tweak the metadata line handler to handle an edge case in trim history

lib/metadata.js

@@ -211,12 +211,14 @@ Meta.createLineHandler = function (ref, errorHandler) {
                 line: JSON.stringify(line),
             });
         }
+
+        // the case above is special, everything else should increment the index
+        var index = ref.index++;
         if (typeof(line) === 'undefined') { return; }
 
         if (Array.isArray(line)) {
             try {
                 handleCommand(ref.meta, line);
-                ref.index++;
             } catch (err2) {
                 errorHandler("METADATA_COMMAND_ERR", {
                     error: err2.stack,
@@ -226,8 +228,15 @@ Meta.createLineHandler = function (ref, errorHandler) {
             return;
         }
 
-        if (ref.index === 0 && typeof(line) === 'object') {
+        // the first line of a channel is processed before the dedicated metadata log.
-            ref.index++;
+        // it can contain a map, in which case it should be used as the initial state.
+        // it's possible that a trim-history command was interrupted, in which case
+        // this first message might exist in parallel with the more recent metadata log
+        // which will contain the computed state of the previous metadata log
+        // which has since been archived.
+        // Thus, accept both the first and second lines you process as valid initial state
+        // preferring the second if it exists
+        if (index < 2 && line typeof(line) === 'object') {
             // special case!
             ref.meta = line;
             return;
@@ -235,7 +244,7 @@ Meta.createLineHandler = function (ref, errorHandler) {
 
         errorHandler("METADATA_HANDLER_WEIRDLINE", {
             line: line,
-            index: ref.index++,
+            index: index,
         });
     };
 };