[PATCH v4] log: Add rate-limiting macros for log messages

[Anshu Kumari <anskuma@redhat.com>]

Currently, some log messages that would be useful at info or warn level
are kept at debug level because there is no way to throttle them, and a
guest could otherwise flood the host logs.

Add a logmsg_ratelimit() macro that uses per-call-site static variables
to independently track each call site's rate.  It allows up to
LOG_RATELIMIT_BURST (5) messages per LOG_RATELIMIT_INTERVAL (1 second)
window, then prints a suppression notice.  When a new window opens and
messages were suppressed, the count is reported after the next allowed
message.

Link: https://bugs.passt.top/show_bug.cgi?id=134
Signed-off-by: Anshu Kumari <anskuma@redhat.com>
---
v4:
   - Print suppression notice immediately after the last allowed message,
     not on the next call, to avoid confusion with unrelated messages
     in between.
   - Add Link: tag before Signed-off-by
---
 log.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/log.h b/log.h
index 6ceb686..dbab006 100644
--- a/log.h
+++ b/log.h
@@ -48,6 +48,50 @@ void logmsg_perror(int pri, const char *format, ...)
 		passt_exit(EXIT_FAILURE);				\
 	} while (0)
 
+#define LOG_RATELIMIT_INTERVAL	1	/* Default rate limit window in seconds */
+#define LOG_RATELIMIT_BURST	5	/* Max messages per window per call site */
+
+/**
+ * logmsg_ratelimit() - Log a message with rate limiting
+ * @fn:		Logging function name (e.g. warn, info, debug)
+ * @now:	Current timestamp
+ */
+#define logmsg_ratelimit(fn, now, ...)					\
+	do {								\
+		static unsigned int rl_suppressed_;			\
+		static unsigned int rl_printed_;			\
+		static time_t rl_last_;					\
+									\
+		if ((now)->tv_sec - rl_last_ > LOG_RATELIMIT_INTERVAL) {\
+			rl_last_ = (now)->tv_sec;			\
+			rl_printed_ = 0;				\
+		}							\
+									\
+		if (rl_printed_ < LOG_RATELIMIT_BURST) {		\
+			fn(__VA_ARGS__);				\
+			if (rl_suppressed_) {				\
+				fn("(suppressed %u similar messages)",	\
+				   rl_suppressed_);			\
+				rl_suppressed_ = 0;			\
+			}						\
+			rl_printed_++;					\
+			if (rl_printed_ == LOG_RATELIMIT_BURST)		\
+				fn("(suppressing further similar"	\
+				   " messages)");			\
+		} else {						\
+			rl_suppressed_++;				\
+		}							\
+	} while (0)
+
+#define err_ratelimit(now, ...)						\
+	logmsg_ratelimit(err, now, __VA_ARGS__)
+#define warn_ratelimit(now, ...)					\
+	logmsg_ratelimit(warn, now, __VA_ARGS__)
+#define info_ratelimit(now, ...)					\
+	logmsg_ratelimit(info, now, __VA_ARGS__)
+#define debug_ratelimit(now, ...)					\
+	logmsg_ratelimit(debug, now, __VA_ARGS__)
+
 extern int log_file;
 extern int log_trace;
 extern bool log_conf_parsed;
-- 
2.53.0