test(compat): add deterministic local fixtures

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

integration/agentcompat/internal/fixture/payload_hash.go

@@ -0,0 +1,43 @@
+package fixture
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+
+	"github.com/nezhahq/nezha/integration/agentcompat/internal/contract"
+)
+
+const verifierBufferBytes = 1024 * 1024
+
+type PayloadDigest struct {
+	Bytes  uint64
+	SHA256 [sha256.Size]byte
+}
+
+func (d PayloadDigest) Hex() string {
+	return hex.EncodeToString(d.SHA256[:])
+}
+
+func VerifyPayload(reader io.Reader, expectedBytes uint64) (PayloadDigest, error) {
+	if expectedBytes > contract.TransferBytes {
+		return PayloadDigest{}, ErrPayloadOverrun
+	}
+	hash := sha256.New()
+	buffer := make([]byte, verifierBufferBytes)
+	limited := io.LimitReader(reader, int64(expectedBytes)+1)
+	written, err := io.CopyBuffer(hash, limited, buffer)
+	if err != nil {
+		return PayloadDigest{}, fmt.Errorf("verify payload: %w", err)
+	}
+	if uint64(written) > expectedBytes {
+		return PayloadDigest{}, ErrPayloadOverrun
+	}
+	if uint64(written) != expectedBytes {
+		return PayloadDigest{}, ErrPayloadSizeMismatch
+	}
+	digest := PayloadDigest{Bytes: uint64(written)}
+	copy(digest.SHA256[:], hash.Sum(nil))
+	return digest, nil
+}