Bug#26837 Return value ignored for packet->append() call within Log_event::read_log_event

 - Improve error handling for "out of memory" problems when master is
   sending logs to slave. If memory allocation fails the log should
   now  report error "memory allocation failed reading log event"  


sql/log_event.cc:
  Return LOG_READ_MEM from "Log_event::read_log_event" if memory
  allocation  fails.

sql/log_event.cc

@@ -669,19 +669,34 @@ int Log_event::read_log_event(IO_CACHE* file, String* packet,
 	     LOG_READ_TOO_LARGE);
     goto end;
   }
-  packet->append(buf, sizeof(buf));
+
+  /* Append the log event header to packet */
+  if (packet->append(buf, sizeof(buf)))
+  {
+    /* Failed to allocate packet */
+    result= LOG_READ_MEM;
+    goto end;
+  }
   data_len-= LOG_EVENT_MINIMAL_HEADER_LEN;
   if (data_len)
   {
+    /* Append rest of event, read directly from file into packet */
     if (packet->append(file, data_len))
     {
       /*
-	Here if we hit EOF it's really an error: as data_len is >=0
-        there's supposed to be more bytes available. 
-	EOF means we are reading the event partially, which should
-	never happen: either we read badly or the binlog is truncated.
+        Fatal error occured when appending rest of the event
+        to packet, possible failures:
+	1. EOF occured when reading from file, it's really an error
+           as data_len is >=0 there's supposed to be more bytes available.
+           file->error will have been set to number of bytes left to read
+        2. Read was interrupted, file->error would normally be set to -1
+        3. Failed to allocate memory for packet, my_errno
+           will be ENOMEM(file->error shuold be 0, but since the
+           memory allocation occurs before the call to read it might
+           be uninitialized)
       */
-      result= file->error >= 0 ? LOG_READ_TRUNC: LOG_READ_IO;
+      result= (my_errno == ENOMEM ? LOG_READ_MEM :
+               (file->error >= 0 ? LOG_READ_TRUNC: LOG_READ_IO));
       /* Implicit goto end; */
     }
   }