Fix issue where gem is installed inside another gem's path

I ran into this issue in the tests being run on GitHub Actions,
since it installs our dependencies inside the dd-trace-rb folder.

It's unclear to me if it can happen in actual customer setups,
but I've decided to fix it anyway.

lib/ddtrace/profiling/collectors/code_provenance.rb

@@ -58,12 +58,32 @@ def record_library(library)
           libraries_by_path[library.path] = library
         end
 
+        # Ruby hash maps are guaranteed to keep the insertion order of keys. Here, we sort @libraries_by_path so
+        # that the hash can be iterated in reverse order of paths.
+        #
+        # Why we do this: We do this to make sure that if there are libraries with paths that are prefixes of other
+        # libraries, e.g. '/home/foo' and '/home/foo/bar', we match to the longest path first.
+        # When reverse sorting paths as strings, '/home/foo/bar' will come before '/home/foo'.
+        #
+        # This way, when we iterate the @libraries_by_path hash, we know the first hit will also be the longest.
+        #
+        # Alternatively/in the future we could instead use a trie to match paths, but I doubt for the data sizes we're
+        # looking at that a trie is that much faster than using Ruby's built-in native collections.
+        def sort_libraries_by_longest_path_first
+          @libraries_by_path = @libraries_by_path.sort.reverse!.to_h
+        end
+
         def record_loaded_specs(loaded_specs)
+          recorded_library = false
+
           loaded_specs.each do |spec|
             next if libraries_by_name.key?(spec.name)
 
             record_library(Library.new(type: 'library', name: spec.name, version: spec.version, path: spec.gem_dir))
+            recorded_library = true
           end
+
+          sort_libraries_by_longest_path_first if recorded_library
         end
 
         def record_loaded_files(loaded_files)

spec/ddtrace/profiling/collectors/code_provenance_spec.rb

@@ -74,6 +74,34 @@
     it 'returns self' do
       expect(code_provenance.refresh).to be code_provenance
     end
+
+    context "when a gem's path is inside another gem's path" do
+      # I'm not entirely sure if this can happen in end-user apps, but can happen in CI if bundler is configured to
+      # install dependencies into a subfolder of ddtrace. In particular GitHub Actions does this.
+
+      it 'matches the loaded file to the longest matching path' do
+        code_provenance.refresh(
+          loaded_files: ['/dd-trace-rb/vendor/bundle/ruby/2.7.0/gems/byebug-11.1.3/lib/byebug.rb'],
+          loaded_specs: [
+            instance_double(
+              Gem::Specification,
+              name: 'ddtrace',
+              version: '1.2.3',
+              gem_dir: '/dd-trace-rb'
+            ),
+            instance_double(
+              Gem::Specification,
+              name: 'byebug',
+              version: '4.5.6',
+              gem_dir: '/dd-trace-rb/vendor/bundle/ruby/2.7.0/gems/byebug-11.1.3'
+            )
+          ],
+        )
+
+        expect(code_provenance.generate).to have(1).item
+        expect(code_provenance.generate.first).to have_attributes(name: 'byebug')
+      end
+    end
   end
 
   describe '#generate_json' do