fix: unreachable error check in retraction (#15433)

Co-authored-by: Liangsheng Yin <lsyincs@gmail.com>

python/sglang/srt/managers/schedule_batch.py

@@ -1746,21 +1746,7 @@ class ScheduleBatch(ScheduleBatchDisaggregationDecodeMixin):
             )
         ):
             if len(sorted_indices) == 1:
-                # Corner case: only one request left
+                # Always keep at least one request
-                if self.is_hybrid_swa:
-                    full_available_size = (
-                        self.token_to_kv_pool_allocator.full_available_size()
-                    )
-                    swa_available_size = (
-                        self.token_to_kv_pool_allocator.swa_available_size()
-                    )
-                    assert (
-                        full_available_size > 0 and swa_available_size > 0
-                    ), f"No space left for only one request in SWA mode {full_available_size=}, {swa_available_size=}"
-                else:
-                    assert (
-                        self.token_to_kv_pool_allocator.available_size() > 0
-                    ), f"No space left for only one request, {self.token_to_kv_pool_allocator.available_size()=}"
                 break
 
             first_iter = False
@@ -1770,10 +1756,12 @@ class ScheduleBatch(ScheduleBatchDisaggregationDecodeMixin):
             # release memory and don't insert into the tree because we need the space instantly
             self.release_req(idx, len(sorted_indices), server_args)
 
-            if len(retracted_reqs) == 0:
+        if len(sorted_indices) <= 1 and not self.check_decode_mem(
-                # Corner case: only one request left
+            selected_indices=sorted_indices, buf_multiplier=buf_multiplier
+        ):
+            # Retracting loops ends and still not enough memory
             raise ValueError(
-                    "Failed to retract any request. No space left for only one request."
+                "Out of memory even after retracting all other requests in the decode batch."
             )
 
         self.filter_batch(keep_indices=sorted_indices)