fix: decode only the viewed bytes of Uint8Array log data (GoogleCloudPlatform#772)

Co-authored-by: dylayed <dylayed@users.noreply.github.com>

Author: dylayed

src/logger.ts

@@ -163,7 +163,15 @@ function processData(data: Uint8Array | string, encoding?: BufferEncoding) {
   let decodedData;
   try {
     if (data instanceof Uint8Array) {
-      decodedData = Buffer.from(data.buffer).toString();
+      // Decode only the bytes the view actually spans. A Uint8Array (e.g. a
+      // Buffer carved from Node's shared pool) is often a view into a larger
+      // ArrayBuffer, so reading `data.buffer` directly would include unrelated
+      // bytes outside the view's byteOffset/byteLength range.
+      decodedData = Buffer.from(
+        data.buffer,
+        data.byteOffset,
+        data.byteLength,
+      ).toString();
     } else {
       decodedData = Buffer.from(data, encoding).toString();
     }

test/logger.ts

@@ -147,6 +147,18 @@ describe('getModifiedData', () => {
     assert.equal(modifiedData, expectedTextOutput);
   });
 
+  it('uint8Array that is a view into a larger (pooled) buffer', () => {
+    // Node.js allocates most Buffers from a shared internal pool, so a Buffer
+    // written to stdout/stderr is frequently a view with a non-zero byteOffset
+    // into a much larger ArrayBuffer. The logger must honor the view's
+    // byteOffset/byteLength rather than decoding the entire backing buffer.
+    const pool = Buffer.alloc(64, 0x2e /* '.' */);
+    const view = pool.subarray(8, 8 + sampleText.length);
+    view.write(sampleText);
+    const modifiedData = getModifiedData(view);
+    assert.equal(modifiedData, expectedTextOutput);
+  });
+
   it('simple text with error', () => {
     const modifiedData = getModifiedData(sampleText, undefined, true);
     const expectedOutput =