roaring64: honor copy-on-write in Bitmap.Or

When two bitmaps share a container key, the in-place Or was calling
getContainerAtIndex, which returns the raw container without consulting
needCopyOnWrite. If rb's container at that index was shared with another
bitmap (e.g. produced by Clone() on a CoW-enabled bitmap), the Or would
mutate the shared container in place and silently corrupt the other
bitmap.

Switch to getWritableContainerAtIndex, which clones the container first
when needCopyOnWrite is set. This matches what the sibling AndNot/And
methods in the same file already do, and what the 32-bit Bitmap.Or does
via getUnionedWritableContainer.

Author: ostafen

roaring64/roaring64.go

@@ -700,7 +700,7 @@ main:
 				}
 				s2 = x2.highlowcontainer.getKeyAtIndex(pos2)
 			} else {
-				rb.highlowcontainer.getContainerAtIndex(pos1).Or(x2.highlowcontainer.getContainerAtIndex(pos2))
+				rb.highlowcontainer.getWritableContainerAtIndex(pos1).Or(x2.highlowcontainer.getContainerAtIndex(pos2))
 				pos1++
 				pos2++
 				if (pos1 == length1) || (pos2 == length2) {

roaring64/roaring64cow_test.go

@@ -1768,3 +1768,32 @@ func TestCloneCOWContainers(t *testing.T) {
 
 	//assert.EqualValues(t, rb.ToArray(), newRb1.ToArray())
 }
+
+// TestOrCOWSharedContainer verifies that in-place Or on a CoW-cloned bitmap
+// does not mutate containers shared with the clone source.
+func TestOrCOWSharedContainer(t *testing.T) {
+	rb1 := NewBitmap()
+	rb1.SetCopyOnWrite(true)
+	// Populate a single high-key container with values in [0, 1000).
+	for i := uint64(0); i < 1000; i++ {
+		rb1.Add(i)
+	}
+
+	// Clone: rb2 shares rb1's container, with needCopyOnWrite set on both sides.
+	rb2 := rb1.Clone()
+
+	// x has values in the same high-key container that are not already in rb1/rb2.
+	x := NewBitmap()
+	for i := uint64(1000); i < 2000; i++ {
+		x.Add(i)
+	}
+
+	// In-place Or into rb1. Must NOT mutate the container shared with rb2.
+	rb1.Or(x)
+
+	assert.EqualValues(t, 2000, rb1.GetCardinality())
+	assert.EqualValues(t, 1000, rb2.GetCardinality(),
+		"rb2 was corrupted: Or on rb1 mutated a CoW-shared container")
+	assert.False(t, rb2.Contains(1500),
+		"rb2 was corrupted: contains a value that was only added to rb1")
+}