chore(runtime): Read configuration from disk, if present (#196)

packages/celest/lib/src/config/env.dart

@@ -9,12 +9,12 @@ sealed class ConfigurationValue implements ContextKey<String> {
 
   @override
   String? read(Context context) {
-    return context.platform.environment[name];
+    return context[this] as String? ?? context.platform.environment[name];
   }
 
   @override
   void set(Context context, String? value) {
-    throw UnsupportedError('Cannot set a configuration value at runtime.');
+    context[this] = value;
   }
 }
 
@@ -40,6 +40,14 @@ final class env extends ConfigurationValue {
 
   @override
   String toString() => 'env($name)';
+
+  @override
+  bool operator ==(Object other) {
+    return identical(this, other) || other is env && other.name == name;
+  }
+
+  @override
+  int get hashCode => Object.hash(env, name);
 }
 
 final class _staticEnv extends env {
@@ -60,4 +68,12 @@ final class secret extends ConfigurationValue {
 
   @override
   String toString() => 'secret($name)';
+
+  @override
+  bool operator ==(Object other) {
+    return identical(this, other) || other is secret && other.name == name;
+  }
+
+  @override
+  int get hashCode => Object.hash(secret, name);
 }

packages/celest/lib/src/core/context.dart

@@ -88,12 +88,7 @@ final class Context {
   Traceparent get currentTrace => expect(ContextKey.currentTrace);
 
   /// The Celest [Environment] of the running service.
-  Environment get environment {
-    return switch (get(env.environment)) {
-      final env? => Environment(env),
-      _ => throw StateError('No environment set in context'),
-    };
-  }
+  Environment get environment => expect(env.environment) as Environment;
 
   /// The HTTP client for the current context.
   http.Client get httpClient =>

packages/celest/lib/src/runtime/http/environment.dart

@@ -0,0 +1,67 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:celest/src/config/env.dart';
+import 'package:celest/src/core/context.dart';
+import 'package:celest_core/_internal.dart';
+import 'package:logging/logging.dart';
+
+/// Configures the environment in which Celest is running.
+Future<void> configureEnvironment() async {
+  final script = Platform.script;
+  final configUri = script.resolve('./config.json');
+  if (!FileSystemEntity.isFileSync(configUri.toFilePath())) {
+    if (kReleaseMode) {
+      Logger.root.finer(
+        'No config.json file found. Skipping environment configuration.',
+      );
+    }
+    return;
+  }
+
+  final configFile = await File.fromUri(configUri)
+      .readAsString()
+      .onError((e, _) => throw StateError('Failed to load config.json: $e'));
+
+  final configJson = jsonDecode(configFile);
+  if (configJson is! Map<String, Object?>) {
+    throw FormatException(
+      'Invalid config.json: $configJson. '
+      'Expected Map<String, Object?>, got ${configJson.runtimeType}',
+    );
+  }
+  final environmentVariables = configJson['environmentVariables'];
+  if (environmentVariables is! Map<String, Object?>) {
+    throw FormatException(
+      'Invalid "environmentVariables" in config.json: $environmentVariables. '
+      'Expected Map<String, String>, got ${environmentVariables.runtimeType}',
+    );
+  }
+  final secrets = configJson['secrets'];
+  if (secrets is! Map<String, Object?>) {
+    throw FormatException(
+      'Invalid "secrets" in config.json: $secrets. '
+      'Expected Map<String, String>, got ${secrets.runtimeType}',
+    );
+  }
+
+  for (final MapEntry(key: name, :value) in environmentVariables.entries) {
+    if (value is! String) {
+      throw FormatException(
+        'Invalid value for environment variable "$name" in config.json: $value. '
+        'Expected String, got ${value.runtimeType}',
+      );
+    }
+    Context.root.put(env(name), value);
+  }
+
+  for (final MapEntry(key: name, :value) in secrets.entries) {
+    if (value is! String) {
+      throw FormatException(
+        'Invalid value for secret "$name" in config.json: $value. '
+        'Expected String, got ${value.runtimeType}',
+      );
+    }
+    Context.root.put(secret(name), value);
+  }
+}

packages/celest/lib/src/runtime/serve.dart

@@ -8,6 +8,7 @@ import 'package:async/async.dart';
 import 'package:celest/celest.dart';
 import 'package:celest/src/runtime/gcp/gcp.dart';
 import 'package:celest/src/runtime/http/cloud_middleware.dart';
+import 'package:celest/src/runtime/http/environment.dart';
 import 'package:celest/src/runtime/http/logging.dart';
 import 'package:celest/src/runtime/http/middleware.dart';
 import 'package:celest/src/runtime/json_utils.dart';
@@ -35,6 +36,7 @@ Future<void> serve({
   required Map<String, CloudFunctionTarget> targets,
 }) async {
   configureLogging();
+  await configureEnvironment();
   final projectId = await googleCloudProject();
   if (projectId != null) {
     Context.root.put(googleCloudProjectKey, projectId);