manifest: fix incorrect Annotator pointer aggregation

#3760 contained a bug which causes Annotator
values to be incorrectly aggregated when pointer values
should be overwritten. This is because the `vtyped`
variable was not being modified due to being on the
stack.

This change fixes this and adds a unit test for
`PickFileAggregator` to catch this issue in the future.

#3759 should already not be affected by
this due to the different way it handles aggregation.

internal/manifest/annotator.go

@@ -84,33 +84,36 @@ func (a *Annotator[T]) findAnnotation(n *node) *annotation {
 // nodeAnnotation computes this annotator's annotation of this node across all
 // files in the node's subtree. The second return value indicates whether the
 // annotation is stable and thus cacheable.
-func (a *Annotator[T]) nodeAnnotation(n *node) (v *T, cacheOK bool) {
+func (a *Annotator[T]) nodeAnnotation(n *node) (_ *T, cacheOK bool) {
 	annot := a.findAnnotation(n)
-	vtyped := annot.v.(*T)
+	t := annot.v.(*T)
 	// If the annotation is already marked as valid, we can return it without
 	// recomputing anything.
 	if annot.valid {
-		return vtyped, true
+		return t, true
 	}
 
-	annot.v = a.Aggregator.Zero(vtyped)
-	annot.valid = true
+	t = a.Aggregator.Zero(t)
+	valid := true
 
 	for i := int16(0); i <= n.count; i++ {
 		if !n.leaf {
 			v, ok := a.nodeAnnotation(n.children[i])
-			annot.v = a.Aggregator.Merge(v, vtyped)
-			annot.valid = annot.valid && ok
+			t = a.Aggregator.Merge(v, t)
+			valid = valid && ok
 		}
 
 		if i < n.count {
-			v, ok := a.Aggregator.Accumulate(n.items[i], vtyped)
-			annot.v = v
-			annot.valid = annot.valid && ok
+			var ok bool
+			t, ok = a.Aggregator.Accumulate(n.items[i], t)
+			valid = valid && ok
 		}
 	}
 
-	return annot.v.(*T), annot.valid
+	annot.v = t
+	annot.valid = valid
+
+	return t, annot.valid
 }
 
 // InvalidateAnnotation removes any existing cached annotations from this

internal/manifest/annotator_test.go

@@ -56,3 +56,26 @@ func BenchmarkNumFilesAnnotator(b *testing.B) {
 		require.EqualValues(b, uint64(i), numFiles)
 	}
 }
+
+func TestPickFileAggregator(t *testing.T) {
+	const count = 1000
+	a := Annotator[FileMetadata]{
+		Aggregator: PickFileAggregator{
+			Filter: func(f *FileMetadata) (eligible bool, cacheOK bool) {
+				return true, true
+			},
+			Compare: func(f1 *FileMetadata, f2 *FileMetadata) bool {
+				return base.DefaultComparer.Compare(f1.Smallest.UserKey, f2.Smallest.UserKey) < 0
+			},
+		},
+	}
+
+	lm, files := makeTestLevelMetadata(1)
+
+	for i := 1; i <= count; i++ {
+		lm.tree.Insert(newItem(key(i)))
+		pickedFile := a.LevelAnnotation(lm)
+		// The picked file should always be the one with the smallest key.
+		require.Same(t, files[0], pickedFile)
+	}
+}