add invalidate() method

[jcupitt]

useful if you're modifying image memory arrays outside libvips, eg. with
numpy

see #316

[jcupitt]

I've not added a test, so any testing would be great!

[erdmann]

Great!

When I first tried this, I got an error about

--> 736     vips_lib.vips_image_invalidate_all(self.pointer)

AttributeError: cffi library '_libvips' has no function, constant or global variable named 'vips_image_invalidate_all'

But on updating libvips and recompiling and then reinstalling pyvips it works.

Here's a little test:

diff --git a/tests/test_image.py b/tests/test_image.py
index ba69275..da07c60 100644
--- a/tests/test_image.py
+++ b/tests/test_image.py
@@ -168,3 +168,21 @@ class TestImage:
             wrongargtype = dict(a=1, b=2)
             x = im[wrongargtype]
 
+
+    def test_invalidate(self):
+        try:
+            import numpy as np
+        except ImportError:
+            pytest.skip('numpy not available')
+
+        a = np.zeros((1,1))
+        p = pyvips.Image.new_from_memory(a.data, 1, 1, 1, 'double')
+        v = p(0, 0)
+        assert v == [0]
+        a[0,0] = 1
+        v = p(0, 0)
+        assert v == [0]
+        p.invalidate()
+        v = p(0, 0)
+        assert v == [1]

[jcupitt]

Oh hmm that's bad. Do you remember which libvips it failed on? I thought invalidate_all() had been around for years.

[jcupitt]

Yes, looks like it has been around since 8.0. Perhaps you just needed to force the rebuild of your FFI binding.

I'll paste the test in, thanks!

[erdmann]

Do you remember which libvips it failed on? I thought invalidate_all() had been around for years.

I was using a build from git back in February, so it was almost certainly just a stale binding. I'm not sure what I did to force the refresh, since I installed your branch just like any other, but anyway all is right now.

[jcupitt]

OK, let's merge then.