Dereference refs when reading and writing

how_to/Change_33.md

@@ -0,0 +1,17 @@
+data: Dereference refs when reading and writing
+
+Now we'll dereference symbolic refs not only when reading them but also when
+writing them.
+
+We'll implement a helper function called `_get_ref_internal` which will return
+the path and the value of the last ref pointed by a symbolic ref. In simple words:
+
+- When given a non-symbolic ref, `_get_ref_internal` will return the ref name
+and value.
+- When given a symbolic ref, `_get_ref_internal` will dereference the ref
+recursively, and then return the name of the last (non-symbolic) ref that points
+to an OID, plus its value.
+
+Now `update_ref` will use `_get_ref_internal` to know which ref it needs to update.
+
+Additionally, we'll use `_get_ref_internal` in `get_ref`.

ugit/data.py

@@ -18,6 +18,7 @@ RefValue = namedtuple("RefValue", ["symbolic", "value"])
 
 def update_ref(ref, value):
     assert not value.symbolic
+    ref = _get_ref_internal(ref)[0]
     ref_path = f"{GIT_DIR}/{ref}"
     Path.mkdir(ref_path, exist_ok=True)
     with open(ref_path, "w") as f:
@@ -25,16 +26,22 @@ def update_ref(ref, value):
 
 
 def get_ref(ref):
+    return _get_ref_internal(ref)[1]
+
+
+def _get_ref_internal(ref):
     ref_path = f"{GIT_DIR}/{ref}"
     value = None
     if Path.is_file(ref_path):
         with open(ref_path) as f:
             value = f.read().strip()
 
-    if value and value.startswith("ref:"):
+    symbolic = bool(value) and value.startswith("ref")
-        return get_ref(value.split(":", 1)[1].strip())
+    if symbolic:
+        value = value.split(":", 1)[1].strip()
+        return _get_ref_internal(value)
 
-    return RefValue(symbolic=False, value=value)
+    return ref, RefValue(symbolic=False, value=value)
 
 
 def iter_refs():