feat: connect to the most recently successfully used SMTP transport first

docs/schema.sql

@@ -456,6 +456,21 @@ CREATE TABLE smtp_status_updates (
     descr TEXT NOT NULL -- text to send along with the updates
 );
 
+-- Table to record the successful usage transports for sending.
+-- Sorting the table by rowid in descending order
+-- returns most recently successfully used transport first.
+CREATE TABLE smtp_success (
+    -- Sequentially increasing ID of the success.
+    -- Transport with the highest ID is to be used first.
+    id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
+
+    -- ID of the transport that was used to send a message.
+    transport_id INTEGER UNIQUE NOT NULL,
+
+    -- Delete `smtp_success` rows when the transport is deleted.
+    FOREIGN KEY(transport_id) REFERENCES transports(id) ON DELETE CASCADE
+) STRICT;
+
 -- Table of "sync items" to be grouped into sync messages
 -- and sent to own devices.
 CREATE TABLE multi_device_sync (

src/smtp.rs

@@ -54,6 +54,39 @@ pub(crate) struct Smtp {
     pub(crate) last_send_error: Option<String>,
 }
 
+/// Returns transports with their IDs in the order in which they should be tried.
+async fn sorted_transports(context: &Context) -> Result<Vec<(u32, ConfiguredLoginParam)>> {
+    context
+        .sql
+        .query_map_vec(
+            "SELECT transports.id, configured_param FROM transports
+             LEFT JOIN smtp_success ON smtp_success.transport_id=transports.id
+             ORDER BY IFNULL(smtp_success.id, 0) DESC, transports.id ASC",
+            (),
+            |row| {
+                let id: u32 = row.get(0)?;
+                let json: String = row.get(1)?;
+                let param = ConfiguredLoginParam::from_json(&json)?;
+                Ok((id, param))
+            },
+        )
+        .await
+}
+
+/// Records successful use of SMTP transport so it is tried first next time we connect to SMTP.
+async fn record_success(context: &Context, transport_id: u32) -> Result<()> {
+    // INSERT OR REPLACE essentially replaces rowid of the row
+    // if the row exists already, so it becomes the highest rowid in the table.
+    context
+        .sql
+        .execute(
+            "INSERT OR REPLACE INTO smtp_success (transport_id) VALUES (?)",
+            (transport_id,),
+        )
+        .await?;
+    Ok(())
+}
+
 impl Smtp {
     /// Create a new Smtp instances.
     pub fn new() -> Self {
@@ -101,13 +134,7 @@ impl Smtp {
 
         self.connectivity.set_connecting(context);
         let proxy_config = ProxyConfig::load(context).await?;
-        let transports = ConfiguredLoginParam::load_all(context).await?;
-
-        // Try to connect to the newest transport first. If sending is unreliable,
-        // user can configure a new transport and it will be the one used.
-        // Conversely, if user just added a new transport and sending got less reliable,
-        // user can restore old state by removing the just added transport.
-        for (transport_id, lp) in transports.into_iter().rev() {
+        for (transport_id, lp) in sorted_transports(context).await? {
             info!(context, "Trying to connect to transport {transport_id}.");
             match self
                 .connect(
@@ -327,6 +354,18 @@ pub(crate) async fn smtp_send(
         Ok(()) => SendResult::Success,
     };
 
+    if matches!(status, SendResult::Success) {
+        debug_assert!(smtp.transport_id.is_some());
+        if let Some(transport_id) = smtp.transport_id
+            && let Err(err) = record_success(context, transport_id).await
+        {
+            warn!(
+                context,
+                "Failed to record successful use of transport {transport_id} in smtp_success table: {err:#}."
+            );
+        }
+    }
+
     if let SendResult::Failure(err) = &status
         && let Some(msg_id) = msg_id
     {
@@ -858,3 +897,6 @@ pub(crate) async fn add_self_recipients(
 
     Ok(())
 }
+
+#[cfg(test)]
+mod smtp_tests;

src/smtp/smtp_tests.rs

@@ -0,0 +1,49 @@
+use anyhow::Result;
+
+use crate::test_utils::TestContextManager;
+use crate::transport;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_smtp_candidates() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let t = &tcm.unconfigured().await;
+
+    transport::add_pseudo_transport(t, "foo@example.net").await?;
+    transport::add_pseudo_transport(t, "bar@example.net").await?;
+    transport::add_pseudo_transport(t, "baz@example.net").await?;
+
+    let transports = super::sorted_transports(t).await?;
+    let [
+        (transport_id1, ref transport1),
+        (transport_id2, ref transport2),
+        (transport_id3, ref transport3),
+    ] = transports[..]
+    else {
+        panic!("Unexpected number of transports");
+    };
+
+    // By default first added transport is used first.
+    assert_eq!(transport1.addr, "foo@example.net");
+    assert_eq!(transport2.addr, "bar@example.net");
+    assert_eq!(transport3.addr, "baz@example.net");
+
+    super::record_success(t, transport_id3).await?;
+    let transports2 = super::sorted_transports(t).await?;
+    assert_eq!(transports2[0].0, transport_id3);
+    assert_eq!(transports2[1].0, transport_id1);
+    assert_eq!(transports2[2].0, transport_id2);
+
+    super::record_success(t, transport_id2).await?;
+    let transports3 = super::sorted_transports(t).await?;
+    assert_eq!(transports3[0].0, transport_id2);
+    assert_eq!(transports3[1].0, transport_id3);
+    assert_eq!(transports3[2].0, transport_id1);
+
+    super::record_success(t, transport_id3).await?;
+    let transports4 = super::sorted_transports(t).await?;
+    assert_eq!(transports4[0].0, transport_id3);
+    assert_eq!(transports4[1].0, transport_id2);
+    assert_eq!(transports4[2].0, transport_id1);
+
+    Ok(())
+}

src/sql/migrations.rs

@@ -2672,6 +2672,21 @@ CREATE TABLE smtp2 (
             .await?;
     }
 
+    inc_and_check(&mut migration_version, 168)?;
+    if dbversion < migration_version {
+        sql.execute_migration(
+            "
+CREATE TABLE smtp_success (
+    id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
+    transport_id INTEGER UNIQUE NOT NULL,
+    FOREIGN KEY(transport_id) REFERENCES transports(id) ON DELETE CASCADE
+) STRICT;
+",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?