Provide a helper to check registration uniqueness

This wouldn't be sound in a public package, since everyone can touch the
registry map directly. However, since this is internal, I'm only doing
it because I find it more readable.

Author: featherbread

internal/store/bbolt/factory.go

@@ -11,10 +11,7 @@ import (
 )
 
 func init() {
-	registry.Registry["bbolt"] = registry.Entry{
-		EnvironmentKeys: []string{"DB_PATH"},
-		FactoryFromEnv:  FactoryFromEnv,
-	}
+	registry.Provide("bbolt", FactoryFromEnv, "DB_PATH")
 }
 
 // FactoryFromEnv returns a store.Factory whose stores are backed by a local

internal/store/dynamodb/factory.go

@@ -13,10 +13,8 @@ import (
 )
 
 func init() {
-	registry.Registry["dynamodb"] = registry.Entry{
-		EnvironmentKeys: []string{"DYNAMODB", "DYNAMODB_TABLE", "DYNAMODB_ENDPOINT"},
-		FactoryFromEnv:  FactoryFromEnv,
-	}
+	registry.Provide("dynamodb", FactoryFromEnv,
+		"DYNAMODB", "DYNAMODB_TABLE", "DYNAMODB_ENDPOINT")
 }
 
 // FactoryFromEnv returns a store.Factory whose stores are backed by Amazon

internal/store/firestore/factory.go

@@ -12,10 +12,8 @@ import (
 )
 
 func init() {
-	registry.Registry["firestore"] = registry.Entry{
-		EnvironmentKeys: []string{"FIRESTORE_PROJECT_ID", "FIRESTORE_DATABASE_ID"},
-		FactoryFromEnv:  FactoryFromEnv,
-	}
+	registry.Provide("firestore", FactoryFromEnv,
+		"FIRESTORE_PROJECT_ID", "FIRESTORE_DATABASE_ID")
 }
 
 // FactoryFromEnv returns a store.Factory whose stores are backed by a Google

internal/store/registry/registry.go

@@ -4,6 +4,7 @@ package registry
 
 import (
 	"context"
+	"fmt"
 
 	"github.com/ahamlinman/randomizer/internal/randomizer"
 )
@@ -24,3 +25,19 @@ type Entry struct {
 	// variables.
 	FactoryFromEnv func(context.Context) (func(partition string) randomizer.Store, error)
 }
+
+// Provide registers a new store backend, or panics if a backend is already
+// registered under this name.
+func Provide(
+	name string,
+	factoryFromEnv func(context.Context) (func(partition string) randomizer.Store, error),
+	environmentKeys ...string,
+) {
+	if _, ok := Registry[name]; ok {
+		panic(fmt.Errorf("%s already registered as a store backend", name))
+	}
+	Registry[name] = Entry{
+		EnvironmentKeys: environmentKeys,
+		FactoryFromEnv:  factoryFromEnv,
+	}
+}