ndb/txn: inherit active transactions on the same thread

Many different parts of the codebase could be opening transactions when
somewhere higher in the heirarchy on the main thread might already have
an active transaction. This can lead to failed transaction opening which
is bad.

Instead of relying on passing down the transaction to subviews, lets
keep track of the active transactions in a thread-local dictionary. That
way whenever we create a new transaction we can inherit the one that is
already active in the current thread.

Inherited transactions don't end the query when they are garbage
collected, we still expect the first-opened query to do this.

nostrdb/NdbNote.swift

@@ -41,7 +41,7 @@ enum NdbData {
 
 class NdbNote: Encodable, Equatable, Hashable {
     // we can have owned notes, but we can also have lmdb virtual-memory mapped notes so its optional
-    private let owned: Bool
+    let owned: Bool
     let count: Int
     let key: NoteKey?
     let note: UnsafeMutablePointer<ndb_note>

nostrdb/NdbTxn.swift

@@ -16,22 +16,32 @@ class NdbTxn<T> {
     var txn: ndb_txn
     private var val: T!
     var moved: Bool
+    var inherited: Bool
 
     init(ndb: Ndb, with: (NdbTxn<T>) -> T = { _ in () }) {
-        self.txn = ndb_txn()
         #if TXNDEBUG
         txn_count += 1
         print("opening transaction \(txn_count)")
         #endif
-        let _ = ndb_begin_query(ndb.ndb.ndb, &self.txn)
+        if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn {
+            // some parent thread is active, use that instead
+            self.txn = active_txn
+            self.inherited = true
+        } else {
+            self.txn = ndb_txn()
+            let _ = ndb_begin_query(ndb.ndb.ndb, &self.txn)
+            Thread.current.threadDictionary["ndb_txn"] = self.txn
+            self.inherited = false
+        }
         self.moved = false
         self.val = with(self)
     }
 
-    init(txn: ndb_txn, val: T) {
+    private init(txn: ndb_txn, val: T) {
         self.txn = txn
         self.val = val
         self.moved = false
+        self.inherited = false
     }
 
     /// Only access temporarily! Do not store database references for longterm use. If it's a primitive type you
@@ -42,13 +52,16 @@ class NdbTxn<T> {
     }
 
     deinit {
-        if !moved {
-            #if TXNDEBUG
-            txn_count -= 1;
-            print("closing transaction \(txn_count)")
-            #endif
-            ndb_end_query(&self.txn)
+        if moved || inherited {
+            return
         }
+
+        #if TXNDEBUG
+        txn_count -= 1;
+        print("closing transaction \(txn_count)")
+        #endif
+        ndb_end_query(&self.txn)
+        Thread.current.threadDictionary.removeObject(forKey: "ndb_txn")
     }
 
     // functor

nostrdb/Test/NdbTests.swift

@@ -155,6 +155,25 @@ final class NdbTests: XCTestCase {
         let testNote = NdbNote.owned_from_json(json: testJSONWithEscapedSlashes)!
         XCTAssertEqual(testNote.content, "https://cdn.nostr.build/i/5c1d3296f66c2630131bf123106486aeaf051ed8466031c0e0532d70b33cddb2.jpg")
     }
+    
+    func test_inherited_transactions() throws {
+        let ndb = Ndb(path: db_dir)!
+        do {
+            let txn1 = NdbTxn(ndb: ndb)
+            
+            let ntxn = (Thread.current.threadDictionary.value(forKey: "ndb_txn") as? ndb_txn)!
+            XCTAssertEqual(txn1.txn.lmdb, ntxn.lmdb)
+            XCTAssertEqual(txn1.txn.mdb_txn, ntxn.mdb_txn)
+
+            let txn2 = NdbTxn(ndb: ndb)
+
+            XCTAssertEqual(txn1.inherited, false)
+            XCTAssertEqual(txn2.inherited, true)
+        }
+
+        let ndb_txn = Thread.current.threadDictionary.value(forKey: "ndb_txn")
+        XCTAssertNil(ndb_txn)
+    }
 
     func test_decode_perf() throws {
         // This is an example of a performance test case.