libc: make mtime update configurable

By specifying <libc update_mtime="no"...>, the modification-time update
on VFS-sync operations (as issued whenever a written file is closed)
can explicitly be disabled.

Issue #1784

repos/libports/src/lib/libc/internal/kernel.h

@@ -94,8 +94,13 @@ struct Libc::Kernel final : Vfs::Io_response_handler,
 		 */
 		void reset_malloc_heap() override;
 
-		Env_implementation   _libc_env { _env, _heap };
-		Vfs_plugin           _vfs { _libc_env, _heap, *this };
+		Env_implementation _libc_env { _env, _heap };
+
+		bool const _update_mtime = _libc_env.libc_config().attribute_value("update_mtime", true);
+
+		Vfs_plugin _vfs { _libc_env, _heap, *this,
+		                  _update_mtime ? Vfs_plugin::Update_mtime::YES
+		                                : Vfs_plugin::Update_mtime::NO };
 
 		bool  const _cloned = _libc_env.libc_config().attribute_value("cloned", false);
 		pid_t const _pid    = _libc_env.libc_config().attribute_value("pid", 0U);

repos/libports/src/lib/libc/internal/vfs_plugin.h

@@ -37,11 +37,16 @@ namespace Libc { class Vfs_plugin; }
 
 class Libc::Vfs_plugin : public Plugin
 {
+	public:
+
+		enum class Update_mtime { NO, YES };
+
 	private:
 
 		Genode::Allocator        &_alloc;
 		Vfs::File_system         &_root_dir;
 		Vfs::Io_response_handler &_response_handler;
+		Update_mtime        const _update_mtime;
 
 		/**
 		 * Sync a handle and propagate errors
@@ -57,9 +62,12 @@ class Libc::Vfs_plugin : public Plugin
 
 		Vfs_plugin(Libc::Env                &env,
 		           Genode::Allocator        &alloc,
-		           Vfs::Io_response_handler &handler)
+		           Vfs::Io_response_handler &handler,
+		           Update_mtime              update_mtime)
 		:
-			_alloc(alloc), _root_dir(env.vfs()), _response_handler(handler)
+			_alloc(alloc), _root_dir(env.vfs()),
+			_response_handler(handler),
+			_update_mtime(update_mtime)
 		{ }
 
 		~Vfs_plugin() final { }

repos/libports/src/lib/libc/vfs_plugin.cc

@@ -349,6 +349,9 @@ void Libc::Vfs_plugin::_vfs_write_mtime(Vfs::Vfs_handle &handle)
 {
 	struct timespec ts;
 
+	if (_update_mtime == Update_mtime::NO)
+		return;
+
 	/* XXX using  clock_gettime directly is probably not the best idea */
 	if (clock_gettime(CLOCK_REALTIME, &ts) < 0) {
 		ts.tv_sec = 0;