new_from_source keeps a ref to the source

Since it might be a custom source with callabcks, and the python wrapper
for the source class must stay alive.

Added another example of custom sources.

CHANGELOG.rst

@@ -2,7 +2,9 @@
 
 - ensure compatibility with a single shared libvips library [kleisauke]
 - add flags_dict(), enum_dict() for better flags introspection
-- improve generation of enums.py
+- improve generation of `enums.py`
+- add `stream.py` example
+- fix a missing reference issue with custom sources
 
 ## Version 2.2.2 (released 4 Jan 2023)
 

examples/stream.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+
+import sys
+import gc
+import requests
+import pyvips
+
+URL = "https://cdn.filestackcontent.com/bnTGtQw5ShqMPpxH2tMw"
+URLS = [URL] * int(sys.argv[1])
+
+session = requests.Session()
+
+image = pyvips.Image.black(1500, 1500)
+
+for i, url in enumerate(URLS):
+    print(f"loading {url} ...")
+    stream = session.get(url, stream=True).raw
+
+    source = pyvips.SourceCustom()
+    source.on_read((lambda stream: stream.read)(stream))
+
+    tile = pyvips.Image.new_from_source(source, "", access="sequential")
+    image = image.composite2(tile, "over", x= 50 * (i + 1), y= 50 * (i + 1))
+
+print(f"writing output.jpg ...")
+image.write_to_file("output.jpg")
+

pyvips/vimage.py

@@ -664,8 +664,15 @@ def new_from_source(source, options, **kwargs):
             raise Error('unable to load from source')
         name = _to_string(pointer)
 
-        return pyvips.Operation.call(name, source,
-                                     string_options=options, **kwargs)
+        image = pyvips.Operation.call(name, source,
+                                      string_options=options, **kwargs)
+
+        # keep a secret ref to the source object .. we need that to stay
+        # alive, since it might be a custom source that triggers a python
+        # callback
+        image._references.append(source)
+
+        return image
 
     @staticmethod
     def new_temp_file(format):