Backend: check password using only one account

Check password using only one account to speed up the process
that otherwise could take too long, especially with the mobile
frontends (and especially in DEBUG mode under an emulator).

Co-authored-by: Andres G. Aragoneses <[email protected]>

src/GWallet.Backend/Account.fs

@@ -262,23 +262,35 @@ module Account =
                 with
                 | :? InvalidPassword ->
                     false
-        match accountOpt with
-        | Some account -> async { return checkValidPasswordForSingleAccount account }
-        | _ ->
-            let checkJobs =
-                seq {
-                    for account in GetAllActiveAccounts().OfType<NormalAccount>() do
-                        yield async {
-                            return checkValidPasswordForSingleAccount account
-                        }
-                }
-            async {
-                let! aggregateSuccess = Async.Parallel checkJobs
-                if aggregateSuccess.All(fun success -> success = true) then
-                    return true
-                else
-                    return false
-            }
+
+        let account =
+            match accountOpt with
+            | Some account -> account
+            | _ ->
+                // Because password check is expensive operation, only check it for one account.
+                // (Ether accounts take about 2 times longer to check, so rather use BTC/LTC)
+                let maybeBtcAccount =
+                    GetAllActiveAccounts().OfType<NormalAccount>()
+                    |> Seq.tryFind (fun acc -> (acc :> IAccount).Currency = Currency.BTC)
+                match maybeBtcAccount with
+                | Some btcAccount -> btcAccount
+                | None ->
+                    let maybeLtcAccount =
+                        GetAllActiveAccounts().OfType<NormalAccount>()
+                        |> Seq.tryFind (fun acc -> (acc :> IAccount).Currency = Currency.LTC)
+                    match maybeLtcAccount with
+                    | Some ltcAccount -> ltcAccount
+                    | None ->
+                        let maybeSomeAccount =
+                            GetAllActiveAccounts().OfType<NormalAccount>()
+                            |> Seq.tryHead
+                        match maybeSomeAccount with
+                        | Some account -> account
+                        | None ->
+                            // the menu "Options" of GWallet.Frontend.Console shouldn't have shown this feature if no accounts
+                            failwith "No accounts found to check valid password. Please report this bug"
+
+        async { return checkValidPasswordForSingleAccount account }
 
     let private CreateArchivedAccount (currency: Currency) (unencryptedPrivateKey: string): ArchivedAccount =
         let fromUnencryptedPrivateKeyToPublicAddressFunc =