Merge branch 'linux-version-construct-on-first-use' into 'master'

[LinuxVersion] Use "Construct on first use" idiom and add mutex

See merge request !348

emper/lib/LinuxVersion.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
-// Copyright © 2021 Florian Fischer
+// Copyright © 2021-2022 Florian Fischer, Florian Schmaus
 #include "lib/LinuxVersion.hpp"
 
 #include <sys/utsname.h>
 
 #include <cerrno>
 #include <cstdlib>
+#include <mutex>
 
 #include "Common.hpp"
 
@@ -35,19 +36,30 @@ static auto checked_strtol(const std::string& s) -> long {
 
 namespace emper::lib {
 
-std::string LinuxVersion::globalVersion;
-
-LinuxVersion::LinuxVersion() {
-	if (globalVersion.empty()) {
-		struct utsname buf;
-		if (uname(&buf)) {
-			DIE_MSG_ERRNO("Failed to invoke uname()");
+auto LinuxVersion::getUtsRelease() -> const std::string& {
+	// Use "construct (members) on first use idiom" to prevent the "static initialization order
+	// fiasco/problem".
+	static std::string utsRelease;
+	// This mutex will become necessary once we call getUtsRelease(),
+	// and by implication LinuxVersion's constructor, concurrently. So
+	// it can not hurt to have it right now.
+	static std::mutex utsReleaseMutex;
+
+	{
+		const std::lock_guard<std::mutex> lock(utsReleaseMutex);
+		if (utsRelease.empty()) {
+			struct utsname buf;
+			if (uname(&buf)) {
+				DIE_MSG_ERRNO("Failed to invoke uname()");
+			}
+			utsRelease = std::string(buf.release);
 		}
-		globalVersion = std::string(buf.release);
 	}
-	version = globalVersion;
+	return utsRelease;
 }
 
+LinuxVersion::LinuxVersion() { version = getUtsRelease(); }
+
 // Returns 1 if s is smaller, -1 if this is smaller, 0 if equal
 auto LinuxVersion::compare(const std::string& s) -> int {
 	size_t dot_pos, dot_pos2;

emper/lib/LinuxVersion.hpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
-// Copyright © 2021 Florian Fischer
+// Copyright © 2021-2022 Florian Fischer, Florian Schmaus
 #pragma once
 
 #include <string>
@@ -9,7 +9,7 @@ class TestLinuxVersion;
 namespace emper::lib {
 class LinuxVersion {
 	friend class ::TestLinuxVersion;
-	static std::string globalVersion;
+	static auto getUtsRelease() -> const std::string&;
 	std::string version;
 
 	auto compare(const std::string& s) -> int;