permit at most one document export at a time

1 files changed, 23 insertions, 0 deletions

diff --git a/src/api/document.rs b/src/api/document.rs
index 6c10c38..20bbc7b 100644
--- a/src/api/document.rs
+++ b/src/api/document.rs
@@ -5,6 +5,8 @@
 //! it came from is in headers: `X-Continuum-Swarm`, `X-Continuum-Epoch`
 //! (`<generation>:<id hex>`) and `X-Continuum-Heads` (hex, comma-separated).
 
+use std::sync::atomic::{AtomicBool, Ordering};
+
 use axum::{
     extract::State,
     http::{HeaderName, HeaderValue, StatusCode, header},
@@ -15,10 +17,31 @@ use tracing::{info, warn};
 
 use crate::state::AppState;
 
+/// An export in progress: each holds a copy of the whole document and its saved bytes,
+/// so one at a time.
+static EXPORTING: AtomicBool = AtomicBool::new(false);
+
+/// Clears `EXPORTING` when the export ends, however it ends.
+struct Exporting;
+
+impl Drop for Exporting {
+    fn drop(&mut self) {
+        EXPORTING.store(false, Ordering::Release);
+    }
+}
+
 pub(crate) async fn get_document(State(state): State<AppState>) -> Response {
     if !state.is_ready().await {
         return (StatusCode::SERVICE_UNAVAILABLE, "node is not ready\n").into_response();
     }
+    if EXPORTING.swap(true, Ordering::AcqRel) {
+        return (
+            StatusCode::TOO_MANY_REQUESTS,
+            "another export is in progress; try again when it is done\n",
+        )
+            .into_response();
+    }
+    let _exporting = Exporting;
     let swarm_id = state.swarm_id().await.unwrap_or_default();
 
     let start = Instant::now();