Fix issue with old rubygems not detecting musl linux properly

ruby/lib/libdatadog.rb

@@ -11,6 +11,23 @@ def self.available_binaries
   def self.pkgconfig_folder(pkgconfig_file_name = "ddprof_ffi_with_rpath.pc")
     current_platform = Gem::Platform.local.to_s
 
+    if RbConfig::CONFIG["arch"].include?("-musl") && !current_platform.include?("-musl")
+      # Fix/workaround for https://github.com/DataDog/dd-trace-rb/issues/2222
+      #
+      # Old versions of rubygems (for instance 3.0.3) don't properly detect alternative libc implementations on Linux;
+      # in particular for our case, they don't detect musl. (For reference, Rubies older than 2.7 may have shipped with
+      # an affected version of rubygems).
+      # In such cases, we fall back to use RbConfig::CONFIG['arch'] instead.
+      #
+      # Why not use RbConfig::CONFIG['arch'] always? Because Gem::Platform.local.to_s does some normalization we want
+      # in other situations -- for instance, it turns `x86_64-linux-gnu` to `x86_64-linux`. So for now we only add this
+      # workaround in a specific situation where we actually know it is wrong.
+      #
+      # See also https://github.com/rubygems/rubygems/pull/2922 and https://github.com/rubygems/rubygems/pull/4082
+
+      current_platform = RbConfig::CONFIG["arch"]
+    end
+
     return unless available_binaries.include?(current_platform)
 
     pkgconfig_file = Dir.glob("#{vendor_directory}/#{current_platform}/**/#{pkgconfig_file_name}").first

ruby/lib/libdatadog/version.rb

@@ -5,7 +5,7 @@ module Libdatadog
   LIB_VERSION = "0.7.0"
 
   GEM_MAJOR_VERSION = "1"
-  GEM_MINOR_VERSION = "0"
+  GEM_MINOR_VERSION = "1"
   GEM_PRERELEASE_VERSION = "" # remember to include dot prefix, if needed!
   private_constant :GEM_MAJOR_VERSION, :GEM_MINOR_VERSION, :GEM_PRERELEASE_VERSION
 

ruby/spec/libdatadog_spec.rb

@@ -66,15 +66,41 @@
         let(:pkgconfig_folder) { "#{temporary_directory}/#{Gem::Platform.local}/some/folder/containing/the/pkgconfig/file" }
 
         before do
-          FileUtils.mkdir_p(pkgconfig_folder)
-          File.open("#{pkgconfig_folder}/ddprof_ffi_with_rpath.pc", "w") {}
+          create_dummy_pkgconfig_file(pkgconfig_folder)
+        end
+
+        def create_dummy_pkgconfig_file(pkgconfig_folder)
+          begin
+            FileUtils.mkdir_p(pkgconfig_folder)
+          rescue Errno::EEXIST
+            # No problem, a few specs try to create the same folder
+          end
+
+          File.open("#{pkgconfig_folder}/ddprof_ffi_with_rpath.pc", "w+") {}
         end
 
         describe ".pkgconfig_folder" do
           it "returns the folder containing the pkgconfig file" do
             expect(Libdatadog.pkgconfig_folder).to eq pkgconfig_folder
           end
         end
+
+        context "when `RbConfig::CONFIG[\"arch\"]` indicates we're on musl libc, but `Gem::Platform.local.to_s` does not detect it" do
+          # Fix for https://github.com/DataDog/dd-trace-rb/issues/2222
+
+          before do
+            allow(RbConfig::CONFIG).to receive(:[]).with("arch").and_return("x86_64-linux-musl")
+            allow(Gem::Platform).to receive(:local).and_return("x86_64-linux")
+
+            ["x86_64-linux", "x86_64-linux-musl"].each do |arch|
+              create_dummy_pkgconfig_file("#{temporary_directory}/#{arch}/some/folder/containing/the/pkgconfig/file")
+            end
+          end
+
+          it "returns the folder containing the pkgconfig file for the musl variant" do
+            expect(Libdatadog.pkgconfig_folder).to eq "#{temporary_directory}/x86_64-linux-musl/some/folder/containing/the/pkgconfig/file"
+          end
+        end
       end
 
       context "but not for the current platform" do