Make dependency paths relocatable

The generated configurations for both CMake and pkg-config included
absolute paths to dependencies (OpenSSL, MbedTLS). This is contrary
to the general CMake advise to create relocatable packages [1].

Additionally, when building both mbedtls and libevent via CMake's
FetchContent in the same project, loading the project would fail with

  INTERFACE_INCLUDE_DIRECTORIES property contains path:
    "/home/user/project/cmake-build/_deps/mbedtls-build/include"
  which is prefixed in the source directory.

The required changes include:
- Adding the outer includes only to the BUILD_INTERFACE solves the
  makes the CMake paths relocatable and thus solves the FetchContent
  problem.
- Updates to libevent_*.pc.in fixes the relocatable issues for
  pkg-config and properly declares currently missing dependencies.
- Using components for linking to OpenSSL (requiring CMake 3.4)
  and MbedTLS. The new MbedTLS target names now match the component
  names of the MbedTLS' CMake project.
- Use the Threads CMake library reference instead of a direct
  reference to support both built-in pthread and -lpthread.

v2 (azat): get back CMAKE_REQUIRED_LIBRARIES

[1] https://cmake.org/cmake/help/v3.25/manual/cmake-packages.7.html#creating-relocatable-packages

11 files changed, 81 insertions, 76 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7199ab82..c0c1c2fa 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,7 +19,7 @@
 #       start libevent.sln
 #
 
-cmake_minimum_required(VERSION 3.1.2 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.4 FATAL_ERROR)
 
 if (POLICY CMP0054)
     cmake_policy(SET CMP0054 NEW)
@@ -578,10 +578,9 @@ if (NOT EVENT__DISABLE_THREAD_SUPPORT)
         set(PTHREADS_AVAILABLE ON)
 
         set(EVENT__HAVE_PTHREADS 1)
-        list(APPEND LIB_APPS ${CMAKE_THREAD_LIBS_INIT})
-
         # for CHECK_SYMBOLS_EXIST()
         list(APPEND CMAKE_REQUIRED_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
+        list(APPEND LIB_APPS Threads::Threads)
 
         CHECK_TYPE_SIZE(pthread_t EVENT__SIZEOF_PTHREAD_T)
         list(APPEND SYMBOLS_TO_CHECK pthread_mutexattr_setprotocol)
@@ -951,15 +950,14 @@ if (NOT EVENT__DISABLE_OPENSSL)
     find_package(OpenSSL REQUIRED)
 
     set(EVENT__HAVE_OPENSSL 1)
+    set(OPENSSL_TARGETS OpenSSL::SSL)
 
     message(STATUS "OpenSSL include: ${OPENSSL_INCLUDE_DIR}")
     message(STATUS "OpenSSL lib: ${OPENSSL_LIBRARIES}")
 
-    include_directories(${OPENSSL_INCLUDE_DIR})
-
     list(APPEND SRC_OPENSSL bufferevent_openssl.c bufferevent_ssl.c)
     list(APPEND HDR_PUBLIC include/event2/bufferevent_ssl.h)
-    list(APPEND LIB_APPS ${OPENSSL_LIBRARIES})
+    list(APPEND LIB_APPS ${OPENSSL_TARGETS})
 endif()
 
 if (NOT EVENT__DISABLE_MBEDTLS)
@@ -980,15 +978,14 @@ if (NOT EVENT__DISABLE_MBEDTLS)
     find_package(MbedTLS REQUIRED)
 
     set(EVENT__HAVE_MBEDTLS 1)
+    set(MBEDTLS_TARGETS MbedTLS::mbedtls MbedTLS::mbedcrypto MbedTLS::mbedx509)
 
     message(STATUS "mbed TLS include: ${MBEDTLS_INCLUDE_DIR}")
     message(STATUS "mbed TLS lib: ${MBEDTLS_LIBRARIES}")
 
