Combine no longer appends by default

Combine used to always load an existing .coverage file. This lead to confusing
results and extra tox-clean steps.  Now the default is to not load the existing
file, though a new --append switch on coverage combine gets you that behavior
if you need it.

This also pointed up an issue with concurrency=multiprocessing, which is that
the child processes automatically used parallel=True, but the parent process
did not.  Now concurrency=multiprocessing implies parallel=True.

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/cmd.rst b/doc/cmd.rst
index f77738bc..90e21057 100644
--- a/doc/cmd.rst
+++ b/doc/cmd.rst
@@ -212,6 +212,11 @@ suffix.  Here are some examples of data files that can be combined::
     .coverage.20120807T212300
     .coverage.last_good_run.ok
 
+An existing combined data file is ignored and re-written. If you want to use
+**combine** to accumulate results into the .coverage data file over a number of
+runs, use the ``--append`` switch on the **combine** command.  This behavior
+was the default before version 4.2.
+
 The ``run --parallel-mode`` switch automatically creates separate data files
 for each run which can be combined later.  The file names include the machine
 name, the process id, and a random number::