perf(export): stream CSV responses instead of buffering in memory

  Use yield_per(500) and Flask streaming response to avoid loading
  entire query results and building the full CSV string in memory.

Author: nledez

zou/app/blueprints/export/csv/base.py

@@ -40,14 +40,14 @@ def get(self):
         self.prepare_import()
         try:
             self.check_permissions()
-            csv_content = []
-            csv_content.append(self.build_headers())
-            results = self.build_query().all()
-            for result in results:
-                csv_content.append(self.build_row(result))
         except permissions.PermissionDenied:
             abort(403)
 
-        return csv_utils.build_csv_response(
-            csv_content, file_name=self.file_name
+        def row_generator():
+            yield self.build_headers()
+            for result in self.build_query().yield_per(500):
+                yield self.build_row(result)
+
+        return csv_utils.build_csv_stream_response(
+            row_generator(), file_name=self.file_name
         )

zou/app/utils/csv_utils.py

@@ -1,7 +1,7 @@
 import csv
 
 from io import StringIO
-from flask import make_response
+from flask import Response, make_response, stream_with_context
 from slugify import slugify
 
 
@@ -17,6 +17,30 @@ def build_csv_response(csv_content, file_name="export"):
     return csv_response
 
 
+def build_csv_stream_response(row_generator, file_name="export"):
+    """
+    Construct a streaming Flask response from a row generator.
+    Each row is written and flushed immediately to avoid buffering
+    the entire CSV in memory.
+    """
+    file_name = build_csv_file_name(file_name)
+
+    def generate():
+        for row in row_generator:
+            buf = StringIO()
+            csv.writer(buf, delimiter=";").writerow(row)
+            yield buf.getvalue()
+
+    response = Response(
+        stream_with_context(generate()),
+        mimetype="text/csv",
+    )
+    response.headers["Content-Disposition"] = (
+        "attachment; filename=%s.csv" % file_name
+    )
+    return response
+
+
 def build_csv_file_name(file_name):
     """
     Add application name as prefix of the file name.