-    include_directories(${MBEDTLS_INCLUDE_DIR})
-
     list(APPEND SRC_MBEDTLS bufferevent_mbedtls.c bufferevent_ssl.c)
     list(APPEND HDR_PUBLIC include/event2/bufferevent_ssl.h)
-    list(APPEND LIB_APPS ${MBEDTLS_LIBRARIES})
+    list(APPEND LIB_APPS ${MBEDTLS_TARGETS})
 endif()
 
 if (NOT EVENT__DISABLE_TESTS)
@@ -996,10 +993,8 @@ if (NOT EVENT__DISABLE_TESTS)
     find_package(ZLIB)
 
     if (ZLIB_LIBRARY AND ZLIB_INCLUDE_DIR)
-        include_directories(${ZLIB_INCLUDE_DIRS})
-
         set(EVENT__HAVE_LIBZ 1)
-        list(APPEND LIB_APPS ${ZLIB_LIBRARIES})
+        list(APPEND LIB_APPS ZLIB::ZLIB)
     endif()
 endif()
 
@@ -1011,7 +1006,7 @@ set(SRC_EXTRA
     sha1.c
     evrpc.c)
 
-if("${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}" LESS 3.20)
+if(${CMAKE_VERSION} VERSION_LESS "3.20")
   include(TestBigEndian)
   TEST_BIG_ENDIAN(IS_BIG_ENDIAN)
   if(IS_BIG_ENDIAN)
@@ -1089,16 +1084,14 @@ add_event_library(event_extra
 if (NOT EVENT__DISABLE_OPENSSL)
     add_event_library(event_openssl
         INNER_LIBRARIES event_core
-        OUTER_INCLUDES ${OPENSSL_INCLUDE_DIR}
-        LIBRARIES ${OPENSSL_LIBRARIES}
+        LIBRARIES ${OPENSSL_TARGETS}
         SOURCES ${SRC_OPENSSL})
 endif()
 
 if (NOT EVENT__DISABLE_MBEDTLS)
     add_event_library(event_mbedtls
         INNER_LIBRARIES event_core
-        OUTER_INCLUDES ${MBEDTLS_INCLUDE_DIR}
-        LIBRARIES ${MBEDTLS_LIBRARIES}
+        LIBRARIES ${MBEDTLS_TARGETS}
         SOURCES ${SRC_MBEDTLS})
 endif()
 
@@ -1106,6 +1099,7 @@ if (EVENT__HAVE_PTHREADS)
     set(SRC_PTHREADS evthread_pthread.c)
     add_event_library(event_pthreads
         INNER_LIBRARIES event_core
+        LIBRARIES Threads::Threads
         SOURCES ${SRC_PTHREADS})
 endif()
 
diff --git a/cmake/AddEventLibrary.cmake b/cmake/AddEventLibrary.cmake
index ffd7e888..203081b2 100644
--- a/cmake/AddEventLibrary.cmake
+++ b/cmake/AddEventLibrary.cmake
@@ -1,4 +1,5 @@
 include(CMakeParseArguments)
+include(GNUInstallDirs)
 
 set(LIBEVENT_SHARED_LIBRARIES "")
 set(LIBEVENT_STATIC_LIBRARIES "")
@@ -11,10 +12,10 @@ macro(set_event_shared_lib_flags LIB_NAME)
 endmacro()
 
 macro(generate_pkgconfig LIB_NAME)
-    set(prefix      ${CMAKE_INSTALL_PREFIX})
-    set(exec_prefix ${CMAKE_INSTALL_PREFIX})
-    set(libdir      ${CMAKE_INSTALL_LIBDIR})
-    set(includedir  ${CMAKE_INSTALL_INCLUDEDIR})
+    set(prefix      "${CMAKE_INSTALL_PREFIX}")
+    set(exec_prefix "\${prefix}")
+    set(libdir      "\${prefix}/${CMAKE_INSTALL_LIBDIR}")
+    set(includedir  "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
 
     set(VERSION ${EVENT_ABI_LIBVERSION})
 
@@ -23,11 +24,6 @@ macro(generate_pkgconfig LIB_NAME)
         set(LIBS "${LIBS} -l${LIB}")
     endforeach()
 
