From 26de73133b8552f56ba72e025ecd82b08900d796 Mon Sep 17 00:00:00 2001
From: Taku Amano <taku@taaas.jp>
Date: Mon, 10 Aug 2026 10:20:29 +0900
Subject: [PATCH] fix(etag): stabilize digest across stream chunks (#5205)

* fix(etag): stabilize digest across stream chunks

* perf(etag): optimize fixed-size digest chunks
---
 src/middleware/etag/digest.ts     | 54 ++++++++++++++++++++++++++++++-
 src/middleware/etag/index.test.ts | 31 ++++++++++++++++++
 2 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/src/middleware/etag/digest.ts b/src/middleware/etag/digest.ts
index c6021fbca6..32ceb2122e 100644
--- a/src/middleware/etag/digest.ts
+++ b/src/middleware/etag/digest.ts
@@ -13,6 +13,8 @@ const mergeBuffers = (
   return merged
 }
 
+const CHUNK_SIZE = 256 * 1024
+
 export const generateDigest = async (
   stream: ReadableStream<Uint8Array<ArrayBuffer>> | null,
   generator: (body: Uint8Array<ArrayBuffer>) => ArrayBuffer | Promise<ArrayBuffer>
@@ -22,6 +24,12 @@ export const generateDigest = async (
   }
 
   let result: ArrayBuffer | undefined = undefined
+  let chunk: Uint8Array<ArrayBuffer> | undefined
+  let chunkLength = 0
+
+  const digest = async (body: Uint8Array<ArrayBuffer>) => {
+    result = await generator(mergeBuffers(result, body))
+  }
 
   const reader = stream.getReader()
   for (;;) {
@@ -30,7 +38,51 @@ export const generateDigest = async (
       break
     }
 
-    result = await generator(mergeBuffers(result, value))
+    let offset = 0
+    while (offset < value.byteLength) {
+      const remaining = value.byteLength - offset
+
+      if (chunkLength === 0 && remaining >= CHUNK_SIZE) {
+        await digest(value.subarray(offset, offset + CHUNK_SIZE))
+        offset += CHUNK_SIZE
+        continue
+      }
+
+      const requiredLength = chunkLength + remaining
+      if (requiredLength < CHUNK_SIZE) {
+        if (!chunk) {
+          chunk = value.subarray(offset)
+        } else {
+          if (chunk.byteLength < requiredLength) {
+            const nextChunk = new Uint8Array<ArrayBuffer>(
+              new ArrayBuffer(Math.min(CHUNK_SIZE, Math.max(requiredLength, chunk.byteLength * 2)))
+            )
+            nextChunk.set(chunk.subarray(0, chunkLength))
+            chunk = nextChunk
+          }
+          chunk.set(value.subarray(offset), chunkLength)
+        }
+        chunkLength = requiredLength
+        break
+      }
+
+      const length = CHUNK_SIZE - chunkLength
+      if (chunk?.byteLength !== CHUNK_SIZE) {
+        const nextChunk = new Uint8Array<ArrayBuffer>(new ArrayBuffer(CHUNK_SIZE))
+        if (chunk) {
+          nextChunk.set(chunk.subarray(0, chunkLength))
+        }
+        chunk = nextChunk
+      }
+      chunk.set(value.subarray(offset, offset + length), chunkLength)
+      await digest(chunk)
+      chunkLength = 0
+      offset += length
+    }
+  }
+
+  if (chunk && chunkLength > 0) {
+    await digest(chunk.subarray(0, chunkLength))
   }
 
   if (!result) {
diff --git a/src/middleware/etag/index.test.ts b/src/middleware/etag/index.test.ts
index 63d056833a..ab91fe672f 100644
--- a/src/middleware/etag/index.test.ts
+++ b/src/middleware/etag/index.test.ts
@@ -130,6 +130,37 @@ describe('Etag Middleware', () => {
     expect(res.headers.get('ETag')).not.toBe(hash)
   })
 
+  it('Should return the same etag regardless of ReadableStream chunk boundaries', async () => {
+    const app = new Hono()
+    app.use('/etag/*', etag())
+    app.get('/etag/rs1', (c) => {
+      return c.body(
+        new ReadableStream({
+          start(controller) {
+            controller.enqueue(new Uint8Array(1_000_000))
+            controller.close()
+          },
+        })
+      )
+    })
+    app.get('/etag/rs2', (c) => {
+      return c.body(
+        new ReadableStream({
+          start(controller) {
+            controller.enqueue(new Uint8Array(1))
+            controller.enqueue(new Uint8Array(32_768))
+            controller.enqueue(new Uint8Array(967_231))
+            controller.close()
+          },
+        })
+      )
+    })
+
+    const res1 = await app.request('http://localhost/etag/rs1')
+    const res2 = await app.request('http://localhost/etag/rs2')
+    expect(res2.headers.get('ETag')).toBe(res1.headers.get('ETag'))
+  })
+
   it('Should not return etag header when the stream is empty', async () => {
     const app = new Hono()
     app.use('/etag/*', etag())