Improve message queue logging

Logging the pipe buffer size on startup will tell how large it was at the
time when MaxScale read it. If there are some abnormalities in it, this
will make it visible.

Logging the worker ID when the posting of a message fails will tell which
particular worker it was. For example, if the worker in question is the
main worker (i.e. ID 0), we know there's something that's blocking the
processing.

maxutils/maxbase/src/messagequeue.cc

@@ -17,10 +17,12 @@
 #include <string.h>
 #include <unistd.h>
 #include <fstream>
+#include <mutex>
 #include <maxbase/assert.h>
 #include <maxbase/log.h>
 #include <maxbase/string.h>
 #include <maxbase/worker.hh>
+#include <maxbase/format.hh>
 
 namespace
 {
@@ -147,9 +149,20 @@ MessageQueue* MessageQueue::create(Handler* pHandler)
         if (fcntl(fds[0], F_SETPIPE_SZ, this_unit.pipe_max_size) == -1)
         {
             MXB_WARNING("Failed to increase pipe buffer size to '%d': %d, %s",
-                        this_unit.pipe_max_size,
-                        errno,
-                        mxb_strerror(errno));
+                        this_unit.pipe_max_size, errno, mxb_strerror(errno));
+        }
+        else
+        {
+            static int current_pipe_max_size = 0;
+            static std::mutex pipe_size_lock;
+            std::lock_guard<std::mutex> guard(pipe_size_lock);
+
+            if (current_pipe_max_size == 0)
+            {
+                current_pipe_max_size = this_unit.pipe_max_size;
+                MXB_NOTICE("Worker message queue size: %s",
+                           mxb::to_binary_size(this_unit.pipe_max_size).c_str());
+            }
         }
 #endif
         pThis = new(std::nothrow) MessageQueue(pHandler, read_fd, write_fd);
@@ -219,7 +232,8 @@ bool MessageQueue::post(const Message& message) const
 
         if (n == -1)
         {
-            MXB_ERROR("Failed to write message: %d, %s", errno, mxb_strerror(errno));
+            MXB_ERROR("Failed to write message to worker %d: %d, %s",
+                      m_pWorker->id(), errno, mxb_strerror(errno));
 
             static bool warn_pipe_buffer_size = true;