Fix crash on iOS 17

This fixes an arithmetic overflow crash on iOS 17 caused by the fallback
lock.

In iOS 17, Swift Mutexes are not available, so we have a fallback class
for NdbUseLock that does not make use of them. This allows some thread
safety for iOS 17 users, but unfortunately not as much as iOS 18+ users.

This attempts to fix those remaining race conditions and subsequent
crashes by using `NSLock` in the fallback class, which is available on
iOS 17.

Closes: https://github.com/damus-io/damus/issues/3512
Changelog-Fixed: Fixed a crash on iOS 17 that would happen on startup
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/NdbUseLock.swift

@@ -7,6 +7,7 @@
 
 import Dispatch
 import Synchronization
+import Foundation
 
 extension Ndb {
     /// Creates a `sync` mechanism for coordinating usages of ndb (read or write) with the app's ability to close ndb.
@@ -94,6 +95,11 @@ extension Ndb {
     class FallbackUseLock: UseLockProtocol {
         /// Number of functions using the `ndb` object (for reading or writing data)
         private var ndbUserCount: UInt = 0
+        /// Lock for protecting access to `ndbUserCount`
+        private let ndbUserCountLock = NSLock()
+        /// Lock for protecting access to `ndbIsOpen`
+        private let ndbIsOpenLock = NSLock()
+        private var ndbIsOpen: Bool = false
         /// Semaphore for general access to `ndb`. A closing task requires exclusive access. Users of `ndb` (read/write tasks) share the access
         private let ndbAccessSemaphore: DispatchSemaphore = DispatchSemaphore(value: 0)
         /// How long a thread can block before throwing an error
@@ -120,18 +126,29 @@ extension Ndb {
         /// Implementation note: NEVER change this to `async`! This is a blocking operation, so we want to minimize the time of the operation
         func waitUntilNdbCanClose(thenClose operation: () -> Bool, maxTimeout: DispatchTimeInterval = DEFAULT_TIMEOUT) throws {
             try ndbAccessSemaphore.waitOrThrow(timeout: .now() + maxTimeout)
-            let ndbIsOpen = operation()
+            ndbIsOpenLock.lock()
+            ndbIsOpen = operation()
             if ndbIsOpen {
                 ndbAccessSemaphore.signal()
             }
+            ndbIsOpenLock.unlock()
         }
         
         /// Marks `ndb` as open to allow other users to use it. Do not call this more than once
         func markNdbOpen() {
+            ndbIsOpenLock.lock()
+            if !ndbIsOpen {
+                ndbIsOpen = true
                 ndbAccessSemaphore.signal()
             }
+            ndbIsOpenLock.unlock()
+        }
         
         private func incrementUserCount(maxTimeout: DispatchTimeInterval = .seconds(2)) throws {
+            ndbUserCountLock.lock()
+            defer { ndbUserCountLock.unlock() }
+            
+            // Signal that ndb cannot close while we have at least one user using ndb
             if ndbUserCount == 0 {
                 try ndbAccessSemaphore.waitOrThrow(timeout: .now() + maxTimeout)
             }
@@ -139,6 +156,9 @@ extension Ndb {
         }
         
         private func decrementUserCount() {
+            ndbUserCountLock.lock()
+            defer { ndbUserCountLock.unlock() }
+            
             ndbUserCount -= 1
             // Signal that ndb can close if we have zero users using ndb
             if ndbUserCount == 0 {