From 453859194f29719b190d12f7c5c0190c820afdef Mon Sep 17 00:00:00 2001
From: Alan Agius <alanagius@google.com>
Date: Thu, 21 Mar 2024 09:45:41 +0000
Subject: [PATCH] fix: `ContentLength` incorrectly set for empty files

A regression was introduced in the latest version wherein files with zero bytes would have their `ContentLength` erroneously set to `1`. This resulted in requests failing with an `ERR_CONTENT_LENGTH_MISMATCH`.
---
 src/utils/compatibleAPI.js |  4 +++-
 test/middleware.test.js    | 17 ++++++++++++++++-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/utils/compatibleAPI.js b/src/utils/compatibleAPI.js
index b168f040d..4d2f81fdc 100644
--- a/src/utils/compatibleAPI.js
+++ b/src/utils/compatibleAPI.js
@@ -267,7 +267,9 @@ async function send(req, res, filename, start, end, goNext, options) {
           start,
           end,
         });
-      byteLength = end - start + 1;
+
+      // Handle files with zero bytes
+      byteLength = end === 0 ? 0 : end - start + 1;
     } else {
       bufferOrStream = /** @type {import("fs").readFileSync} */ (
         options.outputFileSystem.readFileSync
diff --git a/test/middleware.test.js b/test/middleware.test.js
index ead7886fc..663bb3eba 100644
--- a/test/middleware.test.js
+++ b/test/middleware.test.js
@@ -124,6 +124,11 @@ describe.each([
             "unknown",
           );
 
+          instance.context.outputFileSystem.writeFileSync(
+            path.resolve(outputPath, "empty-file.txt"),
+            "",
+          );
+
           req = request(app);
         });
 
@@ -469,7 +474,7 @@ describe.each([
           );
         });
 
-        it('should return "200" code code for the "GET" request and "Content-Length" to the file with unicode', async () => {
+        it('should return "200" code for the "GET" request and "Content-Length" to the file with unicode', async () => {
           const response = await req.get("/byte-length.html");
 
           expect(response.statusCode).toEqual(200);
@@ -482,6 +487,16 @@ describe.each([
           );
         });
 
+        it('should return "200" code for the "GET" request and "Content-Length" of "0" when file is empty', async () => {
+          const response = await req.get("/empty-file.txt");
+
+          expect(response.statusCode).toEqual(200);
+          expect(response.headers["content-length"]).toEqual("0");
+          expect(response.headers["content-type"]).toEqual(
+            "text/plain; charset=utf-8",
+          );
+        });
+
         it('should return the "200" code for the "GET" request to the "image image.svg" file', async () => {
           const fileData = instance.context.outputFileSystem.readFileSync(
             path.resolve(outputPath, "image image.svg"),