Fix log granularity based on Nick's comments.

Instead of rejecting a value that doesn't divide into 1 second, round to
the nearest divisor of 1 second and warn.

Document that the option only controls the granularity written by Tor to a
file or console log. It does not (for example) "batch up" log messages to
affect times logged by a controller, times attached to syslog messages, or
the mtime fields on log files.

doc/tor.1.txt

@@ -346,7 +346,10 @@ Other options can be specified either on the command-line (--option
 **LogTimeGranularity** __NUM__::
     Set the resolution of timestamps in Tor's logs to NUM milliseconds.
     NUM must be positive and either a divisor or a multiple of 1 second.
-    (Default: 1 second)
+    Note that this option only controls the granularity written by Tor to
+    a file or console log.  Tor does not (for example) "batch up" log
+    messages to affect times logged by a controller, times attached to
+    syslog messages, or the mtime fields on log files.  (Default: 1 second)
 
 **SafeLogging** **0**|**1**|**relay**::
     Tor can scrub potentially sensitive strings from log messages (e.g.

src/or/config.c

@@ -4342,15 +4342,31 @@ options_init_logs(or_options_t *options, int validate_only)
                options->RunAsDaemon;
 #endif
 
-  if (options->LogTimeGranularity > 0 &&
-      (1000 % options->LogTimeGranularity == 0 ||
-       options->LogTimeGranularity % 1000 == 0)) {
-    set_log_time_granularity(options->LogTimeGranularity);
-  } else {
-    log_warn(LD_CONFIG, "Log time granularity '%d' has to be positive "
-             "and either a divisor or a multiple of 1 second.",
+  if (options->LogTimeGranularity <= 0) {
+    log_warn(LD_CONFIG, "Log time granularity '%d' has to be positive.",
              options->LogTimeGranularity);
     return -1;
+  } else if (1000 % options->LogTimeGranularity != 0 &&
+             options->LogTimeGranularity % 1000 != 0) {
+    int granularity = options->LogTimeGranularity;
+    if (granularity < 40) {
+      do granularity++;
+      while (1000 % granularity != 0);
+    } else if (granularity < 1000) {
+      granularity = 1000 / granularity;
+      while (1000 % granularity != 0)
+        granularity--;
+      granularity = 1000 / granularity;
+    } else {
+      granularity = 1000 * ((granularity / 1000) + 1);
+    }
+    log_warn(LD_CONFIG, "Log time granularity '%d' has to be either a "
+                        "divisor or a multiple of 1 second. Changing to "
+                        "'%d'.",
+             options->LogTimeGranularity, granularity);
+    set_log_time_granularity(granularity);
+  } else {
+    set_log_time_granularity(options->LogTimeGranularity);
   }
 
   ok = 1;