Fs_log: new truncate behavior

Files are truncated at the next client session if all previous sessions
associated with the file are closed.

Issue #1538

repos/os/src/server/fs_log/log_file.h

@@ -36,6 +36,7 @@ class Fs_log::Log_file : public List<Log_file>::Element
 		File_system::Session &_fs;
 		File_handle           _handle;
 		seek_off_t            _offset;
+		int                   _clients;
 
 	public:
 
@@ -46,12 +47,14 @@ class Fs_log::Log_file : public List<Log_file>::Element
 		         char const *dir_path, char const *file_name,
 		         seek_off_t offset)
 		:
-			_fs(fs), _handle(handle), _offset(offset)
+			_fs(fs), _handle(handle), _offset(offset), _clients(0)
 		{
 			strncpy(_dir_path,   dir_path,  sizeof(_dir_path));
 			strncpy(_file_name, file_name, sizeof(_file_name));
 		}
 
+		~Log_file() { _fs.close(_handle); }
+
 		bool match(char const *dir, char const *filename) const
 		{
 			return
@@ -59,6 +62,10 @@ class Fs_log::Log_file : public List<Log_file>::Element
 				(strcmp(_file_name, filename, MAX_NAME_LEN) == 0);
 		}
 
+		void incr() { ++_clients; }
+		void decr() { --_clients; }
+		int client_count() const { return _clients; }
+
 		/**
 		 * Write a log message to the packet buffer.
 		 */

repos/os/src/server/fs_log/main.cc

@@ -212,6 +212,16 @@ class Fs_log::Root_component :
 			return new (md_alloc()) Unlabeled_session_component(*file);
 		}
 
+		void _destroy_session(Session_component *session)
+		{
+			Log_file *file = session->file();
+			destroy(md_alloc(), session);
+			if (file->client_count() < 1) {
+				_log_files.remove(file);
+				destroy(env()->heap(), file);
+			}
+		}
+
 	public:
 
 		/**

repos/os/src/server/fs_log/session.h

@@ -33,15 +33,24 @@ namespace Fs_log {
 
 class Fs_log::Session_component : public Rpc_object<Log_session, Unlabeled_session_component>
 {
+	protected:
+
+		Log_file &_log_file;
+
 	public:
+
+		Session_component(Log_file &log_file)
+		: _log_file(log_file) { _log_file.incr(); }
+
+		~Session_component() { _log_file.decr(); }
+
+		Log_file *file() const { return &_log_file; }
+
 		virtual size_t write(String const &string) = 0;
 };
 
 class Fs_log::Unlabeled_session_component : public Session_component
 {
-	private:
-
-		Log_file &_log_file;
 
 	public:
 
@@ -49,7 +58,7 @@ class Fs_log::Unlabeled_session_component : public Session_component
 		 * Constructor
 		 */
 		Unlabeled_session_component(Log_file &log_file)
-		: _log_file(log_file) { }
+		: Session_component(log_file) { }
 
 		/*****************
 		 ** Log session **
@@ -75,7 +84,6 @@ class Fs_log::Labeled_session_component : public Session_component
 
 		char      _label[Log_session::String::MAX_SIZE];
 		size_t    _label_len;
-		Log_file &_log_file;
 
 	public:
 
@@ -83,7 +91,7 @@ class Fs_log::Labeled_session_component : public Session_component
 		 * Constructor
 		 */
 		Labeled_session_component(char const *label, Log_file &log_file)
-		: _log_file(log_file)
+		: Session_component(log_file)
 		{
 			snprintf(_label, sizeof(_label), "[%s] ", label);
 			_label_len = strlen(_label);