Add log to file

Enable logging to a file via

```
options(jupyter.log_level = 3) # log debug
options(jupyter.log_to_file = TRUE) # log to file
options(jupyter.logfile = 'irkernel.log') # default, relative to current dir!
```

The default log is 'irkernel.log' in the current directory. This probably should
change, but I haven't yet looked where the other kernels log to...

R/logging.r

@@ -7,19 +7,53 @@
 stderror <- stderr()
 
 log_debug <- function(...) {
-    if (getOption('jupyter.log_level') >= 3L) {
-        cat('DEBUG: ', sprintf(...), '\n', sep = '', file = stderror)
+    if (isTRUE(getOption('jupyter.log_level') >= 3L)) {
+        log_msg_stderror('DEBUG', sprintf(...))
+        log_msg_logfile('DEBUG', sprintf(...))
     }
 }
 
 log_info <- function(...) {
-    if (getOption('jupyter.log_level') >= 2L) {
-        cat('INFO: ', sprintf(...), '\n', sep = '', file = stderror)
+    if (isTRUE(getOption('jupyter.log_level') >= 2L)) {
+        log_msg_stderror('INFO', sprintf(...))
+        log_msg_logfile('INFO', sprintf(...))
     }
 }
 
 log_error <- function(...) {
-    if (getOption('jupyter.log_level') >= 1L) {
-        cat('INFO: ', sprintf(...), '\n', sep = '', file = stderror)
+    if (isTRUE(getOption('jupyter.log_level') >= 1L)) {
+        log_msg_stderror('ERROR', sprintf(...))
+        log_msg_logfile('ERROR', sprintf(...))
     }
 }
+
+log_msg_stderror <- function(lvl, msg){
+    cat(sprintf('%s: %s\n', lvl, msg) , file = stderror)
+}
+
+.is_changed_logfile <- local({
+	old_logfile <- ""
+	function(logfile) {
+		if (old_logfile != logfile) {
+			old_logfile <<- old_logfile
+			TRUE
+		} else {
+			FALSE
+		}
+	}
+})
+
+log_msg_logfile <- function(lvl, msg){
+	if (isTRUE(getOption('jupyter.log_to_file'))) {
+		# Use the default again, so we don't crash here if a user remove this option
+		# TODO: use a better default somewhere in the users home dir? Wherever jupyter itself logs to..
+		# TODO: this is depended on the current working dir...
+		cur_logfile <- getOption('jupyter.logfile', "irkernel.log")
+		if (.is_changed_logfile(cur_logfile)) {
+			log_msg_stderror('INFO', sprintf('Logging to %s', cur_logfile))
+		}
+		log_con <- file(cur_logfile, open='ab')
+		writeBin(charToRaw(sprintf('%s %s: %s\n', format(Sys.time()), lvl, msg)), log_con, endian='little')
+		close(log_con)
+	}
+}

R/options.r

@@ -12,6 +12,8 @@ getenv_default <- function(varname, default) {
 #' @export
 jupyter_option_defaults <- list(
     jupyter.log_level = as.integer(getenv_default('JUPYTER_LOG_LEVEL', 1L)),
+    jupyter.log_to_file = FALSE,
+    jupyter.logfile = getenv_default('JUPYTER_LOGFILE', 'irkernel.log'),
     jupyter.pager_classes = c(
         'help_files_with_topic'),
     jupyter.plot_mimetypes = c(