Tweaking config instructions and initialization - resolves #365

Configuration instructions encourage the user to use a configuration
file called `development.yaml` (for self-build) or `production.yaml`
(for Docker build) but do not provide instructions to point Mjölnir
to read from these files.

This patch:
- allows Mjölnir to read `production.yaml` if available, without
	additional instructions needed;
- change the instructions for self-build to use `default.yaml`,
	which is read without additional instructions;
- add instructions in case the user wishes to use `development.yaml`;
- tweak the error message in case the config file isn't setup at
	all to clarify this for users.

Author: Yoric

docs/setup_selfbuild.md

@@ -7,9 +7,26 @@ cd mjolnir
 yarn install
 yarn build
 
-# Copy and edit the config. It *is* recommended to change the data path.
+# Edit the config.
+# You probably should change `dataPath`.
+nano config/default.yaml
+
+node lib/index.js
+```
+
+Or, if you wish to use a different configuration file, e.g. `development.yaml`
+
+```bash
+git clone https://github.com/matrix-org/mjolnir.git
+cd mjolnir
+
+yarn install
+yarn build
+
+# Edit the config.
+# You probably should change `dataPath`.
 cp config/default.yaml config/development.yaml
 nano config/development.yaml
 
-node lib/index.js
+NODE_ENV=development node lib/index.js
 ```

src/config.ts

@@ -172,10 +172,34 @@ const defaultConfig: IConfig = {
 
 export function read(): IConfig {
     const config_dir = process.env.NODE_CONFIG_DIR || "./config";
-    const config_file = `${process.env.NODE_ENV || "default"}.yaml`
-
-    const content = fs.readFileSync(path.join(config_dir, config_file), "utf8");
+    let config_path;
+    for (let key of [
+        process.env.NODE_ENV,
+        "production",
+        "default"
+    ]) {
+        if (!key) {
+            continue;
+        }
+        const candidate_path = path.join(config_dir, `${key}.yaml`);
+        if (!fs.existsSync(candidate_path)) {
+            continue;
+        }
+        config_path = candidate_path;
+        break;
+    }
+    if (!config_path) {
+        throw new Error("Could not find any valid configuration file");
+    }
+    const content = fs.readFileSync(config_path, "utf8");
     const parsed = load(content);
     const config = {...defaultConfig, ...(parsed as object)} as IConfig;
+    if (config.accessToken === "YOUR_TOKEN_HERE") {
+        // A small check to simplify the error message in case
+        // the configuration file hasn't been modified.
+        throw new Error(`Invalid access token in configuration file ${config_path}. `
+            + "This usually indicates that Mjölnir's configuration file was not setup "
+            + "or that Mjölnir is using the wrong configuration file.");
+    }
     return config;
 }