MB-29501: Validate cookie before doing a TRACE_BEGIN/TRACE_END

The tracing macros TRACE_BEGIN/TRACE_END require that a valid
cookie is passed in to these macros. So, ensure that the cookie
is valid and then call TRACE_BEGIN in the VBucketBGFetchItem
constructor

Change-Id: I7b1f60082dc4586354933c4bc24fe6e8ade17178
Reviewed-on: http://review.couchbase.org/93719
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: Sriram Ganesan

engines/ep/src/ep_vb.cc

@@ -187,10 +187,12 @@ ENGINE_ERROR_CODE EPVBucket::completeBGFetchForSingleItem(
     updateBGStats(fetched_item.initTime, startTime, fetchEnd);
 
     // Close the BG_WAIT span; and add a BG_LOAD span
-    TRACE_END(fetched_item.cookie, cb::tracing::TraceCode::BG_WAIT, startTime);
-    TRACE_BEGIN(
-            fetched_item.cookie, cb::tracing::TraceCode::BG_LOAD, startTime);
-    TRACE_END(fetched_item.cookie, cb::tracing::TraceCode::BG_LOAD, fetchEnd);
+    if (fetched_item.cookie) {
+        TRACE_END(fetched_item.cookie, cb::tracing::TraceCode::BG_WAIT, startTime);
+        TRACE_BEGIN(
+                  fetched_item.cookie, cb::tracing::TraceCode::BG_LOAD, startTime);
+        TRACE_END(fetched_item.cookie, cb::tracing::TraceCode::BG_LOAD, fetchEnd);
+    }
 
     return status;
 }

engines/ep/src/vbucket_bgfetch_item.h

@@ -36,7 +36,9 @@ class VBucketBGFetchItem {
           cookie(c),
           initTime(init_time),
           metaDataOnly(meta_only) {
-        TRACE_BEGIN(cookie, cb::tracing::TraceCode::BG_WAIT, init_time);
+        if (cookie) {
+            TRACE_BEGIN(cookie, cb::tracing::TraceCode::BG_WAIT, init_time);
+        }
     }
 
     GetValue* value;