Thread Performance Checker warning during signing

[nebooz]

Environment

• C2PA iOS SDK Version: 0.0.7
• iOS Version: 18.2
• Device: iPhone (physical device)
• Xcode: Latest

What's happening

We're using the SDK to sign photos in our app and it's working well, but we're seeing this warning from Xcode's Thread Performance Checker every time we call builder.sign():

Thread Performance Checker: Thread running at User-initiated quality-of-service class
waiting on a lower QoS thread running at Default quality-of-service class.
Investigate ways to avoid priority inversions

Code

We run the signing on a background thread with .userInitiated priority to keep the UI responsive:

Task.detached(priority: .userInitiated) {
    let signer = try Signer(
        certsPEM: certificatePEM,
        privateKeyPEM: privateKeyPEM,
        algorithm: .ps256,
        tsaURL: "http://timestamp.digicert.com"
    )

    let builder = try Builder(manifestJSON: manifestJSON)
    let sourceStream = try Stream(fileURL: inputURL, truncate: false)
    let destStream = try Stream(fileURL: outputURL, truncate: true)

    try builder.sign(
        format: "image/jpeg",
        source: sourceStream,
        destination: destStream,
        signer: signer
    )
}

The warning appears even though we're explicitly running this on a background thread - it's coming from inside the SDK's internal operations.

Backtrace

0   std::thread::Thread::park
1   std::sync::mpmc::list::Channel::recv
2   std::sync::mpmc::list::Channel::recv::{{closure}}
3   c2pa::utils::hash_utils::hash_by_alg
4   c2pa::claim::Claim::calc_assertion_box_hash
5   c2pa::claim::Claim::add_assertion_with_salt
6   c2pa::builder::Builder::to_claim
7   c2pa::builder::Builder::to_store
8   c2pa::builder::Builder::sign
9   c2pa_builder_sign (FFI bridge)
10  C2PA.Builder.sign (Swift wrapper)

Looks like it's happening in the internal hashing code (frames 3-4). The signing works fine and completes in about 1.5 seconds, but thought you'd want to know about this warning.

[redaranj]

@nebooz Thanks for the report. We're taking a look.