fix: handle empty/null Buffer in JsReturn::from_napi_value

napi_get_buffer_info returns data=null with len=0 for empty buffers.
slice::from_raw_parts requires a non-null pointer even for zero-length
slices, which caused a panic when returning empty buffers from host
functions.

- Handle len=0 case specially by returning Vec::new() directly
- Add explicit null check with error for data=null with len > 0
- Add vitest for host returning Buffer.alloc(0) to guest

Author: simongdavies

src/js-host-api/src/lib.rs

@@ -613,6 +613,20 @@ impl FromNapiValue for JsReturn {
                     "Failed to get buffer info",
                 ));
             }
+            // Handle empty buffers: napi_get_buffer_info returns data=null, len=0
+            // for empty buffers. slice::from_raw_parts requires non-null pointer
+            // even for zero-length slices, so we handle this case specially.
+            if len == 0 {
+                return Ok(JsReturn::Buffer(Vec::new()));
+            }
+            // Non-empty buffer: data must be valid and non-null.
+            // If it's null with len > 0, the buffer's backing store was likely
+            // garbage collected (e.g., a Buffer.subarray view whose parent died).
+            if data.is_null() {
+                return Err(napi::Error::from_reason(
+                    "Buffer has null data pointer with non-zero length - backing store may have been garbage collected"
+                ));
+            }
             // SAFETY: data points to len bytes of valid buffer memory.
             let bytes = unsafe { std::slice::from_raw_parts(data as *const u8, len) }.to_vec();
             return Ok(JsReturn::Buffer(bytes));

src/js-host-api/tests/host-functions.test.js

@@ -627,6 +627,28 @@ describe('Binary data support', () => {
         expect(result).toEqual({ len: 0 });
     });
 
+    it('should handle host returning empty Buffer', async () => {
+        // Regression: napi_get_buffer_info returns data=null, len=0 for
+        // empty buffers. JsReturn::from_napi_value must not panic on the
+        // null pointer — it should return an empty Vec instead.
+        const loaded = await buildLoadedSandbox(
+            (proto) => {
+                proto.hostModule('host').register('empty_response', () => {
+                    return Buffer.alloc(0);
+                });
+            },
+            `
+            import * as host from "host:host";
+            function handler() {
+                const data = host.empty_response();
+                return { len: data.length, isUint8: data instanceof Uint8Array };
+            }
+            `
+        );
+        const result = await loaded.callHandler('handler', {});
+        expect(result).toEqual({ len: 0, isUint8: true });
+    });
+
     it('should round-trip binary data (send and receive)', async () => {
         const loaded = await buildLoadedSandbox(
             (proto) => {