Varint Frame Decoder

Decode a length‑prefixed frame from a byte buffer using unsigned varint length encoding.

Function signature

func DecodeFrame(buf []byte, maxLen int) (payload []byte, consumed int, err error)

Requirements

• Length is encoded as unsigned varint (7 bits per byte, continuation bit 0x80).
• If the varint is incomplete, return ErrNeedMore and consume nothing.
• If the varint is malformed or overflows 64 bits, return ErrMalformedVarint.
• If length > maxLen, return ErrFrameTooLarge.
• If the payload is incomplete, return ErrNeedMore and consume nothing.
• On success, return a slice of the input buffer (zero‑copy) and the number of bytes consumed.

Constraints

• maxLen >= 0.
• Do not allocate on the hot path (tests enforce zero allocations).

Notes

• Do not convert to string.
• consumed must be varintBytes + length.