joinmarket: create bitcoind wallet

Starting with 0.21.0, bitcoin no longer automatically creates and loads
a default wallet.
This was being ignored because of a JoinMarket issue [1] in CI builds prior
to this version. Now a watch-only Bitcoin Core wallet is created in ExecStartPost.

[1] JoinMarket-Org/joinmarket-clientserver#812

modules/joinmarket.nix

@@ -27,7 +27,7 @@ let
     rpc_port = ${toString bitcoind.rpc.port}
     rpc_user = ${bitcoind.rpc.users.privileged.name}
     @@RPC_PASSWORD@@
-    ${optionalString (cfg.rpcWalletFile != null) "rpc_wallet_file=${cfg.rpcWalletFile}"}
+    ${optionalString (cfg.rpcWalletFile != null) "rpc_wallet_file = ${cfg.rpcWalletFile}"}
 
     [MESSAGING:server1]
     host = darkirc6tqgpnwd3blln3yfv5ckl47eg7llfxkmtovrv7c7iwohhb6ad.onion
@@ -129,7 +129,7 @@ in {
     };
     rpcWalletFile = mkOption {
       type = types.nullOr types.str;
-      default = null;
+      default = "jm_wallet";
       description = ''
         Name of the watch-only bitcoind wallet the JoinMarket addresses are imported to.
       '';
@@ -244,7 +244,11 @@ in {
             walletname=wallet.jmdat
             wallet=${cfg.dataDir}/wallets/$walletname
             if [[ ! -f $wallet ]]; then
-              echo "Create wallet"
+              ${optionalString (cfg.rpcWalletFile != null) ''
+                echo "Create wallet"
+                ${config.services.bitcoind.cli}/bin/bitcoin-cli \
+                   createwallet "${cfg.rpcWalletFile}"
+              ''}
               pw=$(cat "${secretsDir}"/jm-wallet-password)
               cd ${cfg.dataDir}
               if ! ${nbPkgs.joinmarket}/bin/jm-genwallet --datadir=${cfg.dataDir} $walletname $pw \

modules/versioning.nix

@@ -91,6 +91,21 @@ let
           rm -f "${secretsDir}"/{lnd-seed-mnemonic,jm-wallet-seed}
       '';
     }
+    {
+      version = "0.0.48";
+      condition = config.services.joinmarket.enable;
+      message = ''
+        Starting with 0.21.0, [bitcoind no longer automatically creates and loads a
+        default wallet](bitcoin/bitcoin#15454). Therefore it is necessary to manually
+        create a wallet with bitcoind. If you ran bitcoind & JoinMarket before Bitcoin
+        0.21.0, you likely already have a wallet file named `wallet.dat` that JoinMarket
+        is using.
+
+        If you want to continue using this wallet file like before, add the following to
+        your configuration:
+        services.joinmarket.rpcWalletFile = null;
+      '';
+    }
   ];
 
   incompatibleChanges = optionals