-    set(OPENSSL_LIBS "")
-    foreach(LIB ${OPENSSL_LIBRARIES})
-        set(OPENSSL_LIBS "${OPENSSL_LIBS} -l${LIB}")
-    endforeach()
-
     configure_file("lib${LIB_NAME}.pc.in" "lib${LIB_NAME}.pc" @ONLY)
     install(
         FILES "${CMAKE_CURRENT_BINARY_DIR}/lib${LIB_NAME}.pc"
@@ -37,7 +33,7 @@ endmacro()
 
 # LIB_NAME maybe event_core, event_extra, event_openssl, event_pthreads or event.
 # Targets whose LIB_NAME is not 'event' should be exported and installed.
-macro(export_install_target TYPE LIB_NAME OUTER_INCLUDES)
+macro(export_install_target TYPE LIB_NAME)
     if("${LIB_NAME}" STREQUAL "event")
         install(TARGETS "${LIB_NAME}_${TYPE}"
             LIBRARY DESTINATION "lib" COMPONENT lib
@@ -57,7 +53,6 @@ macro(export_install_target TYPE LIB_NAME OUTER_INCLUDES)
             PUBLIC  "$<INSTALL_INTERFACE:include>"
                     "$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>"
                     "$<BUILD_INTERFACE:${PROJECT_BINARY_DIR}/include>"
-                    ${OUTER_INCS}
         )
         set_target_properties("${LIB_NAME}_${TYPE}" PROPERTIES EXPORT_NAME ${PURE_NAME})
         export(TARGETS "${LIB_NAME}_${TYPE}"
@@ -81,8 +76,7 @@ endmacro()
 # - EVENT_ABI_LIBVERSION_REVISION
 # - EVENT_ABI_LIBVERSION_AGE
 # - EVENT_PACKAGE_RELEASE
-# - CMAKE_THREAD_LIBS_INIT LIB_PLATFORM
-# - OPENSSL_LIBRARIES
+# - LIB_PLATFORM
 # - EVENT_SHARED_FLAGS
 # - EVENT_LIBRARY_STATIC
 # - EVENT_LIBRARY_SHARED
@@ -94,13 +88,10 @@ macro(add_event_library LIB_NAME)
     cmake_parse_arguments(LIB
         "" # Options
         "VERSION" # One val
-        "SOURCES;LIBRARIES;INNER_LIBRARIES;OUTER_INCLUDES" # Multi val
+        "SOURCES;LIBRARIES;INNER_LIBRARIES" # Multi val
         ${ARGN}
     )
 
-    if ("${LIB_OUTER_INCLUDES}" STREQUAL "")
-        set(LIB_OUTER_INCLUDES NONE)
-    endif()
     set(ADD_EVENT_LIBRARY_INTERFACE)
     set(INNER_LIBRARIES)
 
@@ -114,12 +105,11 @@ macro(add_event_library LIB_NAME)
             set(INNER_LIBRARIES "${LIB_INNER_LIBRARIES}_static")
         endif()
         target_link_libraries("${LIB_NAME}_static"
-            ${CMAKE_THREAD_LIBS_INIT}
             ${LIB_PLATFORM}
             ${INNER_LIBRARIES}
             ${LIB_LIBRARIES})
 
-        export_install_target(static "${LIB_NAME}" "${LIB_OUTER_INCLUDES}")
+        export_install_target(static "${LIB_NAME}")
 
         set(ADD_EVENT_LIBRARY_INTERFACE "${LIB_NAME}_static")
     endif()
@@ -131,7 +121,6 @@ macro(add_event_library LIB_NAME)
             set(INNER_LIBRARIES "${LIB_INNER_LIBRARIES}_shared")
         endif()
         target_link_libraries("${LIB_NAME}_shared"
-            ${CMAKE_THREAD_LIBS_INIT}
             ${LIB_PLATFORM}
             ${INNER_LIBRARIES}
             ${LIB_LIBRARIES})
@@ -174,7 +163,7 @@ macro(add_event_library LIB_NAME)
                 WORKING_DIRECTORY "${CMAKE_LIBRARY_OUTPUT_DIRECTORY}")
         endif()
 
