tweak log file name construction, support strftime pattern (#546)

docs/mintty.1

@@ -1124,6 +1124,11 @@ see the note for the -i option above.
 The \fBLog\fP setting can be used to specify a log file that all output is
 copied into.  If it is empty, as it is by default, no logging is done.
 If it contains \fB%d\fP it will be substituted with the process ID.
+If it contains \fB%\fP placeholders other than a single \fB%d\fP, 
+the log file name will be constructed by calling strftime(3) on the 
+pattern; note that this is likely to fail if a placeholder expands with 
+"/" (%D).
+
 See also the \fIscript\fP(1) utility for a more flexible logging solution.
 
 .TP

src/child.c

@@ -208,13 +208,21 @@ child_create(char *argv[], struct winsize *winp)
       char * log = path_win_w_to_posix(cfg.log);
       char * format = strchr(log, '%');
       if (format && * ++ format == 'd' && !strchr(format, '%')) {
-        char logf[strlen(log + 20)];
+        char * logf = newn(char, strlen(log) + 20);
         sprintf(logf, log, getpid());
-        log_fd = open(logf, O_WRONLY | O_CREAT | O_TRUNC, 0600);
+        free(log);
+        log = logf;
+      }
+      else if (format) {
+        struct timeval now;
+        gettimeofday (& now, 0);
+        char * logf = newn(char, MAX_PATH + 1);
+        strftime (logf, MAX_PATH, log, localtime (& now.tv_sec));
+        free(log);
+        log = logf;
       }
-      else
-        log_fd = open(log, O_WRONLY | O_CREAT | O_TRUNC, 0600);
 
+      log_fd = open(log, O_WRONLY | O_CREAT | O_EXCL, 0600);
       if (log_fd < 0) {
         // report message and filename:
         childerror("could not open log file", false);