test(compat): add deterministic local fixtures

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

integration/agentcompat/internal/fixture/payload_reader.go

@@ -0,0 +1,68 @@
+package fixture
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"fmt"
+	"io"
+
+	"github.com/nezhahq/nezha/integration/agentcompat/internal/contract"
+)
+
+const payloadBlockSize = sha256.Size
+
+type Payload struct {
+	seed contract.Seed
+	size uint64
+}
+
+type payloadReader struct {
+	payload Payload
+	offset  uint64
+}
+
+func NewPayload(seed contract.Seed, size uint64) (Payload, error) {
+	if seed == 0 {
+		return Payload{}, fmt.Errorf("payload seed must be nonzero")
+	}
+	if size > contract.TransferBytes {
+		return Payload{}, ErrPayloadOverrun
+	}
+	return Payload{seed: seed, size: size}, nil
+}
+
+func (p Payload) Reader() io.Reader {
+	return &payloadReader{payload: p}
+}
+
+func (r *payloadReader) Read(destination []byte) (int, error) {
+	if r.offset >= r.payload.size {
+		return 0, io.EOF
+	}
+	remaining := r.payload.size - r.offset
+	if uint64(len(destination)) > remaining {
+		destination = destination[:remaining]
+	}
+	written := fillPayload(destination, r.payload.seed, r.offset)
+	r.offset += uint64(written)
+	if r.offset == r.payload.size {
+		return written, io.EOF
+	}
+	return written, nil
+}
+
+func fillPayload(destination []byte, seed contract.Seed, offset uint64) int {
+	written := 0
+	for written < len(destination) {
+		absoluteOffset := offset + uint64(written)
+		blockIndex := absoluteOffset / payloadBlockSize
+		blockOffset := absoluteOffset % payloadBlockSize
+		var input [16]byte
+		binary.BigEndian.PutUint64(input[:8], uint64(seed))
+		binary.BigEndian.PutUint64(input[8:], blockIndex)
+		block := sha256.Sum256(input[:])
+		copied := copy(destination[written:], block[blockOffset:])
+		written += copied
+	}
+	return written
+}