-        export_install_target(shared "${LIB_NAME}" "${LIB_OUTER_INCLUDES}")
+        export_install_target(shared "${LIB_NAME}")
 
         set(ADD_EVENT_LIBRARY_INTERFACE "${LIB_NAME}_shared")
 
diff --git a/cmake/FindMbedTLS.cmake b/cmake/FindMbedTLS.cmake
index 39ff8204..fea0f087 100644
--- a/cmake/FindMbedTLS.cmake
+++ b/cmake/FindMbedTLS.cmake
@@ -151,24 +151,26 @@ find_package_handle_standard_args(MbedTLS
                                   VERSION_VAR MBEDTLS_VERSION)
 
 
-if(NOT TARGET mbedtls)
-    add_library(mbedtls UNKNOWN IMPORTED)
-    set_target_properties(mbedtls PROPERTIES
+if(NOT TARGET MbedTLS::mbedtls)
+    add_library(MbedTLS::mbedtls UNKNOWN IMPORTED)
+    set_target_properties(MbedTLS::mbedtls PROPERTIES
                           INTERFACE_INCLUDE_DIRECTORIES "${MBEDTLS_INCLUDE_DIR}"
                           IMPORTED_LINK_INTERFACE_LANGUAGES "C"
+                          INTERFACE_LINK_LIBRARIES MbedTLS::mbedx509
                           IMPORTED_LOCATION "${MBEDTLS_LIBRARY}")
 endif()
 
-if(NOT TARGET mbedcrypto)
-    add_library(mbedcrypto UNKNOWN IMPORTED)
-    set_target_properties(mbedcrypto PROPERTIES
+if(NOT TARGET MbedTLS::mbedcrypto)
+    add_library(MbedTLS::mbedcrypto UNKNOWN IMPORTED)
+    set_target_properties(MbedTLS::mbedcrypto PROPERTIES
                           IMPORTED_LINK_INTERFACE_LANGUAGES "C"
                           IMPORTED_LOCATION "${MBEDTLS_CRYPTO_LIBRARY}")
 endif()
 
-if(NOT TARGET mbedx509)
-    add_library(mbedx509 UNKNOWN IMPORTED)
-    set_target_properties(mbedx509 PROPERTIES
+if(NOT TARGET MbedTLS::mbedx509)
+    add_library(MbedTLS::mbedx509 UNKNOWN IMPORTED)
+    set_target_properties(MbedTLS::mbedx509 PROPERTIES
                           IMPORTED_LINK_INTERFACE_LANGUAGES "C"
+                          INTERFACE_LINK_LIBRARIES MbedTLS::mbedcrypto
                           IMPORTED_LOCATION "${MBEDTLS_X509_LIBRARY}")
 endif()
diff --git a/cmake/LibeventConfig.cmake.in b/cmake/LibeventConfig.cmake.in
index f9b00363..1ae5e3e3 100644
--- a/cmake/LibeventConfig.cmake.in
+++ b/cmake/LibeventConfig.cmake.in
@@ -16,8 +16,9 @@
 # The following table lists all available components. If none is given, all imported targets will used.
 #  core        - the core functons of libevent
 #  extra       - extra functions, contains http, dns and rpc
-#  pthreads    - multiple threads for libevent, not exists on Windows
-#  openssl     - openssl support for libevent
+#  pthreads    - multiple threads for libevent, does not exist on Windows
+#  openssl     - OpenSSL support for libevent
+#  mbedtls     - MbedTLS support for libevent
 #
 # By default, the shared libraries of libevent will be found. To find the static ones instead,
 # you must set the LIBEVENT_STATIC_LINK variable to TRUE before calling find_package(Libevent ...).
@@ -34,6 +35,14 @@
 
 set(LIBEVENT_VERSION @EVENT_PACKAGE_VERSION@)
 
