pw_allocator: Remove mutable default value

Avoid unintentional sharing of the default value between instances
of HeapUsage. This has always been a bug, but will become a runtime
exception in Python 3.11
(https://docs.python.org/3/library/dataclasses.html#mutable-default-values,
https://stackoverflow.com/q/53632152/1224002).

Bug: 259121920
Change-Id: I0315a917316ae9f3bcf59e1015446f572dbbb7b5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/119312
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/pw_allocator/py/pw_allocator/heap_viewer.py b/pw_allocator/py/pw_allocator/heap_viewer.py
index 56e539674..324fee07f 100644
--- a/pw_allocator/py/pw_allocator/heap_viewer.py
+++ b/pw_allocator/py/pw_allocator/heap_viewer.py
@@ -18,7 +18,7 @@ import sys
 import math
 import logging
 from typing import Optional
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 import coloredlogs  # type: ignore
 
 
@@ -33,7 +33,9 @@ class HeapBlock:
 @dataclass
 class HeapUsage:
     """Contains a linked list of allocated HeapBlocks."""
-    begin: HeapBlock = HeapBlock(0, 0)
+    # Use a default_factory to avoid mutable default value. See
+    # https://docs.python.org/3/library/dataclasses.html#mutable-default-values
+    begin: HeapBlock = field(default_factory=lambda: HeapBlock(0, 0))
 
     def add_block(self, block):
         cur_block = self.begin.next