fix: Change no pending batches 404 error into a success response (#279)

# What ❔

When no pending batches were found on the server API, the response was a
404 which was not handled on the client (gateway) side and was treated
as an HTTP client error, causing a misleading log line:

```2023-10-19T18:30:46.726413Z ERROR zksync_prover_fri_gateway::api_data_fetcher: HTTP request failed due to error: HTTP status client error (404 Not Found) for url (http://127.0.0.1:3320/proof_generation_data)```

When testing out the gpu proving pipeline for the ZK stack effort, the error was hit soon after starting all services.
This PR changes this so in info trace is printed stating that there are currently no pending batches to be proven.  

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## Why ❔

There is some semantic dissonance on receiving an HTTP client error and a log line when calling the API, since it's possible and reasonable that there are no pending batches at some points.

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## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
- [x] Code has been formatted via `zk fmt` and `zk lint`.

core/lib/types/src/prover_server_api/mod.rs

@@ -19,7 +19,7 @@ pub struct ProofGenerationDataRequest {}
 
 #[derive(Debug, Serialize, Deserialize)]
 pub enum ProofGenerationDataResponse {
-    Success(ProofGenerationData),
+    Success(Option<ProofGenerationData>),
     Error(String),
 }
 

core/lib/zksync_core/src/proof_data_handler/request_processor.rs

@@ -31,18 +31,13 @@ pub(crate) struct RequestProcessor {
 }
 
 pub(crate) enum RequestProcessorError {
-    NoPendingBatches,
     ObjectStore(ObjectStoreError),
     Sqlx(SqlxError),
 }
 
 impl IntoResponse for RequestProcessorError {
     fn into_response(self) -> Response {
         let (status_code, message) = match self {
-            Self::NoPendingBatches => (
-                StatusCode::NOT_FOUND,
-                "No pending batches to process".to_owned(),
-            ),
             RequestProcessorError::ObjectStore(err) => {
                 tracing::error!("GCS error: {:?}", err);
                 (
@@ -88,15 +83,19 @@ impl RequestProcessor {
     ) -> Result<Json<ProofGenerationDataResponse>, RequestProcessorError> {
         tracing::info!("Received request for proof generation data: {:?}", request);
 
-        let l1_batch_number = self
+        let l1_batch_number_result = self
             .pool
             .access_storage()
             .await
             .unwrap()
             .proof_generation_dal()
             .get_next_block_to_be_proven(self.config.proof_generation_timeout())
-            .await
-            .ok_or(RequestProcessorError::NoPendingBatches)?;
+            .await;
+
+        let l1_batch_number = match l1_batch_number_result {
+            Some(number) => number,
+            None => return Ok(Json(ProofGenerationDataResponse::Success(None))), // no batches pending to be proven
+        };
 
         let blob = self
             .blob_store
@@ -125,7 +124,9 @@ impl RequestProcessor {
             l1_verifier_config,
         };
 
-        Ok(Json(ProofGenerationDataResponse::Success(proof_gen_data)))
+        Ok(Json(ProofGenerationDataResponse::Success(Some(
+            proof_gen_data,
+        ))))
     }
 
     pub(crate) async fn submit_proof(

prover/prover_fri_gateway/src/api_data_fetcher.rs

@@ -33,6 +33,7 @@ impl PeriodicApiStruct {
         Resp: DeserializeOwned,
     {
         tracing::info!("Sending request to {}", endpoint);
+
         self.client
             .post(endpoint)
             .json(&request)

prover/prover_fri_gateway/src/proof_gen_data_fetcher.rs

@@ -33,6 +33,7 @@ impl PeriodicApiStruct {
 impl PeriodicApi<ProofGenerationDataRequest> for PeriodicApiStruct {
     type JobId = ();
     type Response = ProofGenerationDataResponse;
+
     const SERVICE_NAME: &'static str = "ProofGenDataFetcher";
 
     async fn get_next_request(&self) -> Option<(Self::JobId, ProofGenerationDataRequest)> {
@@ -49,7 +50,10 @@ impl PeriodicApi<ProofGenerationDataRequest> for PeriodicApiStruct {
 
     async fn handle_response(&self, _: (), response: Self::Response) {
         match response {
-            ProofGenerationDataResponse::Success(data) => {
+            ProofGenerationDataResponse::Success(None) => {
+                tracing::info!("There are currently no pending batches to be proven");
+            }
+            ProofGenerationDataResponse::Success(Some(data)) => {
                 tracing::info!("Received proof gen data for: {:?}", data.l1_batch_number);
                 self.save_proof_gen_data(data).await;
             }