MB-36554: Make sure the engine won't reallocate tracing vector

The memory accounting could get out of sync if the underlying
engine tried to add a trace element and the tracer needed to
reallocate memory in the vector. To work around this problem
make sure that there there is at least 10 free elements in
the trace array before calling into the engine.

Change-Id: I69144e14708bf4bcc5ee4d0c19e516cadecfebdf
Reviewed-on: http://review.couchbase.org/116928
Well-Formed: Build Bot <[email protected]>
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: trondn

daemon/mcbp_executors.cc

@@ -879,6 +879,7 @@ void mcbp_execute_packet(Cookie& cookie) {
     if (header.isResponse()) {
         execute_response_packet(cookie, header.getResponse());
     } else {
+        cookie.getTracer().ensureSpace();
         // We've already verified that the packet is a legal packet
         // so it must be a request
         execute_request_packet(cookie, header.getRequest());

tracing/tracer.cc

@@ -92,6 +92,11 @@ void Tracer::clear() {
     vecSpans.clear();
 }
 
+void Tracer::ensureSpace() {
+    const size_t minimumSpaceAvailable = 10;
+    vecSpans.reserve(vecSpans.size() + minimumSpaceAvailable);
+}
+
 } // end namespace tracing
 } // end namespace cb
 

tracing/tracer.h

@@ -110,6 +110,10 @@ class MEMCACHED_PUBLIC_CLASS Tracer {
     friend std::string(::to_string)(const cb::tracing::Tracer& tracer,
                                     bool raw);
 
+    /// Ensure that there is space in the tracer so that we won't have to do
+    /// additional memory allcation when inserting a span
+    void ensureSpace();
+
 protected:
     std::vector<Span> vecSpans;
 };