feat: add configurable limit on loaded database count

Unbounded load() calls can exhaust memory via connection pool
proliferation. Add a configurable max (default 50) to DbInstances,
rejecting new load() calls with TOO_MANY_DATABASES when the limit is
reached.

Convert DbInstances from a tuple struct to named fields (inner, max)
and expose Builder::max_databases() for configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pmorris-dev

src/commands.rs

@@ -112,7 +112,7 @@ pub async fn load<R: Runtime>(
    // Wait for migrations to complete if registered for this database
    await_migrations(&migration_states, &db).await?;
 
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    // Return cached if db was already loaded
    if instances.contains_key(&db) {
@@ -121,7 +121,14 @@ pub async fn load<R: Runtime>(
 
    drop(instances); // Release read lock before acquiring write lock
 
-   let mut instances = db_instances.0.write().await;
+   let mut instances = db_instances.inner.write().await;
+
+   // Check database count limit before creating a new connection.
+   // This check is before entry() to avoid borrow conflicts, and the write lock
+   // prevents races between the len() check and the insert.
+   if !instances.contains_key(&db) && instances.len() >= db_instances.max {
+      return Err(Error::TooManyDatabases(db_instances.max));
+   }
 
    // Use entry API to atomically check and insert, avoiding race conditions
    // where two callers could both create wrappers
@@ -187,7 +194,7 @@ pub async fn execute(
    values: Vec<JsonValue>,
    attached: Option<Vec<AttachedDatabaseSpec>>,
 ) -> Result<(u64, i64)> {
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -214,7 +221,7 @@ pub async fn execute_transaction(
    statements: Vec<Statement>,
    attached: Option<Vec<AttachedDatabaseSpec>>,
 ) -> Result<Vec<WriteQueryResult>> {
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -292,7 +299,7 @@ pub async fn fetch_all(
    values: Vec<JsonValue>,
    attached: Option<Vec<AttachedDatabaseSpec>>,
 ) -> Result<Vec<IndexMap<String, JsonValue>>> {
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -319,7 +326,7 @@ pub async fn fetch_one(
    values: Vec<JsonValue>,
    attached: Option<Vec<AttachedDatabaseSpec>>,
 ) -> Result<Option<IndexMap<String, JsonValue>>> {
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -357,7 +364,7 @@ pub async fn fetch_page(
       ));
    }
 
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -394,7 +401,7 @@ pub async fn close(
 ) -> Result<bool> {
    active_subs.remove_for_db(&db).await;
 
-   let mut instances = db_instances.0.write().await;
+   let mut instances = db_instances.inner.write().await;
 
    if let Some(wrapper) = instances.remove(&db) {
       wrapper.close().await?;
@@ -415,7 +422,7 @@ pub async fn close_all(
 ) -> Result<()> {
    active_subs.abort_all().await;
 
-   let mut instances = db_instances.0.write().await;
+   let mut instances = db_instances.inner.write().await;
 
    // Collect all wrappers to close
    let wrappers: Vec<DatabaseWrapper> = instances.drain().map(|(_, v)| v).collect();
@@ -447,7 +454,7 @@ pub async fn remove(
 ) -> Result<bool> {
    active_subs.remove_for_db(&db).await;
 
-   let mut instances = db_instances.0.write().await;
+   let mut instances = db_instances.inner.write().await;
 
    if let Some(wrapper) = instances.remove(&db) {
       wrapper.remove().await?;
@@ -489,7 +496,7 @@ pub async fn begin_interruptible_transaction(
    initial_statements: Vec<Statement>,
    attached: Option<Vec<AttachedDatabaseSpec>>,
 ) -> Result<TransactionToken> {
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -641,7 +648,7 @@ pub async fn observe(
    // enable_observation() drops the old broker
    active_subs.remove_for_db(&db).await;
 
-   let mut instances = db_instances.0.write().await;
+   let mut instances = db_instances.inner.write().await;
 
    let wrapper = instances
       .get_mut(&db)
@@ -683,7 +690,7 @@ pub async fn subscribe(
    tables: Vec<String>,
    on_event: Channel<TableChangePayload>,
 ) -> Result<String> {
-   let instances = db_instances.0.read().await;
+   let instances = db_instances.inner.read().await;
 
    let wrapper = instances
       .get(&db)
@@ -747,7 +754,7 @@ pub async fn unobserve(
    // Abort all subscriptions for this database first
    active_subs.remove_for_db(&db).await;
 
-   let mut instances = db_instances.0.write().await;
+   let mut instances = db_instances.inner.write().await;
 
    let wrapper = instances
       .get_mut(&db)

src/error.rs

@@ -39,6 +39,10 @@ pub enum Error {
    #[error("observation not enabled for database: {0}")]
    ObservationNotEnabled(String),
 
