work around Ruby 3.2 linking conventions on Darwin

See #87

test/rcd_test/ext/mri/extconf.rb

@@ -23,6 +23,73 @@
     # https://github.com/rake-compiler/rake-compiler-dock/issues/69
     puts "Linking with '-static' flag"
     $LDFLAGS << ' -static'
+  else
+    if RbConfig::CONFIG["target_os"].include?("darwin")
+      ## In ruby 3.2, symbol resolution changed on Darwin, to introduce the `-bundle_loader` flag.
+      ##
+      ## See https://github.com/rake-compiler/rake-compiler-dock/issues/87 for a lot of context, but
+      ## I'll try to summarize here.
+      ##
+      ## > -bundle_loader executable
+      ## >   This specifies the executable that will be loading the bundle output file being linked.
+      ## >   Undefined symbols from the bundle are checked against the specified executable like it
+      ## >   was one of the dynamic libraries the bundle was linked with.
+      ##
+      ## There are good reasons to do this, including faster initialiation/loading as the Darwin
+      ## toolchain gets improved over time.
+      ##
+      ## Unfortunately, this flag prevents us from building a shared object that works with both a
+      ## Ruby compiled with `--enable-shared` and one compiled with `--disabled-shared`. The result
+      ## is usually an "Symbol not found" error about `_rb_cObject`, or a "dyld: missing symbol
+      ## called" error.
+      ##
+      ## There are two workarounds that I know of (there may be others I don't know of), and
+      ## they are ...
+
+      ## ----------------------------------------
+      ## SOLUTION 1, the `-flat_namespace` flag
+      ##
+      ## > Two-level namespace
+      ## >   By default all references resolved to a dynamic library record the library to which
+      ## >   they were resolved. At runtime, dyld uses that information to directly resolve symbols.
+      ## >   The alternative is to use the -flat_namespace option.  With flat namespace, the library
+      ## >   is not recorded.  At runtime, dyld will search each dynamic library in load order when
+      ## >   resolving symbols. This is slower, but more like how other operating systems resolve
+      ## >   symbols.  Two-level namespace By default all references resolved to a dynamic library
+      ## >   record the library to which they were resolved. At runtime, dyld uses that information
+      ## >   to directly resolve symbols.  The alternative is to use the -flat_namespace option.
+      ## >   With flat namespace, the library is not recorded.  At runtime, dyld will search each
+      ## >   dynamic library in load order when resolving symbols. This is slower, but more like how
+      ## >   other operating systems resolve symbols.
+      ##
+      #
+      # puts "Adding '-flat_namespace'"
+      # $LDFLAGS << ' -flat_namespace'
+      #
+      ## This solution unfortunately introduces new behavior that any symbols statically linked into
+      ## the shared object (e.g., libxml2 in nokogiri.bundle) may not be resolved locally from the
+      ## shared object, but instead resolved from a shared object loaded in the main process.
+      ##
+      ## This solution might be good for you if:
+      ## - you don't statically link things into your bundle,
+      ## - or you don't export those symbols,
+      ## - or you can avoid exporting those symbols (e.g., by using `-load_hidden`, or
+      ##   `-exported_symbols_list` or some other mechanism)
+      ##
+
+      ## ----------------------------------------
+      ## BUT ... if that is a problem, try SOLUTION 2, remove the `-bundle-loader` flag
+      ##
+      ## This returns us to the symbol resolution we had in previous Rubies. It feels gross but may
+      ## be a workaround for gem maintainers until we all figure out a better way to deal with this.
+      extdldflags = RbConfig::MAKEFILE_CONFIG["EXTDLDFLAGS"].split
+      if found = extdldflags.index("-bundle_loader")
+        removed_1 = extdldflags.delete_at(found) # flag
+        removed_2 = extdldflags.delete_at(found) # and its argument
+        puts "Removing '#{removed_1} #{removed_2}' from EXTDLDFLAGS"
+      end
+      RbConfig::MAKEFILE_CONFIG["EXTDLDFLAGS"] = extdldflags.join(" ")
+    end
   end
 
   create_makefile("rcd_test/rcd_test_ext")