find ruby gems & make coverage conditional on BUILD_COVERAGE

Summary:
new finding tech:
- find_gem function configures gem-specific FindFoo files and wraps around
  find_package
- FindFoo files look for ruby and then attempt to require the gem name
  i.e. it checks if the gem can be successfully loaded by the interpreter
- since this is based on find_package it may be influenced in all the
  regular ways (e.g. forced found or disabled from finding altogether
  via CMAKE_DISABLE_FIND_PACKAGE_$PKGNAME)

various notes for the future:
- technically this isn't 100% correct because the require name of a gem and
  the gem name may not be the same. e.g. the gem `docker-api` has the
  require name `docker`. for all currently used gems the names are however
  the same and so simply requiring the gem name is expected to work
- the implementation doesn't care about versions, again because we don't
  need it to
- test-unit is a bundled gem, some distributions (e.g. Arch) do split it
  out without making suitable dependency arrangements on a package level
  though

the tech is heavily inspired by Aleix Pol's tech for finding QML modules
as seen in extra-cmake-modules

Test Plan:
- ruby not found: none of the modules found
- test-unit not present: error
- simplecov not prseent: never errors
- having a module installed or not is reported in the cmake output

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19856

1 files changed, 39 insertions, 0 deletions

diff --git a/cmake/FindGem.cmake b/cmake/FindGem.cmake
new file mode 100644
index 000000000..11c9c672b
--- /dev/null
+++ b/cmake/FindGem.cmake
@@ -0,0 +1,39 @@
+#=============================================================================
+# Copyright (c) 2019 Harald Sitter <[email protected]>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+#
+# 1. Redistributions of source code must retain the copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+# 3. The name of the author may not be used to endorse or promote products
+#    derived from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+# OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+# IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#=============================================================================
+
+# In this scope it's the dir we are in, in the function scope it will be the
+# caller's dir. So, keep our dir in a var.
+set(FINDGEM_MODULES_DIR ${CMAKE_CURRENT_LIST_DIR})
+
+function(find_gem GEM_NAME)
+    set(GEM_PACKAGE "Gem:${GEM_NAME}")
+
+    configure_file(${FINDGEM_MODULES_DIR}/FindGem.cmake.in Find${GEM_PACKAGE}.cmake @ONLY)
+
+    set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_BINARY_DIR}" ${CMAKE_MODULE_PATH})
+    find_package(${GEM_PACKAGE} ${ARGN})
+endfunction()