Re-add savepoint method to Transaction

Revives #184.

The rewrite for async/await and Tokio accidentally lost functionality
that allowed users to assign specific names to savepoints when using
nested transactions. This functionality had originally been added
in #184 and had been updated in #374.

This commit revives this functionality using a similar scheme to the
one that existed before. This should allow CockroachDB users to update
to the next patch release of version `0.17`.

postgres/src/test.rs

@@ -151,6 +151,57 @@ fn nested_transactions() {
     assert_eq!(rows[2].get::<_, i32>(0), 4);
 }
 
+#[test]
+fn savepoints() {
+    let mut client = Client::connect("host=localhost port=5433 user=postgres", NoTls).unwrap();
+
+    client
+        .batch_execute("CREATE TEMPORARY TABLE foo (id INT PRIMARY KEY)")
+        .unwrap();
+
+    let mut transaction = client.transaction().unwrap();
+
+    transaction
+        .execute("INSERT INTO foo (id) VALUES (1)", &[])
+        .unwrap();
+
+    let mut savepoint1 = transaction.savepoint("savepoint1").unwrap();
+
+    savepoint1
+        .execute("INSERT INTO foo (id) VALUES (2)", &[])
+        .unwrap();
+
+    savepoint1.rollback().unwrap();
+
+    let rows = transaction
+        .query("SELECT id FROM foo ORDER BY id", &[])
+        .unwrap();
+    assert_eq!(rows.len(), 1);
+    assert_eq!(rows[0].get::<_, i32>(0), 1);
+
+    let mut savepoint2 = transaction.savepoint("savepoint2").unwrap();
+
+    savepoint2
+        .execute("INSERT INTO foo (id) VALUES(3)", &[])
+        .unwrap();
+
+    let mut savepoint3 = savepoint2.savepoint("savepoint3").unwrap();
+
+    savepoint3
+        .execute("INSERT INTO foo (id) VALUES(4)", &[])
+        .unwrap();
+
+    savepoint3.commit().unwrap();
+    savepoint2.commit().unwrap();
+    transaction.commit().unwrap();
+
+    let rows = client.query("SELECT id FROM foo ORDER BY id", &[]).unwrap();
+    assert_eq!(rows.len(), 3);
+    assert_eq!(rows[0].get::<_, i32>(0), 1);
+    assert_eq!(rows[1].get::<_, i32>(0), 3);
+    assert_eq!(rows[2].get::<_, i32>(0), 4);
+}
+
 #[test]
 fn copy_in() {
     let mut client = Client::connect("host=localhost port=5433 user=postgres", NoTls).unwrap();

postgres/src/transaction.rs

@@ -173,7 +173,7 @@ impl<'a> Transaction<'a> {
         CancelToken::new(self.transaction.cancel_token())
     }
 
-    /// Like `Client::transaction`.
+    /// Like `Client::transaction`, but creates a nested transaction via a savepoint.
     pub fn transaction(&mut self) -> Result<Transaction<'_>, Error> {
         let transaction = self.connection.block_on(self.transaction.transaction())?;
         Ok(Transaction {
@@ -181,4 +181,15 @@ impl<'a> Transaction<'a> {
             transaction,
         })
     }
+    /// Like `Client::transaction`, but creates a nested transaction via a savepoint with the specified name.
+    pub fn savepoint<I>(&mut self, name: I) -> Result<Transaction<'_>, Error>
+    where
+        I: Into<String>,
+    {
+        let transaction = self.connection.block_on(self.transaction.savepoint(name))?;
+        Ok(Transaction {
+            connection: self.connection.as_ref(),
+            transaction,
+        })
+    }
 }