Try #1791:

io-engine/src/persistent_store.rs

@@ -9,9 +9,12 @@ use crate::{
     core::Reactor,
     store::{
         etcd::Etcd,
-        store_defs::{DeleteWait, GetWait, PutWait, Store, StoreError, StoreKey, StoreValue},
+        store_defs::{
+            DeleteWait, GetWait, PutWait, Store, StoreError, StoreKey, StoreValue, TxnWait,
+        },
     },
 };
+use etcd_client::{Compare, TxnOp, TxnResponse};
 use futures::channel::oneshot;
 use once_cell::sync::OnceCell;
 use parking_lot::Mutex;
@@ -193,6 +196,25 @@ impl PersistentStore {
         })?
     }
 
+    pub async fn txn(
+        key: &impl StoreKey,
+        cmps: Vec<Compare>,
+        ops_success: Vec<TxnOp>,
+        ops_failure: Option<Vec<TxnOp>>,
+    ) -> Result<TxnResponse, StoreError> {
+        let key_string = key.to_string();
+        let rx = Self::execute_store_op(async move {
+            info!("Executing transaction for key {}.", key_string);
+            Self::backing_store()
+                .txn_kv(&key_string, cmps, ops_success, ops_failure)
+                .await
+        });
+
+        rx.await.context(TxnWait {
+            key: key.to_string(),
+        })?
+    }
+
     /// Retrieves a value, with the given key, from the store.
     pub async fn get(key: &impl StoreKey) -> Result<Value, StoreError> {
         let key_string = key.to_string();

io-engine/src/store/etcd.rs

@@ -2,10 +2,10 @@
 
 use crate::store::store_defs::{
     Connect, Delete, DeserialiseValue, Get, Put, SerialiseValue, Store, StoreError,
-    StoreError::MissingEntry, StoreKey, StoreValue, ValueString,
+    StoreError::MissingEntry, StoreKey, StoreValue, Txn as TxnErr, ValueString,
 };
 use async_trait::async_trait;
-use etcd_client::Client;
+use etcd_client::{Client, Compare, Txn, TxnOp, TxnResponse};
 use serde_json::Value;
 use snafu::ResultExt;
 
@@ -49,6 +49,22 @@ impl Store for Etcd {
         Ok(())
     }
 
+    async fn txn_kv<K: StoreKey>(
+        &mut self,
+        key: &K,
+        cmps: Vec<Compare>,
+        ops_success: Vec<TxnOp>,
+        ops_failure: Option<Vec<TxnOp>>,
+    ) -> Result<TxnResponse, StoreError> {
+        let fops = ops_failure.map_or(vec![], |v| v);
+        self.0
+            .txn(Txn::new().when(cmps).and_then(ops_success).or_else(fops))
+            .await
+            .context(TxnErr {
+                key: key.to_string(),
+            })
+    }
+
     /// 'Get' the value for the given key from etcd.
     async fn get_kv<K: StoreKey>(&mut self, key: &K) -> Result<Value, StoreError> {
         let resp = self.0.get(key.to_string(), None).await.context(Get {

io-engine/src/store/store_defs.rs

@@ -1,7 +1,7 @@
 //! Definition of a trait for a key-value store together with its error codes.
 
 use async_trait::async_trait;
-use etcd_client::Error;
+use etcd_client::{Compare, Error, TxnOp, TxnResponse};
 use serde_json::{Error as SerdeError, Value};
 use snafu::Snafu;
 
@@ -56,6 +56,18 @@ pub enum StoreError {
         key: String,
         source: futures::channel::oneshot::Canceled,
     },
+    /// Failed to complete a 'transaction'.
+    #[snafu(display("Failed to do 'transaction' for key {}. Error {}", key, source))]
+    Txn { key: String, source: Error },
+    /// Failed to wait for 'transaction' operations.
+    #[snafu(display(
+        "Failed to wait for 'transaction' operations to complete for key {}.",
+        key,
+    ))]
+    TxnWait {
+        key: String,
+        source: futures::channel::oneshot::Canceled,
+    },
     /// Failed to 'watch' an entry in the store.
     #[snafu(display("Failed to 'watch' entry with key {}. Error {}", key, source))]
     Watch { key: String, source: Error },
@@ -93,6 +105,14 @@ pub trait Store: Sync + Send + Clone {
         value: &V,
     ) -> Result<(), StoreError>;
 
