fix(storage): allow setting full checksum at finalize for appendable uploads

[v-pratap]

Fixes the checksum validation logic at the end of appendable uploads so users can provide a final expected CRC.

[gemini-code-assist]

Code Review

This pull request updates AsyncWriterConnectionImpl::Finalize to support manually injecting a user-provided CRC32C value via options. The review feedback highlights an inconsistency where current_options is checked in one condition while options_ is checked in another, which could lead to incorrect behavior. Additionally, options_ should be checked for nullptr before dereferencing to prevent potential crashes. A refactored code block is suggested to address these issues.

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[gemini-code-assist]

There is an inconsistency in how options are checked. In line 150, current_options is checked for UseCrc32cValueOption, but in line 156, options_ is checked instead. If the option is set at the connection level (options_) but not in the current thread-local options (current_options), the CRC value will not be injected, and the appendable upload check will also fail to set action = PartialUpload::kFinalize (since !options_->has... will be false). Additionally, options_ should be checked for nullptr before dereferencing to prevent potential crashes.

  auto current_options = google::cloud::internal::CurrentOptions();
  auto has_crc_option = current_options.has<google::cloud::storage::UseCrc32cValueOption>() ||
                        (options_ && options_->has<google::cloud::storage::UseCrc32cValueOption>());

  // Default to letting the internal hash function compute and send the checksum.
  auto action = PartialUpload::kFinalizeWithChecksum;

  if (has_crc_option) {
    // The user provided a final CRC. We manually inject it into the request.
    // We use `kFinalize` so the internal hash function doesn't overwrite it.
    auto crc = current_options.has<google::cloud::storage::UseCrc32cValueOption>()
                   ? current_options.get<google::cloud::storage::UseCrc32cValueOption>()
                   : options_->get<google::cloud::storage::UseCrc32cValueOption>();
    write.mutable_object_checksums()->set_crc32c(std::move(crc));
    action = PartialUpload::kFinalize;
  } else if (is_append) {
    // For appendable uploads, the internal hash function only sees the chunks uploaded
    // in this stream, not the full object. We use `kFinalize` to avoid sending this
    // partial CRC, which would otherwise fail validation.
    action = PartialUpload::kFinalize;
  }