test: assert minimum download throughput to catch DRS throttling

Files >= 20 MB must download at >= 3 MB/s. The DRS delay bug (v3.5.0)
throttled to ~2 MB/s — this floor catches the regression while
tolerating CI network variability.

Author: dvershinin

tests/test_direct_e2e.py

@@ -46,6 +46,11 @@
 # Per-file download timeouts (seconds).
 DOWNLOAD_TIMEOUTS = {"1mb": 60, "20mb": 180, "100mb": 600}
 
+# Minimum acceptable throughput (MB/s) for files >= 20 MB.
+# The DRS delay bug (v3.5.0) throttled downloads to ~2 MB/s.
+# This floor catches the regression while tolerating CI network variability.
+MIN_THROUGHPUT_MBPS = 3.0
+
 
 def _patch_telethon_faketls():
     """Patch TelethonFakeTLS bugs.
@@ -153,6 +158,16 @@ async def _download_test_files(client, transport_label):
             continue
 
         throughput_mb = len(data) / elapsed / 1048576
+
+        # Assert minimum throughput for large files to catch DRS throttling.
+        if info["size"] >= 20 * 1048576 and throughput_mb < MIN_THROUGHPUT_MBPS:
+            print(f"  [{transport_label}] {label}: FAIL — "
+                  f"throughput {throughput_mb:.2f} MB/s < "
+                  f"{MIN_THROUGHPUT_MBPS} MB/s minimum "
+                  f"(DRS delay regression?)")
+            all_ok = False
+            continue
+
         print(f"  [{transport_label}] {label}: OK — "
               f"{elapsed:.1f}s, {throughput_mb:.1f} MB/s")