Make rageshake use less memory

If an individual log is larger than the max size, ignore the rest
of it. Also build the string as we go rather than joining it all
together at the end, that way we only need the whole string + one
chunk in memory at once, rather than the whole string x 2.

src/rageshake/rageshake.js

@@ -241,20 +241,27 @@ class IndexedDBLogStore {
 
         // Returns: a string representing the concatenated logs for this ID.
         function fetchLogs(id) {
-            const o = db.transaction("logs", "readonly").objectStore("logs");
-            return selectQuery(o.index("id"), IDBKeyRange.only(id),
-            (cursor) => {
-                return {
-                    lines: cursor.value.lines,
-                    index: cursor.value.index,
+            const objectStore = db.transaction("logs", "readonly").objectStore("logs");
+
+            return new Promise((resolve, reject) => {
+                const query = objectStore.index("id").openCursor(IDBKeyRange.only(id), 'next');
+                let lines = '';
+                query.onerror = (event) => {
+                    reject(new Error("Query failed: " + event.target.errorCode));
+                };
+                query.onsuccess = (event) => {
+                    const cursor = event.target.result;
+                    if (!cursor) {
+                        resolve(lines);
+                        return; // end of results
+                    }
+                    lines += cursor.value.lines;
+                    if (lines.length >= MAX_LOG_SIZE) {
+                        resolve(lines);
+                    } else {
+                        cursor.continue();
+                    }
                 };
-            }).then((linesArray) => {
-                // We have been storing logs periodically, so string them all
-                // together *in order of index* now
-                linesArray.sort((a, b) => {
-                    return a.index - b.index;
-                });
-                return linesArray.map((l) => l.lines).join("");
             });
         }
 
@@ -322,9 +329,6 @@ class IndexedDBLogStore {
             if (i > 0 && size + lines.length > MAX_LOG_SIZE) {
                 // the remaining log IDs should be removed. If we go out of
                 // bounds this is just []
-                //
-                // XXX: there's nothing stopping the current session exceeding
-                // MAX_LOG_SIZE. We ought to think about culling it.
                 removeLogIds = allLogIds.slice(i + 1);
                 break;
             }