Check for magic number before reading metadata length.

Also make sure that metadata length is available.

src/hyparquet.d.ts

@@ -29,12 +29,19 @@ export async function parquetRead(options: ParquetReadOptions): Promise<void>
  * An AsyncBuffer is like an ArrayBuffer, but the slices are loaded
  * asynchronously, possibly over the network.
  *
+ * You must provide the byteLength of the buffer, typically from a HEAD request.
+ *
+ * In theory, you could use suffix-range requests to fetch the end of the file,
+ * and save a round trip. But in practice, this doesn't work because chrome
+ * deems suffix-range requests as a not-safe-listed header, and will require
+ * a pre-flight. So the byteLength is required.
+ *
  * To make this efficient, we initially request the last 512kb of the file,
  * which is likely to contain the metadata. If the metadata length exceeds the
  * initial fetch, 512kb, we request the rest of the metadata from the AsyncBuffer.
  *
  * This ensures that we either make one 512kb initial request for the metadata,
- * or two requests for exactly the metadata size.
+ * or a second request for up to the metadata size.
  *
  * @param {AsyncBuffer} asyncBuffer parquet file contents
  * @param {number} initialFetchSize initial fetch size in bytes (default 512kb)

src/metadata.js

@@ -31,9 +31,21 @@ export async function parquetMetadataAsync(asyncBuffer, initialFetchSize = 1 <<
   // fetch last bytes (footer) of the file
   const footerOffset = Math.max(0, asyncBuffer.byteLength - initialFetchSize)
   const footerBuffer = await asyncBuffer.slice(footerOffset)
-  // check if metadata size fits inside the initial fetch
+
+  // Check for parquet magic number "PAR1"
   const footerView = new DataView(footerBuffer)
+  if (footerView.getUint32(footerBuffer.byteLength - 4, true) !== 0x31524150) {
+    throw new Error('parquet file invalid (footer != PAR1)')
+  }
+
+  // Parquet files store metadata at the end of the file
+  // Metadata length is 4 bytes before the last PAR1
   const metadataLength = footerView.getUint32(footerBuffer.byteLength - 8, true)
+  if (metadataLength > asyncBuffer.byteLength - 8) {
+    throw new Error(`parquet metadata length ${metadataLength} exceeds available buffer ${asyncBuffer.byteLength - 8}`)
+  }
+
+  // check if metadata size fits inside the initial fetch
   if (metadataLength + 8 > initialFetchSize) {
     // fetch the rest of the metadata
     const metadataOffset = asyncBuffer.byteLength - metadataLength - 8
@@ -65,7 +77,7 @@ export function parquetMetadata(arrayBuffer) {
     throw new Error('parquet file is too short')
   }
   if (view.getUint32(view.byteLength - 4, true) !== 0x31524150) {
-    throw new Error('parquet file invalid magic number')
+    throw new Error('parquet file invalid (footer != PAR1)')
   }
 
   // Parquet files store metadata at the end of the file

test/metadata.test.js

@@ -33,7 +33,14 @@ describe('parquetMetadata', () => {
 
   it('should throw an error for invalid magic number', () => {
     const arrayBuffer = new ArrayBuffer(8)
-    expect(() => parquetMetadata(arrayBuffer)).toThrow('parquet file invalid magic number')
+    expect(() => parquetMetadata(arrayBuffer))
+      .toThrow('parquet file invalid (footer != PAR1)')
+  })
+
+  it('should throw an error for invalid metadata length', () => {
+    const { buffer } = new Uint8Array([255, 255, 255, 255, 80, 65, 82, 49])
+    expect(() => parquetMetadata(buffer))
+      .toThrow('parquet metadata length 4294967295 exceeds available buffer 0')
   })
 })
 
@@ -50,6 +57,18 @@ describe('parquetMetadataAsync', () => {
     const result = await parquetMetadataAsync(asyncBuffer, 1609)
     expect(toJson(result)).containSubset(rowgroupsMetadata)
   })
+
+  it('should throw an error for invalid magic number', () => {
+    const { buffer } = new Uint8Array([255, 255, 255, 255, 255, 255, 255, 255])
+    expect(parquetMetadataAsync(buffer)).rejects
+      .toThrow('parquet file invalid (footer != PAR1)')
+  })
+
+  it('should throw an error for invalid metadata length', () => {
+    const { buffer } = new Uint8Array([255, 255, 255, 255, 80, 65, 82, 49])
+    expect(parquetMetadataAsync(buffer)).rejects
+      .toThrow('parquet metadata length 4294967295 exceeds available buffer 0')
+  })
 })
 
 // Parquet v1 from DuckDB