+   /// Too many databases loaded simultaneously.
+   #[error("cannot load more than {0} databases")]
+   TooManyDatabases(usize),
+
    /// Invalid configuration parameter.
    #[error("invalid configuration: {0}")]
    InvalidConfig(String),
@@ -78,6 +82,7 @@ impl Error {
          Error::PathTraversal(_) => "PATH_TRAVERSAL".to_string(),
          Error::DatabaseNotLoaded(_) => "DATABASE_NOT_LOADED".to_string(),
          Error::ObservationNotEnabled(_) => "OBSERVATION_NOT_ENABLED".to_string(),
+         Error::TooManyDatabases(_) => "TOO_MANY_DATABASES".to_string(),
          Error::InvalidConfig(_) => "INVALID_CONFIG".to_string(),
          Error::Other(_) => "ERROR".to_string(),
       }

src/lib.rs

@@ -22,12 +22,38 @@ pub use sqlx_sqlite_toolkit::{
    TransactionExecutionBuilder, WriteQueryResult,
 };
 
+/// Default maximum number of concurrently loaded databases.
+const DEFAULT_MAX_DATABASES: usize = 50;
+
 /// Database instances managed by the plugin.
 ///
 /// This struct maintains a thread-safe map of database paths to their corresponding
-/// connection wrappers.
-#[derive(Clone, Default)]
-pub struct DbInstances(pub Arc<RwLock<HashMap<String, DatabaseWrapper>>>);
+/// connection wrappers, with a configurable upper limit on how many databases can be
+/// loaded simultaneously.
+#[derive(Clone)]
+pub struct DbInstances {
+   pub(crate) inner: Arc<RwLock<HashMap<String, DatabaseWrapper>>>,
+   pub(crate) max: usize,
+}
+
+impl Default for DbInstances {
+   fn default() -> Self {
+      Self {
+         inner: Arc::new(RwLock::new(HashMap::new())),
+         max: DEFAULT_MAX_DATABASES,
+      }
+   }
+}
+
+impl DbInstances {
+   /// Create a new instance with the given maximum database count.
+   pub fn new(max: usize) -> Self {
+      Self {
+         inner: Arc::new(RwLock::new(HashMap::new())),
+         max,
+      }
+   }
+}
 
 /// Migration status for a database.
 #[derive(Debug, Clone)]
@@ -136,6 +162,8 @@ pub struct Builder {
    migrations: HashMap<String, Arc<Migrator>>,
    /// Timeout for interruptible transactions. Defaults to 5 minutes.
    transaction_timeout: Option<std::time::Duration>,
+   /// Maximum number of concurrently loaded databases. Defaults to 50.
+   max_databases: Option<usize>,
 }
 
 impl Builder {
@@ -144,6 +172,7 @@ impl Builder {
       Self {
          migrations: HashMap::new(),
          transaction_timeout: None,
+         max_databases: None,
       }
    }
 
@@ -182,10 +211,20 @@ impl Builder {
       self
    }
 
+   /// Set the maximum number of databases that can be loaded simultaneously.
+   ///
+   /// Prevents unbounded memory growth from connection pool proliferation.
+   /// Defaults to 50.
+   pub fn max_databases(mut self, max: usize) -> Self {
+      self.max_databases = Some(max);
+      self
+   }
+
    /// Build the plugin with command registration and state management.
    pub fn build<R: Runtime>(self) -> tauri::plugin::TauriPlugin<R> {
       let migrations = Arc::new(self.migrations);
       let transaction_timeout = self.transaction_timeout;
+      let max_databases = self.max_databases;
 
       PluginBuilder::<R>::new("sqlite")
          .invoke_handler(tauri::generate_handler![
@@ -208,7 +247,10 @@ impl Builder {
             commands::unobserve,
          ])
          .setup(move |app, _api| {
-            app.manage(DbInstances::default());
+            app.manage(match max_databases {
+               Some(max) => DbInstances::new(max),
+               None => DbInstances::default(),
+            });
             app.manage(MigrationStates::default());
             app.manage(match transaction_timeout {
                Some(timeout) => ActiveInterruptibleTransactions::new(timeout),
@@ -286,7 +328,7 @@ impl Builder {
 
                         // Close databases (each wrapper's close() disables its own
                         // observer at the crate level, unregistering SQLite hooks)
-                        let mut guard = instances_clone.0.write().await;
+                        let mut guard = instances_clone.inner.write().await;
                         let wrappers: Vec<DatabaseWrapper> =
                            guard.drain().map(|(_, v)| v).collect();
 
@@ -329,7 +371,7 @@ impl Builder {
                   // ExitRequested should have already closed all databases
                   // This is just a safety check
                   let instances = app.state::<DbInstances>();
-                  match instances.0.try_read() {
+                  match instances.inner.try_read() {
                      Ok(guard) => {
                         if !guard.is_empty() {
                            warn!(