Add a generic rate-limited log mechanism, and use it in a few places

Incidentally fixes bug 1042.

2 files changed, 71 insertions, 0 deletions

diff --git a/src/common/util.c b/src/common/util.c
index 6830ef3aa4..3c735e9c7c 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -1578,6 +1578,49 @@ ftime_definitely_before(time_t now, time_t when)
 }
 
 /* =====
+ * Rate limiting
+ * ===== */
+
+/** If the rate-limiter <b>lim</b> is ready at <b>now</b>, return the number
+ * of calls to rate_limit_is_ready (including this one!) since the last time
+ * rate_limit_is_ready returned nonzero.  Otherwise return 0. */
+int
+rate_limit_is_ready(ratelim_t *lim, time_t now)
+{
+  if (lim->rate + lim->last_allowed >= now) {
+    int res = lim->n_calls_since_last_time + 1;
+    lim->last_allowed = now;
+    lim->n_calls_since_last_time = 0;
+    return res;
+  } else {
+    ++lim->n_calls_since_last_time;
+    return 0;
+  }
+}
+
+/** If the rate-limiter <b>lim</b> is ready at <b>now</b>, return a newly
+ * allocated string indicating how many messages were suppressed, suitable to
+ * append to a log message.  Otherwise return NULL. */
+char *
+rate_limit_log(ratelim_t *lim, time_t now)
+{
+  int n;
+  if ((n = rate_limit_is_ready(lim, now))) {
+    if (n == 1) {
+      return tor_strdup("");
+    } else {
+      char *cp=NULL;
+      tor_asprintf(&cp,
+                   " [%d similar message(s) suppressed in last %d seconds]",
+                   n-1, lim->rate);
+      return cp;
+    }
+  } else {
+    return NULL;
+  }
+}
+
+/* =====
  * File helpers
  * ===== */
 
diff --git a/src/common/util.h b/src/common/util.h
index ba38f4c7ed..1e22bd5b1b 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -252,6 +252,34 @@ int ftime_maybe_before(time_t now, time_t when);
 int ftime_definitely_after(time_t now, time_t when);
 int ftime_definitely_before(time_t now, time_t when);
 
+/* Rate-limiter */
+
+/** A ratelim_t remembers how often an event is occurring, and how often
+ * it's allowed to occur.  Typical usage is something like:
+ *
+   <pre>
+    if (possibly_very_frequent_event()) {
+      const int INTERVAL = 300;
+      static ratelim_t warning_limit = RATELIM_INIT(INTERVAL);
+      char *m;
+      if ((m = rate_limit_log(&warning_limit, approx_time()))) {
+        log_warn(LD_GENERAL, "The event occurred!%s", m);
+        tor_free(m);
+      }
+    }
+   </pre>
+ */
+typedef struct ratelim_t {
+  int rate;
+  time_t last_allowed;
+  int n_calls_since_last_time;
+} ratelim_t;
+
+#define RATELIM_INIT(r) { (r), 0, 0 }
+
+int rate_limit_is_ready(ratelim_t *lim, time_t now);
+char *rate_limit_log(ratelim_t *lim, time_t now);
+
 /* File helpers */
 ssize_t write_all(int fd, const char *buf, size_t count, int isSocket);
 ssize_t read_all(int fd, char *buf, size_t count, int isSocket);