decouple link fetching

Signed-off-by: varun-edachali-dbx <varun.edachali@databricks.com>

Author: varun-edachali-dbx

src/databricks/sql/backend/sea/queue.py

@@ -1,7 +1,8 @@
 from __future__ import annotations
 
 from abc import ABC
-from typing import List, Optional, Tuple, Union
+import threading
+from typing import Dict, List, Optional, Tuple, Union
 
 from databricks.sql.cloudfetch.download_manager import ResultFileDownloadManager
 
@@ -111,6 +112,86 @@ def remaining_rows(self) -> List[List[str]]:
         return slice
 
 
+class LinkFetcher:
+    def __init__(
+        self,
+        download_manager: ResultFileDownloadManager,
+        backend: "SeaDatabricksClient",
+        statement_id: str,
+        current_chunk_link: Optional["ExternalLink"] = None,
+    ):
+        self.download_manager = download_manager
+        self.backend = backend
+        self._statement_id = statement_id
+        self._current_chunk_link = current_chunk_link
+
+        self._shutdown_event = threading.Event()
+
+        self._map_lock = threading.Lock()
+        self.chunk_index_to_link: Dict[int, "ExternalLink"] = {}
+
+    def _set_current_chunk_link(self, link: "ExternalLink"):
+        with self._map_lock:
+            self.chunk_index_to_link[link.chunk_index] = link
+
+    def get_chunk_link(self, chunk_index: int) -> Optional["ExternalLink"]:
+        with self._map_lock:
+            return self.chunk_index_to_link.get(chunk_index, None)
+
+    def _convert_to_thrift_link(self, link: "ExternalLink") -> TSparkArrowResultLink:
+        """Convert SEA external links to Thrift format for compatibility with existing download manager."""
+        # Parse the ISO format expiration time
+        expiry_time = int(dateutil.parser.parse(link.expiration).timestamp())
+        return TSparkArrowResultLink(
+            fileLink=link.external_link,
+            expiryTime=expiry_time,
+            rowCount=link.row_count,
+            bytesNum=link.byte_count,
+            startRowOffset=link.row_offset,
+            httpHeaders=link.http_headers or {},
+        )
+
+    def _progress_chunk_link(self):
+        """Progress to the next chunk link."""
+        if not self._current_chunk_link:
+            return None
+
+        next_chunk_index = self._current_chunk_link.next_chunk_index
+
+        if next_chunk_index is None:
+            self._current_chunk_link = None
+            return None
+
+        try:
+            self._current_chunk_link = self.backend.get_chunk_link(
+                self._statement_id, next_chunk_index
+            )
+        except Exception as e:
+            logger.error(
+                "LinkFetcher: Error fetching link for chunk {}: {}".format(
+                    next_chunk_index, e
+                )
+            )
+            self._current_chunk_link = None
+
+    def _worker_loop(self):
+        while not (self._shutdown_event.is_set() or self._current_chunk_link is None):
+            self._set_current_chunk_link(self._current_chunk_link)
+            self.download_manager.add_link(
+                self._convert_to_thrift_link(self._current_chunk_link)
+            )
+
+            self._progress_chunk_link()
+
+    def start(self):
+        self._worker_thread = threading.Thread(target=self._worker_loop)
+        self._worker_thread.start()
+
+    def stop(self):
+        self._shutdown_event.set()
+        self._worker_thread.join()
+
+
 class SeaCloudFetchQueue(CloudFetchQueue):
     """Queue implementation for EXTERNAL_LINKS disposition with ARROW format for SEA backend."""
 
@@ -160,6 +241,7 @@ def __init__(
         initial_link = next((l for l in initial_links if l.chunk_index == 0), None)
         if not initial_link:
             return
+        self.current_chunk_index = initial_link.chunk_index
 
         self.download_manager = ResultFileDownloadManager(
             links=[],
@@ -168,75 +250,23 @@ def __init__(
             ssl_options=ssl_options,
         )
 
-        # Track the current chunk we're processing
-        self._current_chunk_link: Optional["ExternalLink"] = initial_link
-        self._download_current_link()
+        self.link_fetcher = LinkFetcher(
+            self.download_manager, self._sea_client, statement_id, initial_link
+        )
+        self.link_fetcher.start()
 
         # Initialize table and position
         self.table = self._create_next_table()
 
-    def _convert_to_thrift_link(self, link: "ExternalLink") -> TSparkArrowResultLink:
-        """Convert SEA external links to Thrift format for compatibility with existing download manager."""
-        # Parse the ISO format expiration time
-        expiry_time = int(dateutil.parser.parse(link.expiration).timestamp())
-        return TSparkArrowResultLink(
-            fileLink=link.external_link,
-            expiryTime=expiry_time,
-            rowCount=link.row_count,
-            bytesNum=link.byte_count,
-            startRowOffset=link.row_offset,
-            httpHeaders=link.http_headers or {},
-        )
-
-    def _download_current_link(self):
-        """Download the current chunk link."""
-        if not self._current_chunk_link:
-            return None
-
-        if not self.download_manager:
-            logger.debug("SeaCloudFetchQueue: No download manager, returning")
-            return None
-
-        thrift_link = self._convert_to_thrift_link(self._current_chunk_link)
-        self.download_manager.add_link(thrift_link)
-
-    def _progress_chunk_link(self):
-        """Progress to the next chunk link."""
-        if not self._current_chunk_link:
-            return None
-
-        next_chunk_index = self._current_chunk_link.next_chunk_index
-
-        if next_chunk_index is None:
-            self._current_chunk_link = None
-            return None
-
-        try:
-            self._current_chunk_link = self._sea_client.get_chunk_link(
-                self._statement_id, next_chunk_index
-            )
-        except Exception as e:
-            logger.error(
-                "SeaCloudFetchQueue: Error fetching link for chunk {}: {}".format(
-                    next_chunk_index, e
-                )
-            )
-            return None
-
-        logger.debug(
-            f"SeaCloudFetchQueue: Progressed to link for chunk {next_chunk_index}: {self._current_chunk_link}"
-        )
-        self._download_current_link()
-
     def _create_next_table(self) -> Union["pyarrow.Table", None]:
         """Create next table by retrieving the logical next downloaded file."""
-        if not self._current_chunk_link:
-            logger.debug("SeaCloudFetchQueue: No current chunk link, returning")
+        current_chunk_link = self.link_fetcher.get_chunk_link(self.current_chunk_index)
+        if not current_chunk_link:
             return None
 
-        row_offset = self._current_chunk_link.row_offset
+        row_offset = current_chunk_link.row_offset
         arrow_table = self._create_table_at_offset(row_offset)
 
-        self._progress_chunk_link()
+        self.current_chunk_index = current_chunk_link.next_chunk_index
 
         return arrow_table