+# Load the dependencies of all components. As find_dependency propagates the original
+# find_package attributes (i.e. required or not), there's no need to repeat this or filter
+# by component.
+include(CMakeFindDependencyMacro)
+find_dependency(Threads)
+find_dependency(MbedTLS)
+find_dependency(OpenSSL)
+
 # IMPORTED targets from LibeventTargets.cmake
 set(LIBEVENT_STATIC_LIBRARIES "@LIBEVENT_STATIC_LIBRARIES@")
 set(LIBEVENT_SHARED_LIBRARIES "@LIBEVENT_SHARED_LIBRARIES@")
@@ -46,6 +55,12 @@ endif()
 if(${LIBEVENT_STATIC_LINK})
     set(_LIB_TYPE static)
     set(_AVAILABLE_LIBS "${LIBEVENT_STATIC_LIBRARIES}")
+
+    # CMake before 3.15 doesn't link OpenSSL to pthread/dl, do it ourselves instead
+    if (${CMAKE_VERSION} VERSION_LESS "3.15.0" AND ${LIBEVENT_STATIC_LINK} AND ${OPENSSL_FOUND} AND ${Threads_FOUND})
+        set_property(TARGET OpenSSL::Crypto APPEND PROPERTY INTERFACE_LINK_LIBRARIES Threads::Threads)
+        set_property(TARGET OpenSSL::Crypto APPEND PROPERTY INTERFACE_LINK_LIBRARIES ${CMAKE_DL_LIBS})
+    endif ()
 else()
     set(_LIB_TYPE shared)
     set(_AVAILABLE_LIBS "${LIBEVENT_SHARED_LIBRARIES}")
@@ -64,7 +79,7 @@ macro(no_component_msg _comp)
             set(pthreadlib ", pthreads")
         endif()
         message(FATAL_ERROR "Your libevent library does not contain a ${_comp} component!\n"
-                "The valid components are core, extra${pthreadlib} and openssl.")
+                "The valid components are core, extra${pthreadlib}, openssl and mbedtls.")
     else()
         message_if_needed(WARNING "Your libevent library does not contain a ${_comp} component!")
     endif()
diff --git a/libevent_core.pc.in b/libevent_core.pc.in
index 98ab1efe..eca8dabe 100644
--- a/libevent_core.pc.in
+++ b/libevent_core.pc.in
@@ -8,9 +8,6 @@ includedir=@includedir@
 Name: libevent_core
 Description: libevent_core
 Version: @VERSION@
-Requires:
-Conflicts:
-Libs: -L${libdir} -levent_core
+Libs: -L${libdir} -levent_core@CMAKE_DEBUG_POSTFIX@
 Libs.private: @LIBS@
 Cflags: -I${includedir}
-
diff --git a/libevent_extra.pc.in b/libevent_extra.pc.in
index b9633998..03deca71 100644
--- a/libevent_extra.pc.in
+++ b/libevent_extra.pc.in
@@ -8,9 +8,7 @@ includedir=@includedir@
 Name: libevent_extra
 Description: libevent_extra
 Version: @VERSION@
-Requires:
-Conflicts:
-Libs: -L${libdir} -levent_extra
+Requires: libevent_core
+Libs: -L${libdir} -levent_extra@CMAKE_DEBUG_POSTFIX@
 Libs.private: @LIBS@
 Cflags: -I${includedir}
-
diff --git a/libevent_mbedtls.pc.in b/libevent_mbedtls.pc.in
index 3f8d40d9..89479a13 100644
--- a/libevent_mbedtls.pc.in
+++ b/libevent_mbedtls.pc.in
@@ -9,8 +9,6 @@ Name: libevent_mbedtls
 Description: libevent_mbedtls adds mbedtls-based TLS support to libevent
 Version: @VERSION@
 Requires: libevent_core
-Conflicts:
-Libs: -L${libdir} -levent_mbedtls
-Libs.private: @LIBS@ @MBEDTLS_LIBS@
-Cflags: -I${includedir} @MBEDTLS_INCS@
-
+Libs: -L${libdir} -levent_mbedtls@CMAKE_DEBUG_POSTFIX@
