Fetching remote configuration on app boot (Vapor 3)

1_DispatchBox.swift

import Dispatch

// Utility class for thread-safe access to an "app global" variable
final class DispatchBox<T>: Service {
    private let queue = DispatchQueue(label: "DispatchBox_\(T.self)_Queue")
    private var _value: T?

    var value: T? {
        get { return queue.sync { self._value }}
        set { queue.async { self._value = newValue }}
    }
}

2_SampleConfiguration.swift

import Foundation

// This sample class can be used to parse a JSON like:
// { "secret": "FOOBAR" }
struct SampleConfiguration: Decodable {
    let secret: String
}

3_configure.swift

import Vapor

public func configure(_ config: inout Config, _ env: inout Environment, _ services: inout Services) throws {
    // ... other configuration ...
    
    // Creating a holder for SampleConfiguration
    let holder = DispatchBox<SampleConfiguration>()
    services.register(holder)
}

4_boot.swift

import Vapor

/// Called after your application has initialized.
public func boot(_ app: Application) throws {
    let client = try app.make(Client.self)
    let holder = try app.make(DispatchBox<SampleConfiguration>.self)

    let config = try client.get("https://api.jsonbin.io/b/5b5f00c4f24d8943d0505d0a")
        .flatMap { try $0.content.decode(SampleConfiguration.self) }
        .wait()

    holder.value = config
}

5_routes.swift

import Foundation
import Vapor

/// Register your application's routes here.
public func routes(_ router: Router) throws {
    router.get("secret") { req -> String in
        let holder = try req.make(DispatchBox<SampleConfiguration>.self)
        return holder.value?.secret ?? ""
    }
}

With All exceptions turned on I receive an exception with the code returning the config from jsonbin.io. If I turn exceptions off it all works as expected, just a little worried about what's happening there. Tried to put it into a do-catch but couldn't get it to hit that.

It's an exception internal to URLSession, probably related to the jsonbin.io site's certificates.
I don't think we can do anything about it, nor that you should worry about it.

#0	0x00007fff4fc941f6 in __cxa_throw ()
#1	0x00007fff35eccf0f in Security::UnixError::throwMeNoLogging(int) ()
#2	0x00007fff35c78f18 in Security::safeCopyFile(char const*, unsigned int, char const*, unsigned short) ()
#3	0x00007fff35c77ce1 in Security::MDSSession::updateDataBases() ()
#4	0x00007fff35cc9ff5 in Security::MDSSession::DbOpen(char const*, cssm_net_address const*, unsigned int, Security::AccessCredentials const*, void const*, long&) ()
#5	0x00007fff35cc9e74 in mds_DbOpen(long, char const*, cssm_net_address const*, unsigned int, cssm_access_credentials const*, void const*, long*) ()
#6	0x00007fff35c77223 in Security::MDSClient::Directory::cdsa() const ()
#7	0x00007fff35d8a9a0 in Security::MDSClient::Directory::dlGetFirst(cssm_query const&, cssm_db_record_attribute_data&, cssm_data*, cssm_db_unique_record*&) ()
#8	0x00007fff35c76cec in Security::CssmClient::Table<Security::MDSClient::Common>::startQuery(Security::CssmQuery const&, bool) ()
#9	0x00007fff35c769e1 in Security::CssmClient::Table<Security::MDSClient::Common>::fetch(Security::CssmClient::Query const&, int) ()
#10	0x00007fff35c761bc in MdsComponent::MdsComponent(Security::Guid const&) ()
#11	0x00007fff35c75df8 in CssmManager::loadModule(Security::Guid const&, unsigned int, Security::ModuleCallback const&) ()
#12	0x00007fff35c75b97 in CSSM_ModuleLoad ()
#13	0x00007fff35c756ab in Security::CssmClient::ModuleImpl::activate() ()
#14	0x00007fff35c7545e in Security::CssmClient::AttachmentImpl::activate() ()
#15	0x00007fff35c75362 in Security::KeychainCore::Certificate::clHandle() ()
#16	0x00007fff35c9a74a in Security::KeychainCore::Certificate::copyFirstFieldValue(cssm_data const&) ()
#17	0x00007fff35c9a5e4 in Security::KeychainCore::Certificate::publicKey() ()
#18	0x00007fff35c9a4b2 in SecCertificateCopyPublicKey ()
#19	0x00007fff4fb5d2dd in boringssl_context_extract_certificates_from_x509_store ()
#20	0x00007fff4fb551a7 in _boringssl_context_SSL_ctx_verify_callback ()
#21	0x00007fff4fbcb301 in ssl_verify_cert_chain ()
#22	0x00007fff4fb6b521 in ssl3_connect ()
#23	0x00007fff4fbcdb4e in SSL_do_handshake ()
#24	0x00007fff4fb53f5c in boringssl_session_handshake_continue ()
#25	0x00007fff4fb53b0d in boringssl_context_handshake_negotiate ()
#26	0x00007fff4fb5ed51 in nw_protocol_boringssl_handshake_negotiate ()
#27	0x00000001019b9cae in _dispatch_call_block_and_release ()
#28	0x00000001019b1d8f in _dispatch_client_callout ()
#29	0x00000001019c7bbb in _dispatch_queue_serial_drain ()
#30	0x00000001019b9801 in _dispatch_queue_invoke ()
#31	0x00000001019c8fca in _dispatch_root_queue_drain_deferred_wlh ()
#32	0x00000001019cdea3 in _dispatch_workloop_worker_thread ()
#33	0x0000000101a2dfd6 in _pthread_wqthread ()
#34	0x0000000101a2dbed in start_wqthread ()

Thanks 👍