Fixed #25072 -- Prevented GDALRaster memory to be uncollectable

Setting GDALRaster.bands as a cached property was creating a circular
reference with objects having __del__ methods, which means the memory
could never be freed.
Thanks Daniel Wiesmann for the report and test, and Tim Graham for the review.

3 files changed, 32 insertions, 12 deletions

diff --git a/django/contrib/gis/gdal/raster/band.py b/django/contrib/gis/gdal/raster/band.py
index f1eb50e506..bd273b2985 100644
--- a/django/contrib/gis/gdal/raster/band.py
+++ b/django/contrib/gis/gdal/raster/band.py
@@ -1,10 +1,12 @@
 from ctypes import byref, c_int
 
 from django.contrib.gis.gdal.base import GDALBase
+from django.contrib.gis.gdal.error import GDALException
 from django.contrib.gis.gdal.prototypes import raster as capi
 from django.contrib.gis.shortcuts import numpy
 from django.utils import six
 from django.utils.encoding import force_text
+from django.utils.six.moves import range
 
 from .const import GDAL_INTEGER_TYPES, GDAL_PIXEL_TYPES, GDAL_TO_CTYPES
 
@@ -148,3 +150,22 @@ class GDALBand(GDALBase):
                 return list(data_array)
         else:
             self.source._flush()
+
+
+class BandList(list):
+    def __init__(self, source):
+        self.source = source
+        list.__init__(self)
+
+    def __iter__(self):
+        for idx in range(1, len(self) + 1):
+            yield GDALBand(self.source, idx)
+
+    def __len__(self):
+        return capi.get_ds_raster_count(self.source._ptr)
+
+    def __getitem__(self, index):
+        try:
+            return GDALBand(self.source, index + 1)
+        except GDALException:
+            raise GDALException('Unable to get band index %d' % index)
diff --git a/django/contrib/gis/gdal/raster/source.py b/django/contrib/gis/gdal/raster/source.py
index e56b442fee..3b4eb183b1 100644
--- a/django/contrib/gis/gdal/raster/source.py
+++ b/django/contrib/gis/gdal/raster/source.py
@@ -6,7 +6,7 @@ from django.contrib.gis.gdal.base import GDALBase
 from django.contrib.gis.gdal.driver import Driver
 from django.contrib.gis.gdal.error import GDALException
 from django.contrib.gis.gdal.prototypes import raster as capi
-from django.contrib.gis.gdal.raster.band import GDALBand
+from django.contrib.gis.gdal.raster.band import BandList
 from django.contrib.gis.gdal.raster.const import GDAL_RESAMPLE_ALGORITHMS
 from django.contrib.gis.gdal.srs import SpatialReference, SRSException
 from django.contrib.gis.geometry.regex import json_regex
@@ -15,7 +15,6 @@ from django.utils.encoding import (
     force_bytes, force_text, python_2_unicode_compatible,
 )
 from django.utils.functional import cached_property
-from django.utils.six.moves import range
 
 
 class TransformPoint(list):
@@ -108,9 +107,10 @@ class GDALRaster(GDALBase):
 
             # Set band data if provided
             for i, band_input in enumerate(ds_input.get('bands', [])):
-                self.bands[i].data(band_input['data'])
+                band = self.bands[i]
+                band.data(band_input['data'])
                 if 'nodata_value' in band_input:
-                    self.bands[i].nodata_value = band_input['nodata_value']
+                    band.nodata_value = band_input['nodata_value']
 
             # Set SRID
             self.srs = ds_input.get('srid')
@@ -273,15 +273,9 @@ class GDALRaster(GDALBase):
 
         return xmin, ymin, xmax, ymax
 
-    @cached_property
+    @property
     def bands(self):
-        """
-        Returns the bands of this raster as a list of GDALBand instances.
-        """
-        bands = []
-        for idx in range(1, capi.get_ds_raster_count(self._ptr) + 1):
-            bands.append(GDALBand(self, idx))
-        return bands
+        return BandList(self)
 
     def warp(self, ds_input, resampling='NearestNeighbour', max_error=0.0):
         """
diff --git a/tests/gis_tests/rasterapp/test_rasterfield.py b/tests/gis_tests/rasterapp/test_rasterfield.py
index d1c015f499..03691ef0b3 100644
--- a/tests/gis_tests/rasterapp/test_rasterfield.py
+++ b/tests/gis_tests/rasterapp/test_rasterfield.py
@@ -23,6 +23,11 @@ class RasterFieldTest(TransactionTestCase):
         r.refresh_from_db()
         self.assertIsNone(r.rast)
 
+    def test_access_band_data_directly_from_queryset(self):
+        RasterModel.objects.create(rast=JSON_RASTER)
+        qs = RasterModel.objects.all()
+        qs[0].rast.bands[0].data()
+
     def test_model_creation(self):
         """
         Test RasterField through a test model.