The asseturl filter should register a build dependency on the asset

Fixes lektor#1020

lektor/context.py

@@ -23,14 +23,10 @@ def get_asset_url(asset):
     ctx = get_ctx()
     if ctx is None:
         raise RuntimeError("No context found")
-    asset = site_proxy.get_asset(asset)
+    asset = ctx.pad.get_asset(asset)
     if asset is None:
         return Undefined("Asset not found")
-    info = ctx.build_state.get_file_info(asset.source_filename)
-    return "%s?h=%s" % (
-        ctx.source.url_to("!" + asset.url_path),
-        info.checksum[:8],
-    )
+    return ctx.get_asset_url(asset)
 
 
 @LocalProxy
@@ -168,6 +164,18 @@ def url_to(self, path, alt=None, absolute=None, external=None):
         rv = self.source.url_to(path, alt=alt, absolute=True)
         return self.pad.make_url(rv, self.base_url, absolute, external)
 
+    def get_asset_url(self, asset):
+        """Calculates the asset URL relative to the current record."""
+        if self.source is None:
+            raise RuntimeError(
+                "Can only generate paths to assets if "
+                "the context has a source document set."
+            )
+        asset_url = self.source.url_to("!" + asset.url_path)
+        info = self.build_state.get_file_info(asset.source_filename)
+        self.record_dependency(asset.source_filename)
+        return f"{asset_url}?h={info.checksum[:8]}"
+
     def sub_artifact(self, *args, **kwargs):
         """Decorator version of :func:`add_sub_artifact`."""
 

tests/test_builder.py

@@ -1,3 +1,5 @@
+from pathlib import Path
+
 from markers import imagemagick
 
 
@@ -271,3 +273,28 @@ def test_slug_contains_slash(pad, builder):
     prog, _ = builder.build(record)
     (artifact,) = prog.artifacts
     assert artifact.artifact_name == "extra/long/path/index.html"
+
+
+def test_asseturl_dependency_tracking_integration(
+    scratch_project_data, scratch_pad, scratch_builder
+):
+    scratch_project_data.joinpath("templates/page.html").write_text(
+        "{{ '/asset.txt'|asseturl }}", "utf-8"
+    )
+    asset_txt = scratch_project_data / "assets/asset.txt"
+    asset_txt.parent.mkdir(exist_ok=True)
+    asset_txt.write_text("an asset\n", "utf-8")
+
+    prog, build_state = scratch_builder.build(scratch_pad.root)
+    assert len(build_state.updated_artifacts) == 1
+    output = Path(prog.artifacts[0].dst_filename)
+    asset_url = output.read_text(encoding="utf-8").rstrip()
+
+    prog, build_state = scratch_builder.build(scratch_pad.root)
+    assert len(build_state.updated_artifacts) == 0
+
+    asset_txt.write_text("updated asset\n", "utf-8")
+    prog, build_state = scratch_builder.build(scratch_pad.root)
+    updated_asset_url = output.read_text(encoding="utf-8").rstrip()
+    assert updated_asset_url != asset_url
+    assert len(build_state.updated_artifacts) == 1