cmake: update threads

For consistency with other backend/provider selection, allow
`USE_THREADS` to select the threads provider.

CMakeLists.txt

@@ -25,10 +25,8 @@ option(BUILD_CLI               "Build the command-line interface"
 option(BUILD_EXAMPLES          "Build library usage example apps"                      OFF)
 option(BUILD_FUZZERS           "Build the fuzz targets"                                OFF)
 
-# Suggested functionality that may not be available on a per-platform basis
-option(USE_THREADS             "Use threads for parallel processing when possible"      ON)
-
 # Feature enablement and backend selection
+   set(USE_THREADS             "" CACHE STRING "Use threads for parallel processing when possible. One of ON, OFF, or a specific provider: pthreads or win32. (Defaults to ON.)")
    set(USE_SSH                 "" CACHE STRING "Enables SSH support and optionally selects provider. One of ON, OFF, or a specific provider: libssh2 or exec. (Defaults to OFF.)")
    set(USE_HTTPS               "" CACHE STRING "Enable HTTPS support and optionally selects the provider. One of ON, OFF, or a specific provider: OpenSSL, OpenSSL-FIPS, OpenSSL-Dynamic, mbedTLS, SecureTransport, Schannel, or WinHTTP. (Defaults to ON.)")
    set(USE_SHA1                "" CACHE STRING "Selects SHA1 provider. One of builtin, HTTPS, or a specific provider. (Defaults to builtin.)")

cmake/SelectThreads.cmake

@@ -0,0 +1,41 @@
+include(SanitizeBool)
+
+sanitizebool(USE_THREADS)
+
+if(NOT WIN32)
+	find_package(Threads)
+endif()
+
+if((USE_THREADS STREQUAL ON OR USE_THREADS STREQUAL "") AND THREADS_FOUND)
+	set(USE_THREADS "pthreads")
+elseif((USE_THREADS STREQUAL ON OR USE_THREADS STREQUAL "") AND WIN32)
+	set(USE_THREADS "win32")
+elseif(USE_THREADS STREQUAL "")
+	set(USE_THREADS OFF)
+endif()
+
+if(USE_THREADS STREQUAL "pthreads")
+	if(NOT THREADS_FOUND)
+		message(FATAL_ERROR "pthreads were requested but not found")
+	endif()
+
+	list(APPEND LIBGIT2_SYSTEM_LIBS ${CMAKE_THREAD_LIBS_INIT})
+	list(APPEND LIBGIT2_PC_LIBS ${CMAKE_THREAD_LIBS_INIT})
+
+	set(GIT_THREADS 1)
+	set(GIT_THREADS_PTHREADS 1)
+	add_feature_info("Threads" ON "using pthreads")
+elseif(USE_THREADS STREQUAL "win32")
+	if(NOT WIN32)
+		message(FATAL_ERROR "Win32 API support is not available on this platform")
+	endif()
+
+	set(GIT_THREADS 1)
+	set(GIT_THREADS_WIN32 1)
+	add_feature_info("Threads" ON "using Win32 APIs")
+elseif(USE_THREADS STREQUAL OFF)
+	set(GIT_THREADS 0)
+	add_feature_info("Threads" OFF "threads support is disabled")
+else()
+	message(FATAL_ERROR "unknown threads option: ${USE_THREADS}")
+endif()

src/CMakeLists.txt

@@ -36,6 +36,7 @@ add_feature_info(debugopen GIT_DEBUG_STRICT_OPEN "path validation in open")
 # Optional feature enablement
 #
 
+include(SelectThreads)
 include(SelectNsec)
 include(SelectHTTPSBackend)
 include(SelectHashes)
@@ -141,19 +142,6 @@ if(AMIGA)
 	add_definitions(-DNO_ADDRINFO -DNO_READDIR_R -DNO_MMAP)
 endif()
 
-# threads
-
-if(USE_THREADS)
-	if(NOT WIN32)
-		find_package(Threads REQUIRED)
-		list(APPEND LIBGIT2_SYSTEM_LIBS ${CMAKE_THREAD_LIBS_INIT})
-		list(APPEND LIBGIT2_PC_LIBS ${CMAKE_THREAD_LIBS_INIT})
-	endif()
-
-	set(GIT_THREADS 1)
-endif()
-add_feature_info(threadsafe USE_THREADS "threadsafe support")
-
 #
 # Include child projects
 #

src/libgit2/libgit2.c

@@ -116,10 +116,12 @@ const char *git_libgit2_feature_backend(git_feature_t feature)
 {
 	switch (feature) {
 	case GIT_FEATURE_THREADS:
-#if defined(GIT_THREADS) && defined(GIT_WIN32)
+#if defined(GIT_THREADS_PTHREADS)
+		return "pthread";
+#elif defined(GIT_THREADS_WIN32)
 		return "win32";
 #elif defined(GIT_THREADS)
-		return "pthread";
+		GIT_ASSERT_WITH_RETVAL(!"Unknown threads backend", NULL);
 #endif
 		break;
 

src/util/git2_features.h.in

@@ -6,6 +6,9 @@
 #cmakedefine GIT_DEBUG_STRICT_OPEN 1
 
 #cmakedefine GIT_THREADS 1
+#cmakedefine GIT_THREADS_PTHREADS 1
+#cmakedefine GIT_THREADS_WIN32 1
+
 #cmakedefine GIT_WIN32_LEAKCHECK 1
 
 #cmakedefine GIT_ARCH_64 1

tests/libgit2/core/features.c

@@ -82,10 +82,12 @@ void test_core_features__backends(void)
 	const char *sha1 = git_libgit2_feature_backend(GIT_FEATURE_SHA1);
 	const char *sha256 = git_libgit2_feature_backend(GIT_FEATURE_SHA256);
 
-#if defined(GIT_THREADS) && defined(GIT_WIN32)
+#if defined(GIT_THREADS_WIN32)
 	cl_assert_equal_s("win32", threads);
-#elif defined(GIT_THREADS)
+#elif defined(GIT_THREADS_PTHREADS)
 	cl_assert_equal_s("pthread", threads);
+#elif defined(GIT_THREADS)
+	cl_assert(0);
 #else
 	cl_assert(threads == NULL);
 #endif