Poison initial Arena block with MSan.

We should always initialize it before we use it.

PiperOrigin-RevId: 693436567

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs

@@ -1,17 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-//
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file or at
-// https://developers.google.com/open-source/licenses/bsd
-#endregion
-
-namespace Google.Protobuf.Reflection;
-
-internal sealed partial class FeatureSetDescriptor
-{
-    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
-    private const string DefaultsBase64 =
-        "ChMYhAciACoMCAEQAhgCIAMoATACChMY5wciACoMCAIQARgBIAIoATABChMY6AciDAgBEAEYASACKAEwASoAIOYHKOgH";
-}

src/google/protobuf/BUILD.bazel

@@ -446,6 +446,7 @@ cc_library(
         ":string_block",
         "//src/google/protobuf/stubs:lite",
         "@com_google_absl//absl/base:core_headers",
+        "@com_google_absl//absl/base:dynamic_annotations",
         "@com_google_absl//absl/base:prefetch",
         "@com_google_absl//absl/container:layout",
         "@com_google_absl//absl/log:absl_check",

src/google/protobuf/arena.cc

@@ -16,6 +16,7 @@
 #include <vector>
 
 #include "absl/base/attributes.h"
+#include "absl/base/dynamic_annotations.h"
 #include "absl/base/prefetch.h"
 #include "absl/container/internal/layout.h"
 #include "absl/log/absl_check.h"
@@ -584,6 +585,7 @@ ArenaBlock* ThreadSafeArena::FirstBlock(void* buf, size_t size) {
     return SentryArenaBlock();
   }
   // Record user-owned block.
+  ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(buf, size);
   alloc_policy_.set_is_user_owned_initial_block(true);
   return new (buf) ArenaBlock{nullptr, size};
 }
@@ -600,6 +602,7 @@ ArenaBlock* ThreadSafeArena::FirstBlock(void* buf, size_t size,
   } else {
     mem = {buf, size};
     // Record user-owned block.
+    ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(buf, size);
     alloc_policy_.set_is_user_owned_initial_block(true);
   }
 
@@ -795,6 +798,8 @@ uint64_t ThreadSafeArena::Reset() {
     size_t offset = alloc_policy_.get() == nullptr
                         ? kBlockHeaderSize
                         : kBlockHeaderSize + kAllocPolicySize;
+    ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(static_cast<char*>(mem.p) + offset,
+                                          mem.n - offset);
     first_arena_.Init(new (mem.p) ArenaBlock{nullptr, mem.n}, offset);
   } else {
     first_arena_.Init(SentryArenaBlock(), 0);