Deadlock when using the print function

I found this really strange bug in which the print function causes a deadlock when called within a dispatch queue.

The deadlock occurs at the line 

print("AuthorizationManager.description: BEFORE queue")

in AuthorizationManager.description, which is indirectly called by

print(self)

Intriguingly, changing the above line to any of the following prevents the deadlock:

 print(self.description)
 print("\(self)")
 print(self, to: &self.someString)

Furthermore, removing the line

print("Refreshing")

also prevents the deadlock.

I suspect that this bug is related to the fact that the print function is synchronized.

I also tested out other locking mechanisms instead of DispatchQueue, including NSLock, NSRecursiveLock and even this fancy implementation from swift-log, and I experienced the exact same bug.

I posted this issue on the swift forums, and a commenter summarized the cause of the bug as follows:

So I found that for a dead lock to happen, you need to repeatedly and concurrently do the following:

1. You run a block in dispatchQueue,
2. The block prints anything [In this case it is the line `print("Refreshing")`],
3. The block runs print(X) on a separated concurrent queue, where
   • X is CustomStringConvertible that uses dispatchQueue inside description.

Here is the code that causes the issue:

import Foundation

class AuthorizationManager: CustomStringConvertible {
    
    let dispatchQueue = DispatchQueue(label: "AuthorizationManager")

    var description: String {
        print("AuthorizationManager.description: BEFORE queue")
        // MARK: - Deadlock Occurs Here -
        return dispatchQueue.sync {
            print("AuthorizationManager.description: INSIDE queue")
            return "I am an authorization manager"
        }
    }

    func refreshTokens() {
        dispatchQueue.sync {
            print("Refreshing")
        }
        DispatchQueue.global().async {
            print(self)
            
        }
    }
    
}

/// Entry Point.
func testDeadlock2() {

    let authorizationManager = AuthorizationManager()
    
    for i in 0..<100 {
        print(i)
        authorizationManager.refreshTokens()
    }
    
    print("\n--- Finished ---\n")
    
}