fix(s3): preserve atomic 1 MiB conditional writes (#6798)

rustfs/src/app/object/put.rs

@@ -35,6 +35,10 @@ const DEFAULT_ZERO_COPY_EAGER_PUT_MAX_SIZE_BYTES: usize = 16 * 1024 * 1024;
 
 const DEFAULT_SMALL_EAGER_PUT_MAX_SIZE_BYTES: usize = 512 * 1024;
 
+// Keep bounded conditional writes eager through the historical 1 MiB boundary
+// so the old object remains readable until the replacement body is complete.
+const CONDITIONAL_SMALL_EAGER_PUT_MAX_SIZE_BYTES: i64 = 1024 * 1024;
+
 const PUT_EAGER_STATUS_ELIGIBLE: &str = "eligible";
 
 const PUT_EAGER_STATUS_EXTRACT: &str = "extract";
@@ -517,6 +521,17 @@ fn should_use_small_eager_put_path_with_max_size(
         return false;
     }
 
+    let has_conditional_write = [http::header::IF_MATCH, http::header::IF_NONE_MATCH]
+        .into_iter()
+        .filter_map(|name| headers.get(name))
+        .filter_map(|value| value.to_str().ok())
+        .any(|value| !value.trim().is_empty());
+    let max_size = if has_conditional_write {
+        max_size.max(CONDITIONAL_SMALL_EAGER_PUT_MAX_SIZE_BYTES)
+    } else {
+        max_size
+    };
+
     if size <= 0 || size > max_size {
         return false;
     }
@@ -2351,6 +2366,31 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn should_use_small_eager_put_path_keeps_conditional_1mb_writes_atomic() {
+        for header in [http::header::IF_MATCH, http::header::IF_NONE_MATCH] {
+            let mut headers = HeaderMap::new();
+            headers.insert(header, HeaderValue::from_static("*"));
+
+            assert!(should_use_small_eager_put_path_with_max_size(
+                1024 * 1024,
+                &headers,
+                false,
+                false,
+                false,
+                512 * 1024,
+            ));
+            assert!(!should_use_small_eager_put_path_with_max_size(
+                1024 * 1024 + 1,
+                &headers,
+                false,
+                false,
+                false,
+                512 * 1024,
+            ));
+        }
+    }
+
     #[test]
     fn should_use_small_eager_put_path_rejects_sse_requests() {
         let headers = HeaderMap::new();