CDRIVER-4637 Remove ENABLE_SRV=AUTO, clean up SRV URI support (#1301)

Remove AUTO for ENABLE_SRV, refactor .pc generation

This AUTO wasn't doing anything, and behaved very similar to plain ON.
Now we define an interface library that provides the resolving library.
Some cleanup to the ResSearch lookup module.

* Use ENABLE_SRV to toggle SRV support rather than detection macros
* Add distinct feature flag for SRV toggle.
* Refactor pkg-config generation to use target properties
  This change uses target properties to fill in the slots in the
  generated pkg-config files. Previously this was unstructured strings
  and flag juggling.
* Tweak method of detecting name resolution
Co-authored-by: Roberto C. Sánchez <[email protected]>

Author: vector-of-bool

CMakeLists.txt

@@ -71,11 +71,9 @@ mongo_bool_setting(ENABLE_EXAMPLES "Build MongoDB C Driver examples")
 mongo_bool_setting(ENABLE_MAN_PAGES "Build the manual pages" DEFAULT VALUE OFF)
 mongo_bool_setting(ENABLE_HTML_DOCS "Build the HTML documentation" DEFAULT VALUE OFF)
 mongo_bool_setting(ENABLE_UNINSTALL "Generate an 'uninstall' script and an 'uninstall' build target")
+mongo_bool_setting(ENABLE_SRV "Enable support for mongodb+srv URIs.")
 
 # Optional features that are ENABLED when necessary dependencies are found:
-mongo_setting(ENABLE_SRV "Enable support for mongodb+srv URIs."
-              OPTIONS ON OFF AUTO
-              DEFAULT VALUE AUTO)
 mongo_setting(ENABLE_SNAPPY "Enable Snappy compression support"
               OPTIONS ON OFF AUTO
               DEFAULT VALUE AUTO)

build/cmake/CMakeLists.txt

@@ -2,7 +2,7 @@ add_subdirectory (make_dist)
 
 set (build_cmake_MODULES
    CheckSchedGetCPU.cmake
-   FindResSearch.cmake
+   ResSearch.cmake
    FindSASL2.cmake
    FindSnappy.cmake
    FindSphinx.cmake

build/cmake/FindResSearch.cmake

@@ -0,0 +1,57 @@
+include(CheckSymbolExists)
+include(CMakePushCheckState)
+
+cmake_push_check_state(RESET)
+
+# The name of the library that performs name resolution, suitable for giving to the "-l" link flag
+set(RESOLVE_LIB_NAME)
+# If TRUE, then the C runtime provides the name resolution that we need
+set(resolve_is_libc FALSE)
+
+if(WIN32)
+    set(RESOLVE_LIB_NAME Dnsapi)
+    set(_MONGOC_HAVE_DNSAPI 1)
+else()
+    # Try to find the search functions for various configurations.
+    # Headers required by minimum on the strictest system: (Tested on FreeBSD 13)
+    set(resolve_headers netinet/in.h sys/types.h arpa/nameser.h resolv.h)
+    set(CMAKE_REQUIRED_LIBRARIES resolv)
+    check_symbol_exists(res_nsearch "${resolve_headers}" _MONGOC_HAVE_RES_NSEARCH_RESOLV)
+    check_symbol_exists(res_search "${resolve_headers}" _MONGOC_HAVE_RES_SEARCH_RESOLV)
+    check_symbol_exists(res_ndestroy "${resolve_headers}" _MONGOC_HAVE_RES_NDESTROY_RESOLV)
+    check_symbol_exists(res_nclose "${resolve_headers}" _MONGOC_HAVE_RES_NCLOSE_RESOLV)
+    if((_MONGOC_HAVE_RES_NSEARCH_RESOLV OR _MONGOC_HAVE_RES_SEARCH_RESOLV)
+        AND (_MONGOC_HAVE_RES_NDESTROY_RESOLV OR _MONGOC_HAVE_RES_NCLOSE_RESOLV))
+        set(RESOLVE_LIB_NAME resolv)
+    else()
+        # Can we use name resolution with just libc?
+        unset(CMAKE_REQUIRED_LIBRARIES)
+        check_symbol_exists(res_nsearch "${resolve_headers}" _MONGOC_HAVE_RES_NSEARCH_NOLINK)
+        check_symbol_exists(res_search "${resolve_headers}" _MONGOC_HAVE_RES_SEARCH_NOLINK)
+        check_symbol_exists(res_ndestroy "${resolve_headers}" _MONGOC_HAVE_RES_NDESTROY_NOLINK)
+        check_symbol_exists(res_nclose "${resolve_headers}" _MONGOC_HAVE_RES_NCLOSE_NOLINK)
+        if((_MONGOC_HAVE_RES_NSEARCH_NOLINK OR _MONGOC_HAVE_RES_SEARCH_NOLINK)
+            AND (_MONGOC_HAVE_RES_NDESTROY_NOLINK OR _MONGOC_HAVE_RES_NCLOSE_NOLINK))
+            set(resolve_is_libc TRUE)
+            message(VERBOSE "Name resolution is provided by the C runtime")
+        endif()
+    endif()
+endif()
+
+_mongo_pick(MONGOC_HAVE_DNSAPI 1 0 _MONGOC_HAVE_DNSAPI)
+_mongo_pick(MONGOC_HAVE_RES_NSEARCH 1 0 [[_MONGOC_HAVE_RES_NSEARCH_NOLINK OR _MONGOC_HAVE_RES_NSEARCH_RESOLV]])
+_mongo_pick(MONGOC_HAVE_RES_SEARCH 1 0 [[_MONGOC_HAVE_RES_SEARCH_NOLINK OR _MONGOC_HAVE_RES_SEARCH_RESOLV]])
+_mongo_pick(MONGOC_HAVE_RES_NDESTROY 1 0 [[_MONGOC_HAVE_RES_NDESTROY_NOLINK OR _MONGOC_HAVE_RES_NDESTROY_RESOLV]])
+_mongo_pick(MONGOC_HAVE_RES_NCLOSE 1 0 [[_MONGOC_HAVE_RES_NCLOSE_NOLINK OR _MONGOC_HAVE_RES_NCLOSE_RESOLV]])
+
+if(RESOLVE_LIB_NAME OR resolve_is_libc)
+    # Define the resolver interface:
+    add_library(_mongoc-resolve INTERFACE)
+    add_library(mongo::detail::c_resolve ALIAS _mongoc-resolve)
+    set_target_properties(_mongoc-resolve PROPERTIES
+        INTERFACE_LINK_LIBRARIES "${RESOLVE_LIB_NAME}"
+        EXPORT_NAME detail::c_resolve)
+    install(TARGETS _mongoc-resolve EXPORT mongoc-targets)
+endif()
+
+cmake_pop_check_state()