Validate absl::StringResizeAndOverwrite op has written bytes as expected.

PiperOrigin-RevId: 820383934
Change-Id: I45be1c15cd2080772cea9bfe267041be5b530832

Author: ckennelly

absl/strings/BUILD.bazel

@@ -150,6 +150,7 @@ cc_library(
     deps = [
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:dynamic_annotations",
         "//absl/base:throw_delegate",
     ],
 )
@@ -161,6 +162,7 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":resize_and_overwrite",
+        "//absl/base:dynamic_annotations",
         "//absl/log:absl_check",
         "@googletest//:gtest",
         "@googletest//:gtest_main",

absl/strings/CMakeLists.txt

@@ -151,6 +151,7 @@ absl_cc_library(
   DEPS
     absl::config
     absl::core_headers
+    absl::dynamic_annotations
     absl::throw_delegate
 )
 
@@ -164,6 +165,7 @@ absl_cc_test(
   DEPS
     absl::strings_resize_and_overwrite
     absl::absl_check
+    absl::dynamic_annotations
     GTest::gmock_main
 )
 

absl/strings/resize_and_overwrite.h

@@ -52,6 +52,7 @@
 #include <utility>
 
 #include "absl/base/config.h"
+#include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/throw_delegate.h"
 #include "absl/base/macros.h"
 #include "absl/base/optimization.h"
@@ -125,7 +126,15 @@ void StringResizeAndOverwriteFallback(T& str, typename T::size_type n, Op op) {
   if (ABSL_PREDICT_FALSE(n > str.max_size())) {
     absl::base_internal::ThrowStdLengthError("absl::StringResizeAndOverwrite");
   }
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+  auto old_size = str.size();
+#endif
   str.resize(n);
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+  if (old_size < n) {
+    ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(str.data() + old_size, n - old_size);
+  }
+#endif
   auto new_size = std::move(op)(str.data(), n);
   ABSL_HARDENING_ASSERT(new_size >= 0 && new_size <= n);
   ABSL_HARDENING_ASSERT(str.data()[n] == typename T::value_type{});
@@ -167,6 +176,10 @@ void StringResizeAndOverwrite(T& str, typename T::size_type n, Op op) {
     strings_internal::StringResizeAndOverwriteFallback(str, n, op);
   }
 #endif
+#if defined(ABSL_HAVE_MEMORY_SANITIZER)
+  auto shadow = __msan_test_shadow(str.data(), str.size());
+  ABSL_ASSERT(shadow == -1);
+#endif
 }
 
 ABSL_NAMESPACE_END

absl/strings/resize_and_overwrite_test.cc

@@ -19,6 +19,7 @@
 #include <string>
 
 #include "gtest/gtest.h"
+#include "absl/base/dynamic_annotations.h"
 #include "absl/log/absl_check.h"
 
 namespace {
@@ -94,6 +95,39 @@ TEST_P(StringResizeAndOverwriteTest, StringResizeAndOverwriteFallback) {
   EXPECT_EQ(s.c_str()[param.final_size], '\0');
 }
 
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+constexpr bool kMSan = true;
+#else
+constexpr bool kMSan = false;
+#endif
+
+TEST_P(StringResizeAndOverwriteTest, Initialized) {
+  if (!kMSan) {
+    GTEST_SKIP() << "Skipping test without MSan.";
+  }
+
+  const auto& param = GetParam();
+  std::string s(param.initial_size, 'a');
+
+  auto op = [&]() {
+    absl::StringResizeAndOverwrite(s, param.requested_capacity,
+                                   [&](char*, size_t) {
+                                     // Fail to initialize the buffer in full.
+                                     return param.final_size;
+                                   });
+  };
+
+  if (param.initial_size < param.final_size) {
+#ifndef NDEBUG
+    EXPECT_DEATH_IF_SUPPORTED(op(), "shadow == -1");
+#endif
+  } else {
+    // The string is fully initialized from the initial constructor, or we skip
+    // the check in optimized builds.
+    op();
+  }
+}
+
 // clang-format off
 INSTANTIATE_TEST_SUITE_P(StringResizeAndOverwriteTestSuite,
                          StringResizeAndOverwriteTest,