起因

新开一条线，需要上传的OTA包里加点内容，好让后台校验它是否是当前这条线(短期最小改动)。

开整

之前看过ota包结构，整包和差分包里都有一个payload_properties.txt文件，所以最简单的就是给这个txt文件里追加点自定义内容，然后测试上传ota包到发布平台上之后，后端通过命令读取这个标志校验，如果校验成功，即可点击发布。
既然思路清楚了，那瞅瞅payload_properties.txt的内容是在哪开始写入的。

grep -rn "payload_properties.txt" *

发现在ota_from_target_files.py里有如下这段代码

 def Sign(self, payload_signer):
    """Generates and signs the hashes of the payload and metadata.

    Args:
      payload_signer: A PayloadSigner() instance that serves the signing work.

    Raises:
      AssertionError: On any failure when calling brillo_update_payload script.
    """
    assert isinstance(payload_signer, PayloadSigner)

    # 1. Generate hashes of the payload and metadata files.
    payload_sig_file = common.MakeTempFile(prefix="sig-", suffix=".bin")
    metadata_sig_file = common.MakeTempFile(prefix="sig-", suffix=".bin")
    cmd = ["brillo_update_payload", "hash",
           "--unsigned_payload", self.payload_file,
           "--signature_size", str(payload_signer.maximum_signature_size),
           "--metadata_hash_file", metadata_sig_file,
           "--payload_hash_file", payload_sig_file]
    self._Run(cmd)

    # 2. Sign the hashes.
    signed_payload_sig_file = payload_signer.Sign(payload_sig_file)
    signed_metadata_sig_file = payload_signer.Sign(metadata_sig_file)

    # 3. Insert the signatures back into the payload file.
    signed_payload_file = common.MakeTempFile(prefix="signed-payload-",
                                              suffix=".bin")
    cmd = ["brillo_update_payload", "sign",
           "--unsigned_payload", self.payload_file,
           "--payload", signed_payload_file,
           "--signature_size", str(payload_signer.maximum_signature_size),
           "--metadata_signature_file", signed_metadata_sig_file,
           "--payload_signature_file", signed_payload_sig_file]
    self._Run(cmd)

    # 4. Dump the signed payload properties.
    properties_file = common.MakeTempFile(prefix="payload-properties-",
                                          suffix=".txt")
    cmd = ["brillo_update_payload", "properties",
           "--payload", signed_payload_file,
           "--properties_file", properties_file]
    self._Run(cmd)

    if self.secondary:
      with open(properties_file, "a") as f:
        f.write("SWITCH_SLOT_ON_REBOOT=0\n")

    if OPTIONS.wipe_user_data:
      with open(properties_file, "a") as f:
        f.write("POWERWASH=1\n")

    self.payload_file = signed_payload_file
    self.payload_properties = properties_file

可以看到最后这里有创建payload_properties.txt，并往里面写入的操作，那在这之前，我们可以加上标志

  # 4. Dump the signed payload properties.
    properties_file = common.MakeTempFile(prefix="payload-properties-",
                                          suffix=".txt")
    cmd = ["brillo_update_payload", "properties",
           "--payload", signed_payload_file,
           "--properties_file", properties_file]
    self._Run(cmd)
    with open(properties_file, "a") as f:
        f.write("Platform_API=1\n")

    if self.secondary:
      with open(properties_file, "a") as f:
        f.write("SWITCH_SLOT_ON_REBOOT=0\n")

    if OPTIONS.wipe_user_data:
      with open(properties_file, "a") as f:
        f.write("POWERWASH=1\n")

    self.payload_file = signed_payload_file
    self.payload_properties = properties_file

编译OTA 整包和差分包，解压后，发现payload_properties.txt里多了这一行，并且也丝毫没有影响正常升级。

读取

unzip -p oriUpdate.zip payload_properties.txt

如下图：

收工！！！