common: Add a default socket name feature.

* common/logging.c (log_set_socket_dir_cb): New.
(socket_dir_cb): New.
(set_file_fd): Allow "socket://".
(fun_writer): Implement default socket name.
* common/init.c (_init_common_subsystems): Register default socket.
--

This change allows the use of

log-file socket://

in any configuration file.

Signed-off-by: Werner Koch <wk@gnupg.org>

doc/dirmngr.texi

@@ -163,7 +163,8 @@ verbose commands to @sc{dirmngr}, such as @option{-vv}.
 @item --log-file @var{file}
 @opindex log-file
 Append all logging output to @var{file}.  This is very helpful in
-seeing what the agent actually does.
+seeing what the agent actually does.  Use @file{socket://} to log to
+socket.
 
 @item --debug-level @var{level}
 @opindex debug-level

doc/gpg-agent.texi

@@ -312,11 +312,12 @@ should in general not be used to avoid X-sniffing attacks.
 @item --log-file @var{file}
 @opindex log-file
 @efindex HKCU\Software\GNU\GnuPG:DefaultLogFile
-Append all logging output to @var{file}.  This is very helpful in seeing
-what the agent actually does.  If neither a log file nor a log file
-descriptor has been set on a Windows platform, the Registry entry
-@code{HKCU\Software\GNU\GnuPG:DefaultLogFile}, if set, is used to specify
-the logging output.
+Append all logging output to @var{file}.  This is very helpful in
+seeing what the agent actually does. Use @file{socket://} to log to
+socket.  If neither a log file nor a log file descriptor has been set
+on a Windows platform, the Registry entry
+@code{HKCU\Software\GNU\GnuPG:DefaultLogFile}, if set, is used to
+specify the logging output.
 
 
 @anchor{option --no-allow-mark-trusted}

doc/gpg.texi

@@ -2697,9 +2697,8 @@ Write log output to file descriptor @code{n} and not to STDERR.
 @item --log-file @code{file}
 @itemx --logger-file @code{file}
 @opindex log-file
-Same as @option{--logger-fd}, except the logger data is written to file
-@code{file}.  Note that @option{--log-file} is only implemented for
-GnuPG-2.
+Same as @option{--logger-fd}, except the logger data is written to
+file @code{file}.  Use @file{socket://} to log to socket.
 
 @item --attribute-fd @code{n}
 @opindex attribute-fd

doc/gpgsm.texi

@@ -384,6 +384,7 @@ Do not print a warning when the so called "secure memory" cannot be used.
 @item --log-file @var{file}
 @opindex log-file
 When running in server mode, append all logging output to @var{file}.
+Use @file{socket://} to log to socket.
 
 @end table
 

doc/scdaemon.texi

@@ -239,7 +239,8 @@ debugging.
 @item --log-file @var{file}
 @opindex log-file
 Append all logging output to @var{file}.  This is very helpful in
-seeing what the agent actually does.
+seeing what the agent actually does.  Use @file{socket://} to log to
+socket.
 
 
 @item --pcsc-driver @var{library}

doc/tools.texi

@@ -103,12 +103,14 @@ This waits for connections on the local socket
 @file{/home/foo/.gnupg/S.log} and shows all log entries.  To make this
 work the option @option{log-file} needs to be used with all modules
 which logs are to be shown.  The value for that option must be given
-with a special prefix (e.g. in the conf file):
+with a special prefix (e.g. in the conf files):
 
 @example
 log-file socket:///home/foo/.gnupg/S.log
 @end example
 
+If only @code{socket://} is used a default socket file named
+@file{S.log} in the standard socket directory is used.
 For debugging purposes it is also possible to do remote logging.  Take
 care if you use this feature because the information is send in the
 clear over the network.  Use this syntax in the conf files:
@@ -1737,8 +1739,8 @@ Try to be as quiet as possible.
 
 @item --log-file @var{file}
 @opindex log-file
-Append all logging output to @var{file}.  Default is to write logging
-information to STDERR.
+Append all logging output to @var{file}.  Use @file{socket://} to log
+to socket.  Default is to write logging information to STDERR.
 
 @end table