cover: Fix a race condition in how the server is started as part of a…

… call

Before sending a message to the cover server, the `call` function checks
if the server is running, starting it if it does not.

However, there is no locking/atomicity between the check and the start.
If there is a concurrent call, the server might be started in parallel
by another process and the `start()` call would return `{error,
{already_started, _}}`. This causes the function to fail with a
`badmatch` exception.

The code now accepts that the server is running after seeing the server
as stopped and retrying the call.

V2: Fix coding style by applying a suggested patch from Björn
    Gustavsson.
V3: Fix incorrect matching of `start()` return value: the correct
    pattern in `{error, {already_started, _}}`, not `{already_started, _}`.
    Found by Dialyzer, reported by Björn Gustavsson.

Co-authored-by: Björn Gustavsson <[email protected]>

lib/tools/src/cover.erl

@@ -1143,7 +1143,10 @@ call(Request) ->
     Ref = erlang:monitor(process,?SERVER),
     receive {'DOWN', Ref, _Type, _Object, noproc} -> 
 	    erlang:demonitor(Ref),
-            {ok,_} = start(),
+            case start() of
+                {ok,_} -> ok;
+                {error,{already_started,_}} -> ok
+            end,
 	    call(Request)
     after 0 ->
 	    ?SERVER ! {self(),Request},