Bug: `ValueWriters.FixedByteBufferWriter` uses `writeBytes()` instead of `writeFixed()` for Avro `fixed[N]` fields

Start in core/src/main/java/org/apache/iceberg/avro/ValueWriters.java at the FixedByteBufferWriter inner class, comparing it with FixedWriter. Reproduce the manifest write/read case described for a FIXED(3) identity partition, then add a regression test that preserves the partition bytes and record_count. Done means fixed fields are encoded without a length prefix and the manifest round trip retains both values.

Apache Iceberg version

1.11.0 (latest release)

Query engine

Snowflake

Please describe the bug 🐞

Bug description

ValueWriters.FixedByteBufferWriter.write(ByteBuffer, Encoder) calls encoder.writeBytes(bytes)
instead of encoder.writeFixed(arr, 0, length). For Avro fixed[N] fields this is wrong:
writeBytes prepends a zigzag-encoded length (e.g. 0x06 for a 3-byte value) before the data,
but the Avro reader consumes exactly N bytes for a fixed field and has no length prefix to skip.
The stray prefix byte spills into the next field in the record.

Affected version: 1.11.0 (also present in prior versions)

Where the bug is

core/src/main/java/org/apache/iceberg/avro/ValueWriters.java, inner class FixedByteBufferWriter:

private static class FixedByteBufferWriter implements ValueWriter<ByteBuffer> {
  private final int length;

  @Override
  public void write(ByteBuffer bytes, Encoder encoder) throws IOException {
    Preconditions.checkArgument(bytes.remaining() == length, ...);
    encoder.writeBytes(bytes);   // ← BUG: should be writeFixed
  }
}

The sibling class FixedWriter (which handles byte[]) already does it correctly:

private static class FixedWriter implements ValueWriter<byte[]> {
  @Override
  public void write(byte[] bytes, Encoder encoder) throws IOException {
    Preconditions.checkArgument(bytes.length == length, ...);
    encoder.writeFixed(bytes);   // ← correct
  }
}

Concrete corruption example

Writing a FIXED(3) partition value [AB CD EF] via FixedByteBufferWriter:

Encoding	Output bytes	What the reader sees
writeBytes (current)	06 AB CD EF	fixed[3] consumes 06 AB CD; EF spills into the next field
writeFixed (correct)	AB CD EF	fixed[3] consumes AB CD EF ✓

When the spill byte EF prefixes the subsequent record_count varint 08 (zigzag for 4 rows),
the combined varint EF 08 decodes to raw value 1135, which zigzag-decodes to -568. Every
reader (Java, native, Spark) trusts this value from the manifest and sees record_count = -568
for a file that has 4 rows.

How to reproduce

1. Create a partitioned Iceberg table with a FIXED(3) identity-partition column.
2. Write a data file via the Iceberg Java writer (triggers InternalWriter → FixedByteBufferWriter).
3. Read the written manifest Avro directly and inspect the data_file.record_count field for the
   partition entry — it will be a large negative number instead of the actual row count.
4. Alternatively: observe that data_file.partition.val8 in the manifest contains 06abcd instead
   of abcdef for a [AB CD EF] partition value.

Proposed fix

In FixedByteBufferWriter.write(), extract the bytes from the ByteBuffer and call
encoder.writeFixed():

@Override
public void write(ByteBuffer bytes, Encoder encoder) throws IOException {
  Preconditions.checkArgument(bytes.remaining() == length,
      "Cannot write byte buffer of length %s as fixed[%s]", bytes.remaining(), length);
  byte[] arr = new byte[length];
  bytes.duplicate().get(arr);
  encoder.writeFixed(arr);
}

A regression test should assert that a round-trip write/read of a manifest entry with a FIXED(N)
identity-partition column preserves the exact partition value bytes and the correct record_count.

Willingness to contribute

• I can contribute a fix for this bug independently
• I would be willing to contribute a fix for this bug with guidance from the Iceberg community
• I cannot contribute a fix for this bug at this time