Tests: Test setjmp/sigsetjmp LibC functions

Since there are no real users of these functions in Serenity's
userland and this is my third attempt at this... This time, the great
LibTest test suite will make sure that I do it right!

Tests/LibC/CMakeLists.txt

@@ -6,6 +6,7 @@ set(TEST_SOURCES
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCExec.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCDirEnt.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCInodeWatcher.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCSetjmp.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCString.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestStackSmash.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestIo.cpp

Tests/LibC/TestLibCSetjmp.cpp

@@ -0,0 +1,151 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <setjmp.h>
+#include <signal.h>
+#include <string.h>
+
+TEST_CASE(setjmp)
+{
+    jmp_buf env;
+    volatile int set = 1;
+
+    if (setjmp(env)) {
+        EXPECT_EQ(set, 0);
+        return;
+    }
+
+    EXPECT_EQ(set, 1);
+    set = 0;
+    longjmp(env, 1);
+
+    VERIFY_NOT_REACHED();
+}
+
+TEST_CASE(setjmp_zero)
+{
+    jmp_buf env;
+    volatile int set = 1;
+
+    switch (setjmp(env)) {
+    case 0:
+        EXPECT_EQ(set, 1);
+        set = 0;
+        longjmp(env, 0);
+        VERIFY_NOT_REACHED();
+
+    case 1:
+        EXPECT_EQ(set, 0);
+        break;
+
+    default:
+        VERIFY_NOT_REACHED();
+    };
+}
+
+TEST_CASE(setjmp_value)
+{
+    jmp_buf env;
+    volatile int set = 1;
+
+    switch (setjmp(env)) {
+    case 0:
+        EXPECT_EQ(set, 1);
+        set = 0;
+        longjmp(env, 0x789ABCDE);
+        VERIFY_NOT_REACHED();
+
+    case 0x789ABCDE:
+        EXPECT_EQ(set, 0);
+        break;
+
+    default:
+        VERIFY_NOT_REACHED();
+    };
+}
+
+TEST_CASE(sigsetjmp)
+{
+    sigjmp_buf env;
+    volatile int set = 1;
+
+    if (sigsetjmp(env, 0)) {
+        EXPECT_EQ(set, 0);
+        return;
+    }
+
+    EXPECT_EQ(set, 1);
+    set = 0;
+    siglongjmp(env, 1);
+
+    VERIFY_NOT_REACHED();
+}
+
+TEST_CASE(sigsetjmp_zero)
+{
+    sigjmp_buf env;
+    volatile int set = 1;
+
+    switch (sigsetjmp(env, 0)) {
+    case 0:
+        EXPECT_EQ(set, 1);
+        set = 0;
+        siglongjmp(env, 0);
+        VERIFY_NOT_REACHED();
+
+    case 1:
+        EXPECT_EQ(set, 0);
+        break;
+
+    default:
+        VERIFY_NOT_REACHED();
+    };
+}
+
+TEST_CASE(sigsetjmp_value)
+{
+    sigjmp_buf env;
+    volatile int set = 1;
+
+    switch (sigsetjmp(env, 0)) {
+    case 0:
+        EXPECT_EQ(set, 1);
+        set = 0;
+        siglongjmp(env, 0x789ABCDE);
+        VERIFY_NOT_REACHED();
+
+    case 0x789ABCDE:
+        EXPECT_EQ(set, 0);
+        break;
+
+    default:
+        VERIFY_NOT_REACHED();
+    };
+}
+
+TEST_CASE(sigsetjmp_signal_mask)
+{
+    sigjmp_buf env;
+
+    sigset_t alternative_sigset;
+    sigfillset(&alternative_sigset);
+
+    sigset_t initial_sigset;
+    sigprocmask(SIG_SETMASK, nullptr, &initial_sigset);
+
+    EXPECT_NE(memcmp(&alternative_sigset, &initial_sigset, sizeof(sigset_t)), 0);
+
+    if (sigsetjmp(env, 1)) {
+        sigprocmask(SIG_SETMASK, nullptr, &alternative_sigset);
+        EXPECT_EQ(memcmp(&alternative_sigset, &initial_sigset, sizeof(sigset_t)), 0);
+        return;
+    }
+
+    sigprocmask(SIG_SETMASK, &alternative_sigset, nullptr);
+    siglongjmp(env, 1);
+    VERIFY_NOT_REACHED();
+}