Fix conflict on changing dependencies (#96)

entangled/io/transaction.py

@@ -93,12 +93,15 @@ class Write(WriterBase):
     def conflict(self, fs: AbstractFileCache, db: FileDB) -> Conflict | None:
         if self.target not in fs:
             return None
+        # If the file on disk matches the filedb record, it is exactly what
+        # Entangled wrote last time and was not edited outside of Entangled, so
+        # it is safe to overwrite. Comparing against the filedb is more reliable
+        # than comparing modification times against the current set of sources:
+        # the latter gives false positives when the dependency graph changes,
+        # e.g. when a target no longer depends on one of its former sources
+        # (see issue #96).
         if fs[self.target].stat != db[self.target]:
             return Conflict(self.target, "changed outside the control of Entangled")
-        if self.sources:
-            if all(fs[s].stat < fs[self.target].stat for s in self.sources):
-                return Conflict(self.target, "newer than all of its sources: " + ", ".join(
-                    f"`{s}`" for s in set(self.sources)))
         return None
 
     @override

test/io/test_transaction.py

@@ -1,5 +1,6 @@
 from contextlib import chdir
 from pathlib import Path
+from time import sleep
 
 from entangled.io.transaction import Transaction, Create, Write, Delete
 from entangled.io.filedb import filedb
@@ -46,3 +47,42 @@ def test_transaction(tmp_path: Path):
             assert isinstance(t.actions[1], Delete)
             t.run()
             assert not Path("b").exists()
+
+
+def test_changing_dependencies(tmp_path: Path):
+    """Regression test for issue #96: when a target no longer depends on one of
+    its former sources, Entangled should still be able to update it. The target
+    is then necessarily newer than its remaining sources, but as long as it was
+    not edited outside of Entangled (i.e. it matches the filedb), overwriting it
+    is safe and should not be reported as a conflict."""
+    with chdir(tmp_path):
+        fs = FileCache()
+
+        # the source files are older than anything generated from them
+        with open("input1", "w") as f:
+            _ = f.write("source one")
+        with open("input2", "w") as f:
+            _ = f.write("source two")
+        sleep(0.01)
+
+        # first run: `output` is generated from both inputs
+        with filedb(fs=fs) as db:
+            t = Transaction(db)
+            t.write(Path("output"), "from one and two",
+                    [Path("input1"), Path("input2")])
+            assert isinstance(t.actions[0], Create)
+            t.run()
+        assert Path("output").exists()
+
+        # `output` is now newer than its sources; `input2` no longer writes to
+        # it, so the content changes while only `input1` remains as a source
+        fs.reset()
+        with filedb(fs=fs) as db:
+            t = Transaction(db)
+            t.write(Path("output"), "from one only", [Path("input1")])
+            assert isinstance(t.actions[0], Write)
+            # `output` was not touched outside of Entangled, so this is fine
+            assert t.all_ok()
+            t.run()
+
+        assert "from one only" in Path("output").read_text()