Make gazelle_python_manifest output deterministic

gazelle/manifest/BUILD.bazel

@@ -5,7 +5,10 @@ go_library(
     srcs = ["manifest.go"],
     importpath = "github.com/bazelbuild/rules_python/gazelle/manifest",
     visibility = ["//visibility:public"],
-    deps = ["@in_gopkg_yaml_v2//:yaml_v2"],
+    deps = [
+        "@com_github_emirpasic_gods//sets/treeset",
+        "@in_gopkg_yaml_v2//:yaml_v2",
+    ],
 )
 
 go_test(

gazelle/manifest/manifest.go

@@ -6,6 +6,8 @@ import (
 	"io"
 	"os"
 
+	"github.com/emirpasic/gods/sets/treeset"
+
 	yaml "gopkg.in/yaml.v2"
 )
 
@@ -94,11 +96,30 @@ func (f *File) Decode(manifestPath string) error {
 	return nil
 }
 
+// ModulesMapping is the type used to map from importable Python modules to
+// the wheel names that provide these modules.
+type ModulesMapping map[string]string
+
+// MarshalYAML makes sure that we sort the module names before marshaling
+// the contents of `ModulesMapping` to a YAML file. This ensures that the
+// file is deterministically generated from the map.
+func (m ModulesMapping) MarshalYAML() (interface{}, error) {
+	var mapslice yaml.MapSlice
+	keySet := treeset.NewWithStringComparator()
+	for key := range m {
+		keySet.Add(key)
+	}
+	for _, key := range keySet.Values() {
+		mapslice = append(mapslice, yaml.MapItem{Key: key, Value: m[key.(string)]})
+	}
+	return mapslice, nil
+}
+
 // Manifest represents the structure of the Gazelle manifest file.
 type Manifest struct {
 	// ModulesMapping is the mapping from importable modules to which Python
 	// wheel name provides these modules.
-	ModulesMapping map[string]string `yaml:"modules_mapping"`
+	ModulesMapping ModulesMapping `yaml:"modules_mapping"`
 	// PipDepsRepositoryName is the name of the pip_install repository target.
 	// DEPRECATED
 	PipDepsRepositoryName string `yaml:"pip_deps_repository_name,omitempty"`

gazelle/manifest/manifest_test.go

@@ -10,7 +10,7 @@ import (
 	"github.com/bazelbuild/rules_python/gazelle/manifest"
 )
 
-var modulesMapping = map[string]string{
+var modulesMapping = manifest.ModulesMapping{
 	"arrow":           "arrow",
 	"arrow.__init__":  "arrow",
 	"arrow.api":       "arrow",
@@ -76,4 +76,4 @@ func TestFile(t *testing.T) {
 			t.FailNow()
 		}
 	})
-}
+}