[libc] Create ErrnoCheckingTest harness to simplify errno tests.

libc/test/UnitTest/CMakeLists.txt

@@ -188,3 +188,12 @@ add_header_library(
     libc.src.__support.StringUtil.error_to_string
     libc.src.errno.errno
 )
+
+add_header_library(
+  ErrnoCheckingTest
+  HDRS
+    ErrnoCheckingTest.h
+  DEPENDS
+    libc.src.__support.common
+    libc.src.errno.errno
+)

libc/test/UnitTest/ErrnoCheckingTest.h

@@ -0,0 +1,40 @@
+//===-- ErrnoCheckingTest.h ------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TEST_UNITTEST_ERRNOCHECKINGTEST_H
+#define LLVM_LIBC_TEST_UNITTEST_ERRNOCHECKINGTEST_H
+
+#include "src/__support/macros/config.h"
+#include "src/errno/libc_errno.h"
+#include "test/UnitTest/Test.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace testing {
+
+// Provides a test fixture for tests that validate modifications of the errno.
+// It clears out the errno at the beginning of the test (e.g. in case it
+// contained the value pre-set by the system), and confirms it's still zero
+// at the end of the test, forcing the test author to explicitly account for all
+// non-zero values.
+class ErrnoCheckingTest : public Test {
+public:
+  void SetUp() override {
+    Test::SetUp();
+    LIBC_NAMESPACE::libc_errno = 0;
+  }
+
+  void TearDown() override {
+    ASSERT_ERRNO_SUCCESS();
+    Test::TearDown();
+  }
+};
+
+} // namespace testing
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_TEST_UNITTEST_ERRNOCHECKINGTEST_H

libc/test/src/unistd/CMakeLists.txt

@@ -357,6 +357,8 @@ add_libc_unittest(
     libc.src.fcntl.open
     libc.src.unistd.close
     libc.src.unistd.unlink
+    libc.test.UnitTest.ErrnoCheckingTest
+    libc.test.UnitTest.ErrnoSetterMatcher
 )
 
 add_libc_unittest(

libc/test/src/unistd/unlink_test.cpp

@@ -6,27 +6,30 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/errno/libc_errno.h"
 #include "src/fcntl/open.h"
 #include "src/unistd/close.h"
 #include "src/unistd/unlink.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
 #include "test/UnitTest/ErrnoSetterMatcher.h"
 #include "test/UnitTest/Test.h"
 
 #include <sys/stat.h>
 
-TEST(LlvmLibcUnlinkTest, CreateAndUnlink) {
+using LlvmLibcUnlinkTest = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+TEST_F(LlvmLibcUnlinkTest, CreateAndUnlink) {
   using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
   constexpr const char *FILENAME = "unlink.test";
   auto TEST_FILE = libc_make_test_file_path(FILENAME);
   int write_fd = LIBC_NAMESPACE::open(TEST_FILE, O_WRONLY | O_CREAT, S_IRWXU);
   ASSERT_ERRNO_SUCCESS();
   ASSERT_GT(write_fd, 0);
+
   ASSERT_THAT(LIBC_NAMESPACE::close(write_fd), Succeeds(0));
   ASSERT_THAT(LIBC_NAMESPACE::unlink(TEST_FILE), Succeeds(0));
 }
 
-TEST(LlvmLibcUnlinkTest, UnlinkNonExistentFile) {
+TEST_F(LlvmLibcUnlinkTest, UnlinkNonExistentFile) {
   using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Fails;
   ASSERT_THAT(LIBC_NAMESPACE::unlink("non-existent-file"), Fails(ENOENT));
 }

utils/bazel/llvm-project-overlay/libc/test/UnitTest/BUILD.bazel

@@ -73,6 +73,17 @@ libc_test_library(
     ],
 )
 
+libc_test_library(
+    name = "errno_test_helpers",
+    hdrs = [
+        "ErrnoCheckingTest.h",
+    ],
+    deps = [
+        ":LibcUnitTest",
+        "//libc:errno",
+    ],
+)
+
 libc_test_library(
     name = "fp_test_helpers",
     srcs = [

utils/bazel/llvm-project-overlay/libc/test/src/unistd/BUILD.bazel

@@ -248,6 +248,9 @@ libc_test(
         "//libc:close",
         "//libc:unlink",
     ],
+    deps = [
+        "//libc/test/UnitTest:errno_test_helpers",
+    ],
 )
 
 # libc_test(
@@ -261,7 +264,6 @@ libc_test(
 #     ],
 # )
 
-
 libc_test(
     name = "getppid_test",
     srcs = ["getppid_test.cpp"],