Allow TaskExecutor to be used in async tests (#3178)

# Description

Since the `TaskExecutor` currently requires a `Weak<Runtime>`, it's impossible to use it in an async test where the `Runtime` is created outside our scope. Whilst we *could* create a new `Runtime` instance inside the async test, dropping that `Runtime` would cause a panic (you can't drop a `Runtime` in an async context).

To address this issue, this PR creates the `enum Handle`, which supports either:

- A `Weak<Runtime>` (for use in our production code)
- A `Handle` to a runtime (for use in testing)

In theory, there should be no change to the behaviour of our production code (beyond some slightly different descriptions in HTTP 500 errors), or even our tests. If there is no change, you might ask *"why bother?"*. There are two PRs (#3070 and #3175) that are waiting on these fixes to introduce some new tests. Since we've added the EL to the `BeaconChain` (for the merge), we are now doing more async stuff in tests.

I've also added a `RuntimeExecutor` to the `BeaconChainTestHarness`. Whilst that's not immediately useful, it will become useful in the near future with all the new async testing.

validator_client/src/http_api/keystores.rs

@@ -14,8 +14,8 @@ use slog::{info, warn, Logger};
 use slot_clock::SlotClock;
 use std::path::PathBuf;
 use std::sync::Arc;
-use std::sync::Weak;
-use tokio::runtime::Runtime;
+use task_executor::TaskExecutor;
+use tokio::runtime::Handle;
 use types::{EthSpec, PublicKeyBytes};
 use validator_dir::Builder as ValidatorDirBuilder;
 use warp::Rejection;
@@ -59,7 +59,7 @@ pub fn import<T: SlotClock + 'static, E: EthSpec>(
     request: ImportKeystoresRequest,
     validator_dir: PathBuf,
     validator_store: Arc<ValidatorStore<T, E>>,
-    runtime: Weak<Runtime>,
+    task_executor: TaskExecutor,
     log: Logger,
 ) -> Result<ImportKeystoresResponse, Rejection> {
     // Check request validity. This is the only cases in which we should return a 4xx code.
@@ -122,14 +122,14 @@ pub fn import<T: SlotClock + 'static, E: EthSpec>(
                 ImportKeystoreStatus::Error,
                 format!("slashing protection import failed: {:?}", e),
             )
-        } else if let Some(runtime) = runtime.upgrade() {
+        } else if let Some(handle) = task_executor.handle() {
             // Import the keystore.
             match import_single_keystore(
                 keystore,
                 password,
                 validator_dir.clone(),
                 &validator_store,
-                runtime,
+                handle,
             ) {
                 Ok(status) => Status::ok(status),
                 Err(e) => {
@@ -159,7 +159,7 @@ fn import_single_keystore<T: SlotClock + 'static, E: EthSpec>(
     password: ZeroizeString,
     validator_dir_path: PathBuf,
     validator_store: &ValidatorStore<T, E>,
-    runtime: Arc<Runtime>,
+    handle: Handle,
 ) -> Result<ImportKeystoreStatus, String> {
     // Check if the validator key already exists, erroring if it is a remote signer validator.
     let pubkey = keystore
@@ -198,7 +198,7 @@ fn import_single_keystore<T: SlotClock + 'static, E: EthSpec>(
     let voting_keystore_path = validator_dir.voting_keystore_path();
     drop(validator_dir);
 
-    runtime
+    handle
         .block_on(validator_store.add_validator_keystore(
             voting_keystore_path,
             password,
@@ -214,7 +214,7 @@ fn import_single_keystore<T: SlotClock + 'static, E: EthSpec>(
 pub fn delete<T: SlotClock + 'static, E: EthSpec>(
     request: DeleteKeystoresRequest,
     validator_store: Arc<ValidatorStore<T, E>>,
-    runtime: Weak<Runtime>,
+    task_executor: TaskExecutor,
     log: Logger,
 ) -> Result<DeleteKeystoresResponse, Rejection> {
     // Remove from initialized validators.
@@ -225,8 +225,11 @@ pub fn delete<T: SlotClock + 'static, E: EthSpec>(
         .pubkeys
         .iter()
         .map(|pubkey_bytes| {
-            match delete_single_keystore(pubkey_bytes, &mut initialized_validators, runtime.clone())
-            {
+            match delete_single_keystore(
+                pubkey_bytes,
+                &mut initialized_validators,
+                task_executor.clone(),
+            ) {
                 Ok(status) => Status::ok(status),
                 Err(error) => {
                     warn!(
@@ -244,8 +247,8 @@ pub fn delete<T: SlotClock + 'static, E: EthSpec>(
     // Use `update_validators` to update the key cache. It is safe to let the key cache get a bit out
     // of date as it resets when it can't be decrypted. We update it just a single time to avoid
     // continually resetting it after each key deletion.
-    if let Some(runtime) = runtime.upgrade() {
-        runtime
+    if let Some(handle) = task_executor.handle() {
+        handle
             .block_on(initialized_validators.update_validators())
             .map_err(|e| custom_server_error(format!("unable to update key cache: {:?}", e)))?;
     }
@@ -278,14 +281,14 @@ pub fn delete<T: SlotClock + 'static, E: EthSpec>(
 fn delete_single_keystore(
     pubkey_bytes: &PublicKeyBytes,
     initialized_validators: &mut InitializedValidators,
-    runtime: Weak<Runtime>,
+    task_executor: TaskExecutor,
 ) -> Result<DeleteKeystoreStatus, String> {
-    if let Some(runtime) = runtime.upgrade() {
+    if let Some(handle) = task_executor.handle() {
         let pubkey = pubkey_bytes
             .decompress()
             .map_err(|e| format!("invalid pubkey, {:?}: {:?}", pubkey_bytes, e))?;
 
-        match runtime.block_on(initialized_validators.delete_definition_and_keystore(&pubkey, true))
+        match handle.block_on(initialized_validators.delete_definition_and_keystore(&pubkey, true))
         {
             Ok(_) => Ok(DeleteKeystoreStatus::Deleted),
             Err(e) => match e {