+    async fn txn_kv<K: StoreKey>(
+        &mut self,
+        key: &K,
+        cmps: Vec<Compare>,
+        ops_success: Vec<TxnOp>,
+        ops_failure: Option<Vec<TxnOp>>,
+    ) -> Result<TxnResponse, StoreError>;
+
     /// Get an entry from the store.
     async fn get_kv<K: StoreKey>(&mut self, key: &K) -> Result<Value, StoreError>;
 

io-engine/tests/persistence.rs

@@ -8,17 +8,22 @@ use common::compose::{
         },
         GrpcConnect, RpcHandle,
     },
-    Binary, Builder, ComposeTest, ContainerSpec,
+    Binary, Builder, ComposeTest, ContainerSpec, MayastorTest,
 };
-use etcd_client::Client;
-
-use io_engine::bdev::nexus::{ChildInfo, NexusInfo};
+use etcd_client::{Client, Compare, CompareOp, TxnOp};
 
+use io_engine::{
+    bdev::nexus::{ChildInfo, NexusInfo},
+    core::MayastorCliArgs,
+    persistent_store::{PersistentStore, PersistentStoreBuilder},
+};
 use std::{convert::TryFrom, thread::sleep, time::Duration};
 use url::Url;
 
 pub mod common;
+use once_cell::sync::OnceCell;
 
+static MAYASTOR: OnceCell<MayastorTest> = OnceCell::new();
 static ETCD_ENDPOINT: &str = "0.0.0.0:2379";
 static CHILD1_UUID: &str = "d61b2fdf-1be8-457a-a481-70a42d0a2223";
 static CHILD2_UUID: &str = "094ae8c6-46aa-4139-b4f2-550d39645db3";
@@ -519,3 +524,83 @@ pub(crate) fn uuid(uri: &str) -> String {
     }
     panic!("URI does not contain a uuid query parameter.");
 }
+
+// This test does a successful etcd transaction that upon successful CompareOp, modifies
+// an existing key and adds a new key. Then does another transaction which is
+// fails the CompareOp and hence that transaction is expected to fail.
+#[tokio::test]
+async fn pstor_txn_api() {
+    let dummy_key1 = "dummy_key_1".to_string();
+    let dummy_key2 = "dummy_key_2".to_string();
+    let pre_txn_value_k1 = "pre_txn_value_key1".to_string();
+    let post_txn_value_k1 = "post_txn_value_key1".to_string();
+    let post_txn_value_k2 = "post_txn_value_key2".to_string();
+    common::composer_init();
+
+    let _test = Builder::new()
+        .name("etcd_txn_test")
+        .add_container_spec(
+            ContainerSpec::from_binary(
+                "etcd",
+                Binary::from_path(env!("ETCD_BIN")).with_args(vec![
+                    "--data-dir",
+                    "/tmp/etcd-data",
+                    "--advertise-client-urls",
+                    "http://0.0.0.0:2379",
+                    "--listen-client-urls",
+                    "http://0.0.0.0:2379",
+                ]),
+            )
+            .with_portmap("2379", "2379")
+            .with_portmap("2380", "2380"),
+        )
+        .build()
+        .await
+        .unwrap();
+
+    PersistentStoreBuilder::new()
+        .with_endpoint(ETCD_ENDPOINT)
+        .connect()
+        .await;
+
+    // create the mayastor test instance
+    let ms = MayastorTest::new(MayastorCliArgs {
+        log_components: vec!["all".into()],
+        reactor_mask: "0x3".to_string(),
+        no_pci: true,
+        ..Default::default()
+    });
+
+    let _ms = MAYASTOR.get_or_init(|| ms);
+    MAYASTOR
+        .get()
+        .unwrap()
+        .spawn(async move {
+            let _ = PersistentStore::put(&dummy_key1, &pre_txn_value_k1).await;
+            let expect = serde_json::to_vec(&pre_txn_value_k1).unwrap();
+
+            // Transaction - expected to succeed.
+            let cmp = vec![Compare::value(
+                dummy_key1.to_string(),
+                CompareOp::Equal,
+                expect,
+            )];
+            let ops = vec![
+                TxnOp::put(dummy_key1.to_string(), post_txn_value_k1, None),
+                TxnOp::put(dummy_key2.to_string(), post_txn_value_k2, None),
+            ];
+            let txn_resp = PersistentStore::txn(&dummy_key1, cmp.clone(), ops, None)
+                .await
+                .unwrap();
+            assert!(txn_resp.succeeded());
+
+            // A new transaction - expected to fail. Execute fops upon compare failure.
+            let ops = vec![TxnOp::delete(dummy_key1.to_string(), None)];
+            let fops = vec![TxnOp::delete(dummy_key2.to_string(), None)];
+            let txn_resp = PersistentStore::txn(&dummy_key1, cmp, ops, Some(fops))
+                .await
+                .unwrap();
+            assert!(!txn_resp.succeeded());
+        })
+        .await;
+}