In MemoryRegion, use swap on move-assignment.

PiperOrigin-RevId: 716714484
Change-Id: I309bbfb68f31941f60fad4c01cb62ffe992f8e29

Author: laramiel

tensorstore/internal/flat_cord_builder.h

@@ -38,7 +38,6 @@ namespace internal {
 /// likely to be retained.
 class FlatCordBuilder {
  public:
-  FlatCordBuilder() = default;
   explicit FlatCordBuilder(size_t size) : FlatCordBuilder(size, size) {}
 
   explicit FlatCordBuilder(internal_os::MemoryRegion region)

tensorstore/internal/os/BUILD

@@ -297,6 +297,7 @@ tensorstore_cc_test(
     srcs = ["memory_region_test.cc"],
     deps = [
         ":memory_region",
+        "@com_google_absl//absl/strings:cord",
         "@com_google_googletest//:gtest_main",
     ],
 )

tensorstore/internal/os/memory_region.h

@@ -40,16 +40,21 @@ class MemoryRegion {
   MemoryRegion(const MemoryRegion&) = delete;
   MemoryRegion& operator=(const MemoryRegion&) = delete;
 
-  MemoryRegion(MemoryRegion&& other) { *this = std::move(other); }
+  MemoryRegion(MemoryRegion&& other)
+      : data_(std::exchange(other.data_, nullptr)),
+        size_(std::exchange(other.size_, 0)),
+        unmap_fn_(other.unmap_fn_) {}
+
   MemoryRegion& operator=(MemoryRegion&& other) {
-    data_ = std::exchange(other.data_, nullptr);
-    size_ = std::exchange(other.size_, 0);
-    unmap_fn_ = std::exchange(other.unmap_fn_, nullptr);
+    std::swap(data_, other.data_);
+    std::swap(size_, other.size_);
+    std::swap(unmap_fn_, other.unmap_fn_);
     return *this;
   }
 
   size_t size() const { return size_; }
-  char* data() const { return data_; }
+  const char* data() const { return data_; }
+  char* data() { return data_; }
 
   std::string_view as_string_view() const {
     return std::string_view(data_, size_);

tensorstore/internal/os/memory_region_test.cc

@@ -16,15 +16,35 @@
 
 #include <stddef.h>
 
+#include <utility>
+
 #include <gtest/gtest.h>
+#include "absl/strings/cord.h"
 
 using ::tensorstore::internal_os::AllocateHeapRegion;
 
 namespace {
 
+TEST(MemoryRegionTest, EmptyRegion) {
+  auto region = AllocateHeapRegion(0);
+  EXPECT_EQ(region.as_string_view().size(), 0);
+  absl::Cord a = std::move(region).as_cord();
+}
+
+TEST(MemoryRegionTest, Assignment) {
+  auto region = AllocateHeapRegion(0);
+  for (int i = 0; i < 10; ++i) region = AllocateHeapRegion(i);
+}
+
 TEST(MemoryRegionTest, AllocateHeapRegion) {
   auto region = AllocateHeapRegion(16 * 1024 * 1024);
   EXPECT_EQ(region.as_string_view().size(), 16 * 1024 * 1024);
+
+  // Verify that assignment doesn't leak.
+  region = AllocateHeapRegion(16);
+  EXPECT_EQ(region.as_string_view().size(), 16);
+
+  absl::Cord a = std::move(region).as_cord();
 }
 
 }  // namespace