feat: allow reading config from a file

Author: himanshusinghs

src/common/config/createUserConfig.ts

@@ -95,6 +95,10 @@ function parseUserConfigSources(cliArguments: string[]): {
         ...OPTIONS,
         // This helps parse the relevant environment variables.
         envPrefix: "MDB_MCP_",
+        // This is the name of key that yargs-parser will look up in CLI
+        // arguments (--config) and ENV variables (MDB_MCP_CONFIG) to load an
+        // initial configuration from.
+        config: "config",
         configuration: {
             ...OPTIONS.configuration,
             // Setting this to true will populate `_` variable which is

tests/fixtures/config-with-invalid-value.json

@@ -0,0 +1,4 @@
+{
+  "connectionString": "mongodb://invalid-value-json-localhost:1000",
+  "loggers": "stderr,stderr"
+}

tests/fixtures/valid-config.json

@@ -0,0 +1,4 @@
+{
+  "connectionString": "mongodb://valid-json-localhost:1000",
+  "loggers": "stderr"
+}

tests/unit/common/config.test.ts

@@ -1,3 +1,4 @@
+import path from "path";
 import { describe, it, expect, vi, beforeEach, afterEach, type MockedFunction } from "vitest";
 import { type UserConfig, UserConfigSchema } from "../../../src/common/config/userConfig.js";
 import { type CreateUserConfigHelpers, createUserConfig } from "../../../src/common/config/createUserConfig.js";
@@ -25,6 +26,11 @@ function createEnvironment(): {
     };
 }
 
+const CONFIG_FIXTURES = {
+    VALID: path.resolve(import.meta.dirname, "..", "..", "fixtures", "valid-config.json"),
+    WITH_INVALID_VALUE: path.resolve(import.meta.dirname, "..", "..", "fixtures", "config-with-invalid-value.json"),
+};
+
 describe("config", () => {
     it("should generate defaults from UserConfigSchema that match expected values", () => {
         // Expected hardcoded values (what we had before)
@@ -489,6 +495,87 @@ describe("config", () => {
         });
     });
 
+    describe("loading a config file", () => {
+        let warn: MockedFunction<CreateUserConfigHelpers["onWarning"]>;
+        let error: MockedFunction<CreateUserConfigHelpers["onError"]>;
+        let exit: MockedFunction<CreateUserConfigHelpers["closeProcess"]>;
+
+        beforeEach(() => {
+            warn = vi.fn();
+            error = vi.fn();
+            exit = vi.fn();
+        });
+
+        describe("through env variable MDB_MCP_CONFIG", () => {
+            const { setVariable, clearVariables } = createEnvironment();
+            afterEach(() => {
+                clearVariables();
+            });
+
+            it("should load a valid config file without troubles", () => {
+                setVariable("MDB_MCP_CONFIG", CONFIG_FIXTURES.VALID);
+                const config = createUserConfig({
+                    onWarning: warn,
+                    onError: error,
+                    closeProcess: exit,
+                });
+                expect(warn).not.toBeCalled();
+                expect(error).not.toBeCalled();
+                expect(exit).not.toBeCalled();
+
+                expect(config.connectionString).toBe("mongodb://valid-json-localhost:1000");
+                expect(config.loggers).toStrictEqual(["stderr"]);
+            });
+
+            it("should attempt loading config file with wrong value and exit", () => {
+                setVariable("MDB_MCP_CONFIG", CONFIG_FIXTURES.WITH_INVALID_VALUE);
+                createUserConfig({
+                    onWarning: warn,
+                    onError: error,
+                    closeProcess: exit,
+                });
+                expect(warn).not.toBeCalled();
+                expect(error).toBeCalledWith(
+                    expect.stringContaining("Invalid configuration for the following fields:")
+                );
+                expect(error).toBeCalledWith(expect.stringContaining("loggers - Duplicate loggers found in config"));
+                expect(exit).toBeCalledWith(1);
+            });
+        });
+
+        describe("through cli argument --config", () => {
+            it("should load a valid config file without troubles", () => {
+                const config = createUserConfig({
+                    onWarning: warn,
+                    onError: error,
+                    closeProcess: exit,
+                    cliArguments: ["--config", CONFIG_FIXTURES.VALID],
+                });
+                expect(warn).not.toBeCalled();
+                expect(error).not.toBeCalled();
+                expect(exit).not.toBeCalled();
+
+                expect(config.connectionString).toBe("mongodb://valid-json-localhost:1000");
+                expect(config.loggers).toStrictEqual(["stderr"]);
+            });
+
+            it("should attempt loading config file with wrong value and exit", () => {
+                createUserConfig({
+                    onWarning: warn,
+                    onError: error,
+                    closeProcess: exit,
+                    cliArguments: ["--config", CONFIG_FIXTURES.WITH_INVALID_VALUE],
+                });
+                expect(warn).not.toBeCalled();
+                expect(error).toBeCalledWith(
+                    expect.stringContaining("Invalid configuration for the following fields:")
+                );
+                expect(error).toBeCalledWith(expect.stringContaining("loggers - Duplicate loggers found in config"));
+                expect(exit).toBeCalledWith(1);
+            });
+        });
+    });
+
     describe("precedence rules", () => {
         const { setVariable, clearVariables } = createEnvironment();
 
