tools/oxenstored: Render backtraces more nicely in Syslog

fallback_exception_handler feeds a string with embedded newlines directly into
syslog().  While this is an improvement on getting nothing, syslogd escapes
all control characters it gets, and emits one (long) log line.

Fix the problem generally in the syslog stub.  As we already have a local copy
of the string, split it in place and emit one syslog() call per line.

Also tweak Logging.msg_of to avoid putting an extra newline on a string which
already ends with one.

Fixes: ee7815f49f ("tools/oxenstored: Set uncaught exception handler")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>

tools/ocaml/xenstored/logging.ml

@@ -344,7 +344,7 @@ let watch_not_fired ~con perms path =
 	access_logging ~tid:0 ~con ~data Watch_not_fired ~level:Info
 
 let msg_of exn bt =
-	Printf.sprintf "Fatal exception: %s\n%s\n" (Printexc.to_string exn)
+	Printf.sprintf "Fatal exception: %s\n%s" (Printexc.to_string exn)
 		(Printexc.raw_backtrace_to_string bt)
 
 let fallback_exception_handler exn bt =

tools/ocaml/xenstored/syslog_stubs.c

@@ -37,14 +37,34 @@ value stub_syslog(value facility, value level, value msg)
 {
 	CAMLparam3(facility, level, msg);
 	char *c_msg = strdup(String_val(msg));
+	char *s = c_msg, *ss;
 	int c_facility = __syslog_facility_table[Int_val(facility)]
 	               | __syslog_level_table[Int_val(level)];
 
 	if ( !c_msg )
 		caml_raise_out_of_memory();
-	caml_enter_blocking_section();
-	syslog(c_facility, "%s", c_msg);
-	caml_leave_blocking_section();
+
+	/*
+	 * syslog() doesn't like embedded newlines, and c_msg generally
+	 * contains them.
+	 *
+	 * Split the message in place by converting \n to \0, and issue one
+	 * syslog() call per line, skipping the final iteration if c_msg ends
+	 * with a newline anyway.
+	 */
+	do {
+		ss = strchr(s, '\n');
+		if ( ss )
+			*ss = '\0';
+		else if ( *s == '\0' )
+			break;
+
+		caml_enter_blocking_section();
+		syslog(c_facility, "%s", s);
+		caml_leave_blocking_section();
+
+		s = ss + 1;
+	} while ( ss );
 
 	free(c_msg);
 	CAMLreturn(Val_unit);