fix: concurrent Lazy::Get should get correct result

ci/scripts/build_example.sh

@@ -26,7 +26,7 @@ mkdir ${build_dir}
 pushd ${build_dir}
 
 is_windows() {
-    [[ "${OSTYPE}" == "msys" || "${OSTYPE}" == "win32" ]]
+    [[ "${OSTYPE}" == "msys" || "${OSTYPE}" == "win32" || "${OSTYPE}" == "cygwin" ]]
 }
 
 CMAKE_ARGS=(

ci/scripts/build_iceberg.sh

@@ -28,7 +28,7 @@ mkdir ${build_dir}
 pushd ${build_dir}
 
 is_windows() {
-    [[ "${OSTYPE}" == "msys" || "${OSTYPE}" == "win32" ]]
+    [[ "${OSTYPE}" == "msys" || "${OSTYPE}" == "win32" || "${OSTYPE}" == "cygwin" ]]
 }
 
 CMAKE_ARGS=(

src/iceberg/test/CMakeLists.txt

@@ -116,6 +116,7 @@ add_iceberg_test(util_test
                  decimal_test.cc
                  endian_test.cc
                  formatter_test.cc
+                 lazy_test.cc
                  location_util_test.cc
                  roaring_position_bitmap_test.cc
                  position_delete_index_test.cc

src/iceberg/test/lazy_test.cc

@@ -0,0 +1,68 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include "iceberg/util/lazy.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "iceberg/result.h"
+#include "iceberg/test/matchers.h"
+
+namespace {
+
+struct NonDefaultConstructible {
+  explicit NonDefaultConstructible(int value) : value(value) {}
+  NonDefaultConstructible() = delete;
+
+  int value;
+};
+
+iceberg::Result<NonDefaultConstructible> InitNonDefaultConstructible(int value) {
+  return NonDefaultConstructible(value);
+}
+
+iceberg::Result<int> InitAlwaysFails() { return iceberg::Invalid("init failed"); }
+
+}  // namespace
+
+TEST(LazyTest, SupportsNonDefaultConstructibleValues) {
+  const iceberg::Lazy<InitNonDefaultConstructible> lazy;
+
+  auto first = lazy.Get(42);
+  ASSERT_THAT(first, iceberg::IsOk());
+  EXPECT_EQ(first->get().value, 42);
+
+  auto second = lazy.Get(13);
+  ASSERT_THAT(second, iceberg::IsOk());
+  EXPECT_EQ(second->get().value, 42);
+  EXPECT_EQ(&first->get(), &second->get());
+}
+
+TEST(LazyTest, ReusesInitializationError) {
+  const iceberg::Lazy<InitAlwaysFails> lazy;
+
+  auto first = lazy.Get();
+  EXPECT_THAT(first, iceberg::IsError(iceberg::ErrorKind::kInvalid));
+  EXPECT_THAT(first, iceberg::HasErrorMessage("init failed"));
+
+  auto second = lazy.Get();
+  EXPECT_THAT(second, iceberg::IsError(iceberg::ErrorKind::kInvalid));
+  EXPECT_THAT(second, iceberg::HasErrorMessage("init failed"));
+}

src/iceberg/test/meson.build

@@ -89,6 +89,7 @@ iceberg_tests = {
             'decimal_test.cc',
             'endian_test.cc',
             'formatter_test.cc',
+            'lazy_test.cc',
             'location_util_test.cc',
             'position_delete_index_test.cc',
             'retry_util_test.cc',

src/iceberg/util/lazy.h

@@ -25,6 +25,7 @@
 #include <concepts>
 #include <functional>
 #include <mutex>
+#include <utility>
 
 #include "iceberg/result.h"
 #include "iceberg/util/macros.h"
@@ -48,19 +49,14 @@ class Lazy {
     requires std::invocable<decltype(InitFunc), Args...> &&
              std::same_as<std::invoke_result_t<decltype(InitFunc), Args...>, Result<T>>
   Result<std::reference_wrapper<T>> Get(Args&&... args) const {
-    Result<T> result;
-    std::call_once(flag_, [&result, this, &args...]() {
-      result = InitFunc(std::forward<Args>(args)...);
-      if (result) {
-        this->value_ = std::move(result.value());
-      }
-    });
-    ICEBERG_RETURN_UNEXPECTED(result);
-    return std::ref(value_);
+    std::call_once(
+        flag_, [this, &args...]() { value_ = InitFunc(std::forward<Args>(args)...); });
+    ICEBERG_RETURN_UNEXPECTED(value_);
+    return std::ref(*value_);
   }
 
  private:
-  mutable T value_;
+  mutable Result<T> value_ = Invalid("Lazy value has not been initialized");
   mutable std::once_flag flag_;
 };