+Libs.private: -lmbedtls -lmbedx509 @LIBS@
+Cflags: -I${includedir}
diff --git a/libevent_openssl.pc.in b/libevent_openssl.pc.in
index ed08ffe3..f9da5ead 100644
--- a/libevent_openssl.pc.in
+++ b/libevent_openssl.pc.in
@@ -9,8 +9,7 @@ Name: libevent_openssl
 Description: libevent_openssl adds openssl-based TLS support to libevent
 Version: @VERSION@
 Requires: libevent_core
-Conflicts:
-Libs: -L${libdir} -levent_openssl
-Libs.private: @LIBS@ @OPENSSL_LIBS@
-Cflags: -I${includedir} @OPENSSL_INCS@
-
+Requires.private: libssl
+Libs: -L${libdir} -levent_openssl@CMAKE_DEBUG_POSTFIX@
+Libs.private: @LIBS@
+Cflags: -I${includedir}
diff --git a/libevent_pthreads.pc.in b/libevent_pthreads.pc.in
index dee58251..22f713df 100644
--- a/libevent_pthreads.pc.in
+++ b/libevent_pthreads.pc.in
@@ -9,8 +9,6 @@ Name: libevent_pthreads
 Description: libevent_pthreads adds pthreads-based threading support to libevent
 Version: @VERSION@
 Requires: libevent_core
-Conflicts:
-Libs: -L${libdir} -levent_pthreads
-Libs.private: @LIBS@ @PTHREAD_LIBS@
-Cflags: -I${includedir} @PTHREAD_CFLAGS@
-
+Libs: -L${libdir} -levent_pthreads@CMAKE_DEBUG_POSTFIX@
+Libs.private: @CMAKE_THREAD_LIBS_INIT@ @LIBS@
+Cflags: -I${includedir}
diff --git a/test-export/CMakeLists.txt b/test-export/CMakeLists.txt
index 89c1806c..84e15181 100644
--- a/test-export/CMakeLists.txt
+++ b/test-export/CMakeLists.txt
@@ -1,7 +1,8 @@
-cmake_minimum_required(VERSION 3.1.2)
+cmake_minimum_required(VERSION 3.4)
 if (POLICY CMP0074)
     cmake_policy(SET CMP0074 NEW)
 endif()
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/../cmake")
 project(verify)
 # set(CMAKE_VERBOSE_MAKEFILE 1)
 if(NOT ${EVENT__CODE_COMPONENT} STREQUAL "")
diff --git a/test-export/test-export.py b/test-export/test-export.py
index 84824350..efeac0cc 100644
--- a/test-export/test-export.py
+++ b/test-export/test-export.py
@@ -46,10 +46,24 @@ def link_and_run(link, code):
     """
     exec_cmd("cmake --build . -v --target clean", True)
     arch = ''
+    vcpkg = ''
+    openssldir = ''
+    mbedtlsdir = ''
     if platform.system() == "Windows":
         arch = '-A x64'
-    cmd = 'cmake .. %s -DEVENT__LINK_COMPONENT=%s -DEVENT__CODE_COMPONENT=%s' % (
-        arch, link, code)
+        vcpkg_root = os.environ.get('VCPKG_ROOT')
+        if vcpkg_root is not None:
+            vcpkg = f"-DCMAKE_TOOLCHAIN_FILE={vcpkg_root}/scripts/buildsystems/vcpkg.cmake"
+    elif platform.system() == "Darwin":
+        openssldir = '-DOPENSSL_ROOT_DIR=/usr/local/opt/openssl'
+        mbedtlsdir = '-DMBEDTLS_ROOT_DIR=/usr/local/opt/mbedtls@2'
+    cmd = f"cmake .." \
+          f" {arch}" \
+          f" {vcpkg}" \
+          f" -DEVENT__LINK_COMPONENT={link}" \
+          f" -DEVENT__CODE_COMPONENT={code}" \
+          f" {openssldir}" \
+          f" {mbedtlsdir}"
     if link_type == "static":
         cmd = "".join([cmd, " -DLIBEVENT_STATIC_LINK=1"])
     r = exec_cmd(cmd, True)