MINOR: [perl] skip bytes and fixed relative to current position in decoder

[arib06]

What is the purpose of the change

Avro::BinaryDecoder skips a writer field it does not need with $reader->seek($n, 0), but whence 0 is SEEK_SET (absolute) rather than SEEK_CUR (relative). skip_bytes (also used for skip_string) and skip_fixed therefore jump to an absolute offset equal to the field length instead of advancing past the field.

When a reader schema projects away a bytes/string/fixed field that is not the last writer field, the stream position is left wrong and every following field is decoded from the wrong offset, silently yielding wrong values (or a later marker-mismatch error on data files). The sibling decode_bytes/decode_fixed consume exactly the field bytes, so this only affects the skip path. Using Fcntl->SEEK_CUR makes the skip advance from the current position.

Verifying this change

This change added tests and can be verified as follows:

• Added cases to t/03_bin_decode.t that decode a record with a projecting reader schema where a skipped bytes field and a skipped fixed field are each followed by another field. The trailing field now decodes correctly; on the current code it reads a wrong value.

Documentation

• Does this pull request introduce a new feature? no