feat: add robust path validation and error handling for file output

- Validate output directory exists and is writable before attempting manifest generation
- Provide specific error messages for common failure cases (ENOENT, EACCES)
- Check directory permissions early to fail fast with helpful guidance
- Improve error messages to help users resolve issues quickly

Author: taeold

src/bin/firebase-functions.ts

@@ -25,6 +25,7 @@
 import * as http from "http";
 import * as express from "express";
 import * as fs from "fs/promises";
+import * as path from "path";
 import { loadStack } from "../runtime/loader";
 import { stackToWire } from "../runtime/manifest";
 
@@ -57,20 +58,40 @@ function handleQuitquitquit(req: express.Request, res: express.Response, server:
 
 if (process.env.FUNCTIONS_MANIFEST_OUTPUT_PATH) {
   void (async () => {
+    const outputPath = process.env.FUNCTIONS_MANIFEST_OUTPUT_PATH;
     try {
+      // Validate the output path
+      const dir = path.dirname(outputPath);
+      try {
+        await fs.access(dir, fs.constants.W_OK);
+      } catch (e) {
+        console.error(
+          `Error: Cannot write to directory '${dir}': ${e instanceof Error ? e.message : String(e)}`
+        );
+        console.error("Please ensure the directory exists and you have write permissions.");
+        process.exit(1);
+      }
+
       const stack = await loadStack(functionsDir);
       const wireFormat = stackToWire(stack);
-      await fs.writeFile(
-        process.env.FUNCTIONS_MANIFEST_OUTPUT_PATH,
-        JSON.stringify(wireFormat, null, 2)
-      );
+      await fs.writeFile(outputPath, JSON.stringify(wireFormat, null, 2));
       process.exit(0);
-    } catch (e) {
-      console.error(
-        `Failed to generate manifest from function source: ${
-          e instanceof Error ? e.message : String(e)
-        }`
-      );
+    } catch (e: any) {
+      if (e.code === "ENOENT") {
+        console.error(`Error: Directory '${path.dirname(outputPath)}' does not exist.`);
+        console.error("Please create the directory or specify a valid path.");
+      } else if (e.code === "EACCES") {
+        console.error(`Error: Permission denied writing to '${outputPath}'.`);
+        console.error("Please check file permissions or choose a different location.");
+      } else if (e.message?.includes("Failed to generate manifest")) {
+        console.error(e.message);
+      } else {
+        console.error(
+          `Failed to generate manifest from function source: ${
+            e instanceof Error ? e.message : String(e)
+          }`
+        );
+      }
       if (e instanceof Error && e.stack) {
         console.error(e.stack);
       }