From aeed0e14a04e22adb8c46049605b87319d9c5e04 Mon Sep 17 00:00:00 2001
From: Cezar Augusto <boss@cezaraugusto.net>
Date: Sun, 20 Sep 2026 09:57:16 -0300
Subject: [PATCH] fix: send a one-byte asset with its byte instead of
 Content-Length 0

---
 src/middleware.js              |  2 ++
 src/utils/compatibleAPI.js     |  7 +++++--
 test/middleware.test.js        | 23 +++++++++++++++++++++++
 types/utils/compatibleAPI.d.ts |  2 ++
 4 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/src/middleware.js b/src/middleware.js
index 45cce79..55fa0bf 100644
--- a/src/middleware.js
+++ b/src/middleware.js
@@ -703,6 +703,7 @@ function wrapper(context) {
               extra.outputFileSystem,
               start,
               end,
+              len,
             );
 
             ({ bufferOrStream, byteLength } = result);
@@ -848,6 +849,7 @@ function wrapper(context) {
             extra.outputFileSystem,
             start,
             end,
+            len,
           ));
         } catch (error) {
           await errorHandler(/** @type {NodeJS.ErrnoException} */ (error));
diff --git a/src/utils/compatibleAPI.js b/src/utils/compatibleAPI.js
index 2800981..0175315 100644
--- a/src/utils/compatibleAPI.js
+++ b/src/utils/compatibleAPI.js
@@ -218,6 +218,7 @@ function finish(res, data) {
  * @param {OutputFileSystem} outputFileSystem output file system
  * @param {number} start start
  * @param {number} end end
+ * @param {number} len byte length of the range, or of the whole file
  * @returns {{ bufferOrStream: (Buffer | import("fs").ReadStream), byteLength: number }} result with buffer or stream and byte length
  */
 function createReadStreamOrReadFileSync(
@@ -225,6 +226,7 @@ function createReadStreamOrReadFileSync(
   outputFileSystem,
   start,
   end,
+  len,
 ) {
   /** @type {Buffer | import("fs").ReadStream} */
   let bufferOrStream;
@@ -243,8 +245,9 @@ function createReadStreamOrReadFileSync(
         end,
       });
 
-    // Handle files with zero bytes
-    byteLength = end === 0 ? 0 : end - start + 1;
+    // An empty file and a one-byte file both end at offset 0, so the length
+    // is the caller's byte count. A stat without a size keeps the arithmetic
+    byteLength = Number.isFinite(len) ? len : end - start + 1;
   } else {
     bufferOrStream = outputFileSystem.readFileSync(filename);
     ({ byteLength } = bufferOrStream);
diff --git a/test/middleware.test.js b/test/middleware.test.js
index 52d24c1..5cb5898 100644
--- a/test/middleware.test.js
+++ b/test/middleware.test.js
@@ -1053,6 +1053,10 @@ describe.each([
             path.resolve(outputPath, "empty-file.txt"),
             "",
           );
+          instance.context.outputFileSystem.writeFileSync(
+            path.resolve(outputPath, "one-byte.txt"),
+            "a",
+          );
         });
 
         afterAll(async () => {
@@ -1463,6 +1467,25 @@ describe.each([
           );
         });
 
+        it('should return "200" code for the "GET" request and "Content-Length" of "1" when file has one byte', async () => {
+          const response = await req.get("/one-byte.txt");
+
+          expect(response.statusCode).toBe(200);
+          expect(response.headers["content-length"]).toBe("1");
+          expect(response.text).toBe("a");
+        });
+
+        it('should return "206" code for the "GET" request with a one-byte range', async () => {
+          const response = await req
+            .get("/one-byte.txt")
+            .set("Range", "bytes=0-0");
+
+          expect(response.statusCode).toBe(206);
+          expect(response.headers["content-range"]).toBe("bytes 0-0/1");
+          expect(response.headers["content-length"]).toBe("1");
+          expect(response.text).toBe("a");
+        });
+
         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"),
diff --git a/types/utils/compatibleAPI.d.ts b/types/utils/compatibleAPI.d.ts
index 01c077e..1cf7b13 100644
--- a/types/utils/compatibleAPI.d.ts
+++ b/types/utils/compatibleAPI.d.ts
@@ -218,6 +218,7 @@ declare function finish<
  * @param {OutputFileSystem} outputFileSystem output file system
  * @param {number} start start
  * @param {number} end end
+ * @param {number} len byte length of the range, or of the whole file
  * @returns {{ bufferOrStream: (Buffer | import("fs").ReadStream), byteLength: number }} result with buffer or stream and byte length
  */
 declare function createReadStreamOrReadFileSync(
@@ -225,6 +226,7 @@ declare function createReadStreamOrReadFileSync(
   outputFileSystem: OutputFileSystem,
   start: number,
   end: number,
+  len: number,
 ): {
   bufferOrStream: Buffer | import("fs").ReadStream;
   byteLength: number;