Add file locking to support parallel runs. (#427)

* Add file locking to support parallel runs.

* Fixed formatting.

* Prevent double locking file.

* Fix SegFault from test 2.

* Remove unnecessary debugging messages.

* Lock the package directory rather than the cache directory.
Only synchronize if CPM_SOURCE_CACHE is defined.

* Lock the version specific cache entry rather than the package specific entry.

* Remove unnecessary arguments in conditional statements.

* Change back to locking entire cache directory.

* Only check CPM_HAS_CACHE_LOCK.

* Lock on a per-package basis rather than the entire cache.

* Clean up the locked file.

* Unlock then remove to fix Windows.

* Specify use of cmake.lock as the lock file.

* - Changed CPM_HAS_CACHE_LOCK to ${CPM_ARGS_NAME}_CPM_HAS_CACHE_LOCK.
- Removed redundant variable initialization.

* Add unit test.

* Actually test if resulting git cache is clean in unit test.

* - Added comments
- Fixed formatting
- Removed unnecessary imports

* convert parallelism test to integration test

* remove comment

* - Removed now unnecessary variable.
 - Only delete file instead of unlocking it then deleting it.

* Forgot to change variable name.

* Add similar changes to the missed section.

* Fixed formatting.

* Unlock the file, but do not delete it.

* Only unlock the file if it exists.

* Changed cache.cmake test to ignore non-directory entries.

* Integration test lib make_project:
* keyword args
* 'name' arg to allow multiple projects from the same test

* - Moved checks to function.
- Fixed small grammatical errors.

* - Fix formatting

* Switch to snake case.

---------

Co-authored-by: Lars Melchior <TheLartians@users.noreply.github.com>
Co-authored-by: Lars Melchior <lars.melchior@gmail.com>
Co-authored-by: Borislav Stanimirov <b.stanimirov@abv.bg>

cmake/CPM.cmake

@@ -712,6 +712,9 @@ function(CPMAddPackage)
     # relative paths.
     get_filename_component(download_directory ${download_directory} ABSOLUTE)
     list(APPEND CPM_ARGS_UNPARSED_ARGUMENTS SOURCE_DIR ${download_directory})
+
+    file(LOCK ${download_directory}/../cmake.lock)
+
     if(EXISTS ${download_directory})
       cpm_store_fetch_properties(
         ${CPM_ARGS_NAME} "${download_directory}"
@@ -790,6 +793,10 @@ function(CPMAddPackage)
     cpm_get_fetch_properties("${CPM_ARGS_NAME}")
   endif()
 
+  if(EXISTS ${download_directory}/../cmake.lock)
+    file(LOCK ${download_directory}/../cmake.lock RELEASE)
+  endif()
+
   set(${CPM_ARGS_NAME}_ADDED YES)
   cpm_export_variables("${CPM_ARGS_NAME}")
 endfunction()