@@ -499,30 +586,34 @@ describe("config", () => {
         it("positional argument takes precedence over all", () => {
             setVariable("MDB_MCP_CONNECTION_STRING", "mongodb://crazyhost1");
             const actual = createUserConfig({
-                cliArguments: ["mongodb://crazyhost2", "--connectionString", "mongodb://localhost"],
+                cliArguments: [
+                    "mongodb://crazyhost2",
+                    "--config",
+                    CONFIG_FIXTURES.VALID,
+                    "--connectionString",
+                    "mongodb://localhost",
+                ],
             });
             expect(actual.connectionString).toBe("mongodb://crazyhost2/?directConnection=true");
         });
 
-        it("cli arguments take precedence over env vars", () => {
-            setVariable("MDB_MCP_CONNECTION_STRING", "mongodb://crazyhost");
+        it("any cli argument takes precedence over env vars, config and defaults", () => {
+            setVariable("MDB_MCP_CONNECTION_STRING", "mongodb://dummyhost");
             const actual = createUserConfig({
-                cliArguments: ["--connectionString", "mongodb://localhost"],
+                cliArguments: ["--config", CONFIG_FIXTURES.VALID, "--connectionString", "mongodb://host-from-cli"],
             });
-            expect(actual.connectionString).toBe("mongodb://localhost");
+            expect(actual.connectionString).toBe("mongodb://host-from-cli");
         });
 
-        it("any cli argument takes precedence over defaults", () => {
-            const actual = createUserConfig({
-                cliArguments: ["--connectionString", "mongodb://localhost"],
-            });
-            expect(actual.connectionString).toBe("mongodb://localhost");
+        it("any env var takes precedence over config and defaults", () => {
+            setVariable("MDB_MCP_CONNECTION_STRING", "mongodb://dummyhost");
+            const actual = createUserConfig({ cliArguments: ["--config", CONFIG_FIXTURES.VALID] });
+            expect(actual.connectionString).toBe("mongodb://dummyhost");
         });
 
-        it("any env var takes precedence over defaults", () => {
-            setVariable("MDB_MCP_CONNECTION_STRING", "mongodb://localhost");
-            const actual = createUserConfig();
-            expect(actual.connectionString).toBe("mongodb://localhost");
+        it("config file takes precendence over defaults", () => {
+            const actual = createUserConfig({ cliArguments: ["--config", CONFIG_FIXTURES.VALID] });
+            expect(actual.connectionString).toBe("mongodb://valid-json-localhost:1000");
         });
     });