn a_refresh_in_progress_does_not_hold_up_a_reader() {
+    let dir = tempfile::tempdir().unwrap();
+    let cache = cache(dir.path(), "a");
+    cache.write(Write::Replace, 1, &index()).unwrap();
+    let mut refresh = rusqlite::Connection::open(cache.path()).unwrap();
+    let tx = refresh
+        .transaction_with_behavior(rusqlite::TransactionBehavior::Exclusive)
+        .unwrap();
+    tx.execute_batch("DROP TABLE rules").unwrap();
+    assert_eq!(value(&cache.read().unwrap().unwrap()), value(&index()));
+}
+
 /// Whether `e` is SQLite giving up on a lock another writer held past the
 /// busy timeout, which leaves the cache as it was.
 fn is_busy(e: &anyhow::Error) -> bool {