race condition in install targets in nmake Makefiles

For example qtbase/src/corelib/Makefile.Debug. There are the targets install_class_headers and install_targ_headers. Both are independent and can be executed in parallel by jom.

Both start with the following command:

@if not exist D:$(INSTALL_ROOT)\dev\qt\5.0-vs2012\qtbase\include\QtCore $(MKDIR) D:$(INSTALL_ROOT)\dev\qt\5.0-vs2012\qtbase\include\QtCore 

There's a race condition:

• both conditions (if not exists) evaluate to true
• one $(MKDIR) call creates the directory
• one $(MKDIR) call fails to create the directory because it already exists

Note that this works on Linux, because mkdir -p doesn't complain, if the directory already exists.

Possible solutions:

• Create a target for every distinct "if not exist $(MKDIR)" call and let the install target that needs this directory depend on it.
• Prevent parallel execution of install targets by chaining them.