Retry interrupted thematic WCS tiles

backend/app/services/thematic_raster_acquisition_service.py

@@ -6,6 +6,7 @@ import math
 import time
 from dataclasses import dataclass
 from datetime import UTC, datetime
+from http.client import HTTPException
 from pathlib import Path
 from typing import Any, Callable
 from urllib.error import HTTPError, URLError
@@ -55,6 +56,8 @@ class ThematicRasterAcquisitionService:
     NODATA = -9999.0
     WCS_TILE_SIDE_M = 10_000.0
     WCS_REQUEST_INTERVAL_SECONDS = 0.5
+    WCS_FETCH_ATTEMPTS = 3
+    WCS_RETRY_DELAY_SECONDS = 1.0
     ATTRIBUTION = "Bron: Departement Omgeving, MercatorNet"
     LICENSE_NOTE = "Publieke GDI-Vlaanderen bron; bronvermelding en productspecifieke gebruiksvoorwaarden blijven van toepassing."
 
@@ -305,6 +308,7 @@ class ThematicRasterAcquisitionService:
     def _fetch(request_url: str, settings: Settings, opener: Callable[..., Any] | None = None) -> tuple[bytes, str]:
         request = Request(request_url, headers={"Accept": "image/tiff,*/*", "User-Agent": "GeoIntel/0.1 bounded-thematic-raster"})
         max_bytes = settings.thematic_raster_max_response_mb * 1024 * 1024
+        for attempt in range(1, ThematicRasterAcquisitionService.WCS_FETCH_ATTEMPTS + 1):
             try:
                 with (opener or urlopen)(request, timeout=settings.thematic_raster_timeout_seconds) as response:
                     content_type = str(response.headers.get("Content-Type", ""))
@@ -312,21 +316,28 @@ class ThematicRasterAcquisitionService:
                     if content_length and int(content_length) > max_bytes:
                         raise AppError(code="THEMATIC_RASTER_RESPONSE_TOO_LARGE", message="Official raster response exceeds the configured size limit", status_code=502)
                     content = response.read(max_bytes + 1)
+                break
             except AppError:
                 raise
             except HTTPError as exc:
                 preview = exc.read(300).decode("utf-8", errors="replace")
+                if attempt < ThematicRasterAcquisitionService.WCS_FETCH_ATTEMPTS and int(exc.code) in {429, 500, 502, 503, 504}:
+                    time.sleep(ThematicRasterAcquisitionService.WCS_RETRY_DELAY_SECONDS * attempt)
+                    continue
                 raise AppError(
                     code="THEMATIC_RASTER_PROVIDER_UNAVAILABLE",
                     message="The official MercatorNet WCS could not complete the bounded request",
-                details={"reason": str(exc), "provider_status_code": int(exc.code), "response_preview": preview},
+                    details={"reason": str(exc), "provider_status_code": int(exc.code), "response_preview": preview, "attempts": attempt},
                     status_code=502,
                 ) from exc
-        except (URLError, TimeoutError, OSError) as exc:
+            except (URLError, TimeoutError, OSError, HTTPException) as exc:
+                if attempt < ThematicRasterAcquisitionService.WCS_FETCH_ATTEMPTS:
+                    time.sleep(ThematicRasterAcquisitionService.WCS_RETRY_DELAY_SECONDS * attempt)
+                    continue
                 raise AppError(
                     code="THEMATIC_RASTER_PROVIDER_UNAVAILABLE",
                     message="The official MercatorNet WCS could not complete the bounded request",
-                details={"reason": str(exc)},
+                    details={"reason": str(exc), "attempts": attempt},
                     status_code=502,
                 ) from exc
         if len(content) > max_bytes:

backend/tests/test_sprint213_thematic_rasters.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from http.client import IncompleteRead
 from pathlib import Path
 from types import SimpleNamespace
 from uuid import uuid4
@@ -89,6 +90,11 @@ class FakeResponse:
         return self.content[:limit]
 
 
+class IncompleteResponse(FakeResponse):
+    def read(self, limit: int):
+        raise IncompleteRead(self.content[:limit])
+
+
 def payload(product_key: str = "space_occupation_2025", *, side_m: float = 1000.0) -> ThematicRasterAcquireRequest:
     transformer = Transformer.from_crs("EPSG:31370", "EPSG:4326", always_xy=True)
     min_x, min_y = transformer.transform(200_000, 210_000)
@@ -171,6 +177,52 @@ def test_complete_kempen_scope_fits_the_tiled_thematic_guardrails() -> None:
     assert exc_info.value.code == "THEMATIC_RASTER_SELECTION_TOO_LARGE"
 
 
+def test_wcs_fetch_retries_an_incomplete_tile_without_accepting_partial_bytes(monkeypatch) -> None:
+    content = b"II*\x00complete-geotiff"
+    responses = [IncompleteResponse(content), FakeResponse(content)]
+    attempts = 0
+
+    def opener(*_args, **_kwargs):
+        nonlocal attempts
+        response = responses[attempts]
+        attempts += 1
+        return response
+
+    monkeypatch.setattr("app.services.thematic_raster_acquisition_service.time.sleep", lambda _seconds: None)
+
+    result, content_type = ThematicRasterAcquisitionService._fetch(
+        "https://example.invalid/wcs",
+        Settings(_env_file=None),
+        opener,
+    )
+
+    assert attempts == 2
+    assert result == content
+    assert content_type == "image/tiff"
+
+
+def test_wcs_fetch_fails_closed_after_bounded_incomplete_tile_retries(monkeypatch) -> None:
+    attempts = 0
+
+    def opener(*_args, **_kwargs):
+        nonlocal attempts
+        attempts += 1
+        return IncompleteResponse(b"II*\x00partial")
+
+    monkeypatch.setattr("app.services.thematic_raster_acquisition_service.time.sleep", lambda _seconds: None)
+
+    with pytest.raises(AppError) as exc_info:
+        ThematicRasterAcquisitionService._fetch(
+            "https://example.invalid/wcs",
+            Settings(_env_file=None),
+            opener,
+        )
+
+    assert attempts == ThematicRasterAcquisitionService.WCS_FETCH_ATTEMPTS
+    assert exc_info.value.code == "THEMATIC_RASTER_PROVIDER_UNAVAILABLE"
+    assert exc_info.value.details["attempts"] == 3
+
+
 def test_binary_and_normalized_products_fail_closed_on_invalid_values() -> None:
     binary = ThematicRasterAcquisitionService._product("space_occupation_2025")
     score = ThematicRasterAcquisitionService._product("service_level_2022")