RUST-1585 Do not perform server selection to determine sessions suppo…

…rt (#854)

src/client/csfle/state_machine.rs

@@ -19,7 +19,7 @@ use crate::{
     error::{Error, Result},
     operation::{RawOutput, RunCommand},
     options::ReadConcern,
-    runtime::{AsyncStream, Process, TlsConfig},
+    runtime::{process::Process, AsyncStream, TlsConfig},
     Client,
     Namespace,
 };

src/client/executor.rs

@@ -51,14 +51,7 @@ use crate::{
         Retryability,
     },
     options::{ChangeStreamOptions, SelectionCriteria},
-    sdam::{
-        HandshakePhase,
-        SelectedServer,
-        ServerType,
-        SessionSupportStatus,
-        TopologyType,
-        TransactionSupportStatus,
-    },
+    sdam::{HandshakePhase, SelectedServer, ServerType, TopologyType, TransactionSupportStatus},
     selection_criteria::ReadPreference,
     ClusterTime,
 };
@@ -349,8 +342,16 @@ impl Client {
                 }
             };
 
-            if session.is_none() {
-                implicit_session = self.start_implicit_session(&op).await?;
+            if !conn.supports_sessions() && session.is_some() {
+                return Err(ErrorKind::SessionsNotSupported.into());
+            }
+
+            if conn.supports_sessions()
+                && session.is_none()
+                && op.supports_sessions()
+                && op.is_acknowledged()
+            {
+                implicit_session = Some(ClientSession::new(self.clone(), None, true).await);
                 session = implicit_session.as_mut();
             }
 
@@ -790,19 +791,6 @@ impl Client {
         })
     }
 
-    /// Start an implicit session if the operation and write concern are compatible with sessions.
-    async fn start_implicit_session<T: Operation>(&self, op: &T) -> Result<Option<ClientSession>> {
-        match self.get_session_support_status().await? {
-            SessionSupportStatus::Supported {
-                logical_session_timeout,
-            } if op.supports_sessions() && op.is_acknowledged() => Ok(Some(
-                self.start_session_with_timeout(logical_session_timeout, None, true)
-                    .await,
-            )),
-            _ => Ok(None),
-        }
-    }
-
     async fn select_data_bearing_server(&self, operation_name: &str) -> Result<()> {
         let topology_type = self.inner.topology.topology_type();
         let criteria = SelectionCriteria::Predicate(Arc::new(move |server_info| {
@@ -814,24 +802,6 @@ impl Client {
         Ok(())
     }
 
-    /// Gets whether the topology supports sessions, and if so, returns the topology's logical
-    /// session timeout. If it has yet to be determined if the topology supports sessions, this
-    /// method will perform a server selection that will force that determination to be made.
-    pub(crate) async fn get_session_support_status(&self) -> Result<SessionSupportStatus> {
-        let initial_status = self.inner.topology.session_support_status();
-
-        // Need to guarantee that we're connected to at least one server that can determine if
-        // sessions are supported or not.
-        match initial_status {
-            SessionSupportStatus::Undetermined => {
-                self.select_data_bearing_server(crate::client::SESSIONS_SUPPORT_OP_NAME)
-                    .await?;
-                Ok(self.inner.topology.session_support_status())
-            }
-            _ => Ok(initial_status),
-        }
-    }
-
     /// Gets whether the topology supports transactions. If it has yet to be determined if the
     /// topology supports transactions, this method will perform a server selection that will force
     /// that determination to be made.

src/client/mod.rs

@@ -46,7 +46,7 @@ use crate::{
         SessionOptions,
     },
     results::DatabaseSpecification,
-    sdam::{server_selection, SelectedServer, SessionSupportStatus, Topology},
+    sdam::{server_selection, SelectedServer, Topology},
     ClientSession,
 };
 
@@ -56,8 +56,6 @@ pub(crate) use session::{ClusterTime, SESSIONS_UNSUPPORTED_COMMANDS};
 use session::{ServerSession, ServerSessionPool};
 
 const DEFAULT_SERVER_SELECTION_TIMEOUT: Duration = Duration::from_secs(30);
-// TODO: RUST-1585 Remove this constant.
-pub(crate) const SESSIONS_SUPPORT_OP_NAME: &str = "Check sessions support status";
 
 /// This is the main entry point for the API. A `Client` is used to connect to a MongoDB cluster.
 /// By default, it will monitor the topology of the cluster, keeping track of any changes, such
@@ -367,14 +365,7 @@ impl Client {
         if let Some(ref options) = options {
             options.validate()?;
         }
-        match self.get_session_support_status().await? {
-            SessionSupportStatus::Supported {
-                logical_session_timeout,
-            } => Ok(self
-                .start_session_with_timeout(logical_session_timeout, options, false)
-                .await),
-            _ => Err(ErrorKind::SessionsNotSupported.into()),
-        }
+        Ok(ClientSession::new(self.clone(), options, false).await)
     }
 
