ndb: add NdbTxn transaction class

This will be used for transactions

damus.xcodeproj/project.pbxproj

@@ -156,6 +156,7 @@
 		4C3BEFE0281DE1ED00B3DE84 /* DamusState.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4C3BEFDF281DE1ED00B3DE84 /* DamusState.swift */; };
 		4C3D52B6298DB4E6001C5831 /* ZapEvent.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4C3D52B5298DB4E6001C5831 /* ZapEvent.swift */; };
 		4C3D52B8298DB5C6001C5831 /* TextEvent.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4C3D52B7298DB5C6001C5831 /* TextEvent.swift */; };
+		4C3DCC762A9FE9EC0091E592 /* NdbTxn.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4C3DCC752A9FC2030091E592 /* NdbTxn.swift */; };
 		4C3EA63D28FF52D600C48A62 /* bolt11.c in Sources */ = {isa = PBXBuildFile; fileRef = 4C3EA63C28FF52D600C48A62 /* bolt11.c */; };
 		4C3EA64128FF553900C48A62 /* hash_u5.c in Sources */ = {isa = PBXBuildFile; fileRef = 4C3EA64028FF553900C48A62 /* hash_u5.c */; };
 		4C3EA64428FF558100C48A62 /* sha256.c in Sources */ = {isa = PBXBuildFile; fileRef = 4C3EA64328FF558100C48A62 /* sha256.c */; };
@@ -694,6 +695,7 @@
 		4C3BEFDF281DE1ED00B3DE84 /* DamusState.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = DamusState.swift; sourceTree = "<group>"; };
 		4C3D52B5298DB4E6001C5831 /* ZapEvent.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ZapEvent.swift; sourceTree = "<group>"; };
 		4C3D52B7298DB5C6001C5831 /* TextEvent.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TextEvent.swift; sourceTree = "<group>"; };
+		4C3DCC752A9FC2030091E592 /* NdbTxn.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = NdbTxn.swift; sourceTree = "<group>"; };
 		4C3EA63B28FF52D600C48A62 /* bolt11.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = bolt11.h; sourceTree = "<group>"; };
 		4C3EA63C28FF52D600C48A62 /* bolt11.c */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.c; path = bolt11.c; sourceTree = "<group>"; };
 		4C3EA63E28FF54BD00C48A62 /* short_types.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = short_types.h; sourceTree = "<group>"; };
@@ -1816,6 +1818,7 @@
 				4CDD1ADF2A6B305F001CD4DF /* NdbTagElem.swift */,
 				4C478E242A9932C100489948 /* Ndb.swift */,
 				4CDD1AE12A6B3074001CD4DF /* NdbTagsIterator.swift */,
+				4C3DCC752A9FC2030091E592 /* NdbTxn.swift */,
 				4CE9FBB82A6B3B26007E485C /* nostrdb.c */,
 				4C4793032A993DB900489948 /* midl.c */,
 				4C4793002A993B9A00489948 /* mdb.c */,
@@ -2478,6 +2481,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				4C3DCC762A9FE9EC0091E592 /* NdbTxn.swift in Sources */,
 				4CEF958D2A9CE650000F901B /* verifier.c in Sources */,
 				4C32B9342A9AD01A00DC3548 /* NdbProfile.swift in Sources */,
 				4C32B9332A99845B00DC3548 /* Ndb.swift in Sources */,

nostrdb/NdbTxn.swift

@@ -0,0 +1,102 @@
+//
+//  NdbTx.swift
+//  damus
+//
+//  Created by William Casarin on 2023-08-30.
+//
+
+import Foundation
+
+#if TXNDEBUG
+fileprivate var txn_count: Int = 0
+#endif
+
+// Would use struct and ~Copyable but generics aren't supported well
+class NdbTxn<T> {
+    var txn: ndb_txn
+    private var val: T!
+    var moved: Bool
+
+    init(ndb: Ndb, with: (NdbTxn<T>) -> T) {
+        self.txn = ndb_txn()
+        #if TXNDEBUG
+        txn_count += 1
+        print("opening transaction \(txn_count)")
+        #endif
+        let _ = ndb_begin_query(ndb.ndb.ndb, &self.txn)
+        self.moved = false
+        self.val = with(self)
+    }
+
+    init(txn: ndb_txn, val: T) {
+        self.txn = txn
+        self.val = val
+        self.moved = false
+    }
+
+    /// Only access temporarily! Do not store database references for longterm use. If it's a primitive type you
+    /// can retrieve this value with `.value`
+    var unsafeUnownedValue: T {
+        precondition(!moved)
+        return val
+    }
+
+    deinit {
+        if !moved {
+            #if TXNDEBUG
+            txn_count -= 1;
+            print("closing transaction \(txn_count)")
+            #endif
+            ndb_end_query(&self.txn)
+        }
+    }
+
+    // functor
+    func map<Y>(_ transform: (T) -> Y) -> NdbTxn<Y> {
+        self.moved = true
+        return .init(txn: self.txn, val: transform(val))
+    }
+
+    // comonad!?
+    // useful for moving ownership of a transaction to another value
+    func extend<Y>(_ with: (NdbTxn<T>) -> Y) -> NdbTxn<Y> {
+        self.moved = true
+        return .init(txn: self.txn, val: with(self))
+    }
+}
+
+protocol OptionalType {
+    associatedtype Wrapped
+    var optional: Wrapped? { get }
+}
+
+extension Optional: OptionalType {
+    typealias Wrapped = Wrapped
+
+    var optional: Wrapped? {
+        return self
+    }
+}
+
+extension NdbTxn where T: OptionalType {
+    func collect() -> NdbTxn<T.Wrapped>? {
+        guard let unwrappedVal: T.Wrapped = val.optional else {
+            return nil
+        }
+        self.moved = true
+        return NdbTxn<T.Wrapped>(txn: self.txn, val: unwrappedVal)
+    }
+}
+
+extension NdbTxn where T == Bool { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == Bool? { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == Int { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == Int? { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == Double { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == Double? { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == UInt64 { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == UInt64? { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == String { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == String? { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == NoteId? { var value: T { return self.unsafeUnownedValue } }
+extension NdbTxn where T == NoteId { var value: T { return self.unsafeUnownedValue } }