Improve parallel-update reject log thread safety

Fixed an issue in which parallel-update did not properly handle
multiple concurrent attempts to write to the reject file, which
could cause its contents to be unparseable or otherwise incorrect.

Author: dirmgr

docs/release-notes.html

@@ -24,6 +24,13 @@ <h3>Version 6.0.10</h3>
                   <br><br>
                 </li>
 
+                <li>
+                  Fixed an issue in which parallel-update did not properly handle multiple
+                  concurrent attempts to write to the reject file, which could cause its contents
+                  to be unparseable or otherwise incorrect.
+                  <br><br>
+                </li>
+
                 <li>
                   Updated the PLAINBindRequest class to add an encodeCredentials method that can be
                   used to retrieve a properly encoded representation of the SASL credentials for a

src/com/unboundid/ldap/sdk/unboundidds/tools/ParallelUpdate.java

@@ -2446,11 +2446,17 @@ void reject(@Nullable final LDIFChangeRecord changeRecord,
             changeRecord.getChangeType().getName(), " REJECT ", resultCode, " ",
              ldapException.getMessage());
 
-        rejectWriter.writeChangeRecord(changeRecord, comment);
+        synchronized (rejectWriter)
+        {
+          rejectWriter.writeChangeRecord(changeRecord, comment);
+        }
       }
       else
       {
-        rejectWriter.writeComment(comment, true, true);
+        synchronized (rejectWriter)
+        {
+          rejectWriter.writeComment(comment, true, true);
+        }
       }
     }
     catch (final Exception e)