util: Add RAII directory lock

This makes it easier for a class or a struct to own a lock on a
directory for the duration of its lifetime. It is used in the next
commit.

src/test/util_tests.cpp

@@ -1291,6 +1291,30 @@ BOOST_AUTO_TEST_CASE(test_LockDirectory)
     BOOST_CHECK_EQUAL(processstatus, 0);
     BOOST_CHECK_EQUAL(util::LockDirectory(dirname, lockname, true), util::LockResult::Success);
 
+    {
+        auto lock{DirectoryLock(dirname, "test")};
+        BOOST_CHECK_THROW(DirectoryLock(dirname, "test"), std::runtime_error);
+    }
+    {
+        BOOST_CHECK_NO_THROW(DirectoryLock(dirname, "test"));
+    }
+
+    {
+        DirectoryLock lock1(dirname, "test");
+        DirectoryLock lock2(std::move(lock1));
+        BOOST_CHECK_THROW(DirectoryLock(dirname, "test"), std::runtime_error);
+    }
+
+    {
+        auto dirname_move = dirname / "move";
+        fs::create_directories(dirname_move);
+        DirectoryLock lock1(dirname, "test");
+        DirectoryLock lock2(dirname_move, "test");
+        lock2 = std::move(lock1);
+        BOOST_CHECK_THROW(DirectoryLock(dirname, "test"), std::runtime_error);
+        BOOST_CHECK_NO_THROW(DirectoryLock(dirname_move, "test"));
+    }
+
     // Restore SIGCHLD
     signal(SIGCHLD, old_handler);
     BOOST_CHECK_EQUAL(close(fd[1]), 0); // Close our side of the socketpair

src/util/fs_helpers.cpp

@@ -11,6 +11,7 @@
 #include <sync.h>
 #include <util/fs.h>
 #include <util/syserror.h>
+#include <util/translation.h>
 
 #include <cerrno>
 #include <fstream>
@@ -91,6 +92,50 @@ void ReleaseDirectoryLocks()
     dir_locks.clear();
 }
 
+DirectoryLock::DirectoryLock(fs::path dir_path, std::string name)
+    : m_path{dir_path},
+      m_name{name}
+{
+    // Ensures only a single lock is taken on the provided directory.
+    switch (util::LockDirectory(m_path, ".lock", false)) {
+    case util::LockResult::ErrorWrite:
+        throw std::runtime_error(strprintf(_("Cannot write to %s directory '%s'; check permissions."), m_name, fs::PathToString(m_path)).original);
+    case util::LockResult::ErrorLock:
+        throw std::runtime_error(strprintf(_("Cannot obtain a lock on %s directory %s. %s is probably already running."), m_name, fs::PathToString(m_path), CLIENT_NAME).original);
+    case util::LockResult::Success:
+        return;
+    } // no default case, so the compiler can warn about missing cases
+    assert(false);
+}
+
+DirectoryLock::DirectoryLock(DirectoryLock&& other) noexcept
+    : m_path{std::move(other.m_path)},
+      m_name{std::move(other.m_name)}
+{
+    other.m_path.clear();
+    other.m_name.clear();
+}
+
+DirectoryLock& DirectoryLock::operator=(DirectoryLock&& other) noexcept
+{
+    if (this != &other) {
+        if (!m_path.empty()) {
+            UnlockDirectory(m_path, ".lock");
+        }
+        m_path = std::move(other.m_path);
+        other.m_path.clear();
+        m_name = std::move(other.m_name);
+        other.m_name.clear();
+    }
+    return *this;
+}
+
+
+DirectoryLock::~DirectoryLock()
+{
+    if (!m_path.empty()) UnlockDirectory(m_path, ".lock");
+}
+
 bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes)
 {
     constexpr uint64_t min_disk_space = 52428800; // 50 MiB

src/util/fs_helpers.h

@@ -45,6 +45,23 @@ enum class LockResult {
 [[nodiscard]] LockResult LockDirectory(const fs::path& directory, const fs::path& lockfile_name, bool probe_only = false);
 } // namespace util
 void UnlockDirectory(const fs::path& directory, const fs::path& lockfile_name);
+
+class DirectoryLock
+{
+    fs::path m_path;
+    std::string m_name;
+
+public:
+    explicit DirectoryLock(fs::path dir_path, std::string name);
+    ~DirectoryLock();
+
+    DirectoryLock(const DirectoryLock&) = delete;
+    DirectoryLock& operator=(const DirectoryLock&) = delete;
+
+    DirectoryLock(DirectoryLock&& other) noexcept;
+    DirectoryLock& operator=(DirectoryLock&& other) noexcept;
+};
+
 bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes = 0);
 
 /** Get the size of a file by scanning it.