     /// Starts a new [`ChangeStream`] that receives events for all changes in the cluster. The
@@ -428,42 +419,11 @@ impl Client {
             .await
     }
 
-    /// Check in a server session to the server session pool.
-    /// If the session is expired or dirty, or the topology no longer supports sessions, the session
-    /// will be discarded.
+    /// Check in a server session to the server session pool. The session will be discarded if it is
+    /// expired or dirty.
     pub(crate) async fn check_in_server_session(&self, session: ServerSession) {
-        let session_support_status = self.inner.topology.session_support_status();
-        if let SessionSupportStatus::Supported {
-            logical_session_timeout,
-        } = session_support_status
-        {
-            self.inner
-                .session_pool
-                .check_in(session, logical_session_timeout)
-                .await;
-        }
-    }
-
-    /// Starts a `ClientSession`.
-    ///
-    /// This method will attempt to re-use server sessions from the pool which are not about to
-    /// expire according to the provided logical session timeout. If no such sessions are
-    /// available, a new one will be created.
-    pub(crate) async fn start_session_with_timeout(
-        &self,
-        logical_session_timeout: Option<Duration>,
-        options: Option<SessionOptions>,
-        is_implicit: bool,
-    ) -> ClientSession {
-        ClientSession::new(
-            self.inner
-                .session_pool
-                .check_out(logical_session_timeout)
-                .await,
-            self.clone(),
-            options,
-            is_implicit,
-        )
+        let timeout = self.inner.topology.logical_session_timeout();
+        self.inner.session_pool.check_in(session, timeout).await;
     }
 
     #[cfg(test)]
@@ -601,7 +561,7 @@ impl Client {
     }
 
     #[cfg(test)]
-    pub(crate) fn topology(&self) -> &crate::sdam::Topology {
+    pub(crate) fn topology(&self) -> &Topology {
         &self.inner.topology
     }
 

src/client/session/mod.rs

@@ -202,13 +202,15 @@ pub(crate) enum TransactionPin {
 }
 
 impl ClientSession {
-    /// Creates a new `ClientSession` wrapping the provided server session.
-    pub(crate) fn new(
-        server_session: ServerSession,
+    /// Creates a new `ClientSession` by checking out a corresponding `ServerSession` from the
+    /// provided client's session pool.
+    pub(crate) async fn new(
         client: Client,
         options: Option<SessionOptions>,
         is_implicit: bool,
     ) -> Self {
+        let timeout = client.inner.topology.logical_session_timeout();
+        let server_session = client.inner.session_pool.check_out(timeout).await;
         Self {
             client,
             server_session,

src/client/session/test/mod.rs

@@ -215,6 +215,8 @@ async fn pool_is_lifo() {
     let _guard: RwLockReadGuard<()> = LOCK.run_concurrently().await;
 
     let client = TestClient::new().await;
+    // Wait for the implicit sessions created in TestClient::new to be returned to the pool.
+    runtime::delay_for(Duration::from_millis(500)).await;
 
     if client.is_standalone() {
         return;

src/cmap/conn/mod.rs

@@ -431,6 +431,14 @@ impl Connection {
     pub(crate) fn is_streaming(&self) -> bool {
         self.more_to_come
     }
+
+    /// Whether the connection supports sessions.
+    pub(crate) fn supports_sessions(&self) -> bool {
+        self.stream_description
+            .as_ref()
+            .and_then(|sd| sd.logical_session_timeout)
+            .is_some()
+    }
 }
 
 impl Drop for Connection {

src/runtime/mod.rs

@@ -3,8 +3,11 @@ mod http;
 #[cfg(feature = "async-std-runtime")]
 mod interval;
 mod join_handle;
-#[cfg(feature = "in-use-encryption-unstable")]
-mod process;
+#[cfg(any(
+    feature = "in-use-encryption-unstable",
+    all(test, not(feature = "sync"), not(feature = "tokio-sync"))
+))]
+pub(crate) mod process;
 mod resolver;
 pub(crate) mod stream;
 mod sync_read_ext;
@@ -16,8 +19,6 @@ mod worker_handle;
 
 use std::{future::Future, net::SocketAddr, time::Duration};
 
-#[cfg(feature = "in-use-encryption-unstable")]
-pub(crate) use self::process::Process;
 pub(crate) use self::{
     acknowledged_message::AcknowledgedMessage,
     join_handle::AsyncJoinHandle,