Merge bitcoin/bitcoin#32489: wallet: Add exportwatchonlywallet RPC to export a watchonly version of a wallet

a15bdc0 doc: update offline-signing-tutorial to use exportwatchonlywallet rpc (Pol Espinasa)
a388076 test: Test for exportwatchonlywallet (Ava Chow)
d053e3e wallet, rpc: Add exportwatchonlywallet RPC (Ava Chow)
444878e wallet: Add CWallet::ExportWatchOnly (Ava Chow)
f9273f0 wallet: Move listdescriptors retrieving from RPC to CWallet (Ava Chow)
a1c8378 wallet: Write new descriptor's cache in AddWalletDescriptor (Ava Chow)
1e99664 wallet: Use Descriptor::CanSelfExpand() in CanGetAddresses() (Ava Chow)
d2ee922 descriptor: Add CanSelfExpand() (Ava Chow)

Pull request description:

  Currently, if a user wants to use an airgapped setup, they need to manually create the watchonly wallet that will live on the online node by importing the public descriptors. This PR introduces `exportwatchonlywallet` which will create a wallet file with the public descriptors to avoid exposing the specific internals to the user. Additionally, this RPC will copy any existing labels, transactions, and wallet flags. This ensures that the exported watchonly wallet is almost entirely a copy of the original wallet but without private keys.

ACKs for top commit:
  polespinasa:
    lgtm ACK a15bdc0
  Sjors:
    re-utACK a15bdc0
  pablomartin4btc:
    re-ACK [a15bdc0](bitcoin/bitcoin@a15bdc0)
  w0xlt:
    lgtm reACK a15bdc0

Tree-SHA512: cfc59415ad9aa13d1445cf2a85db1c051215496b6edcf5a8db463499b2b51b92ee7bf840b709035dff7635f9d0c533423bceb58c851f220500e1ea254d12f3b8

Author: fanquake

doc/offline-signing-tutorial.md

@@ -10,12 +10,12 @@ This workflow uses [Partially Signed Bitcoin Transactions](/doc/psbt.md) (PSBTs)
 > While this tutorial demonstrates the process using `signet` network, you should omit the `-signet` flag in the provided commands when working with `mainnet`.
 
 ## Overview
-In this tutorial we have two hosts, both running Bitcoin v25.0
+In this tutorial we have two hosts, both running Bitcoin v32.0
 
 * `offline` host which is disconnected from all networks (internet, Tor, wifi, bluetooth etc.) and does not have, or need, a copy of the blockchain.
 * `online` host which is a regular online node with a synced blockchain.
 
-We are going to first create an `offline_wallet` on the offline host. We will then create a `watch_only_wallet` on the online host using public key descriptors exported from the `offline_wallet`. Next we will receive some coins into the wallet. In order to spend these coins we'll create an unsigned PSBT using the `watch_only_wallet`, sign the PSBT using the private keys in the `offline_wallet`, and finally broadcast the signed PSBT using the online host.
+We are going to first create an `offline_wallet` on the offline host. We will then create a `watch_only_wallet` on the online host using a wallet file exported from the `offline_wallet`. Next we will receive some coins into the wallet. In order to spend these coins we'll create an unsigned PSBT using the `watch_only_wallet`, sign the PSBT using the private keys in the `offline_wallet`, and finally broadcast the signed PSBT using the online host.
 
 ### Requirements
 - [jq](https://jqlang.github.io/jq/) installation - This tutorial uses jq to process certain fields from JSON RPC responses, but this convenience is optional.
@@ -39,72 +39,30 @@ We are going to first create an `offline_wallet` on the offline host. We will th
 > [!NOTE]
 > The use of a passphrase is crucial to encrypt the wallet.dat file. This encryption ensures that even if an unauthorized individual gains access to the offline host, they won't be able to access the wallet's contents. Further details about securing your wallet can be found in  [Managing the Wallet](/doc/managing-wallets.md#12-encrypting-the-wallet)
 
-2. Export the public key-only descriptors from the offline host to a JSON file named `descriptors.json`. We use `jq` here to extract the `.descriptors` field from the full RPC response.
-
+2. Export the wallet in a watch-only format to a .dat file named `watch_only_wallet.dat`.
 ```sh
-[offline]$ ./build/bin/bitcoin-cli -signet -rpcwallet="offline_wallet" listdescriptors \
-             | jq -r '.descriptors' \
-             >> /path/to/descriptors.json
+[offline]$ ./build/bin/bitcoin-cli -signet -rpcwallet="offline_wallet" -named exportwatchonlywallet \
+             destination=/path/to/watch_only_wallet.dat
 ```
 
 > [!NOTE]
-> The `descriptors.json` file will be transferred to the online machine (e.g. using a USB flash drive) where it can be imported to create a related watch-only wallet.
+> The `watch_only_wallet.dat` file will be transferred to the online machine (e.g. using a USB flash drive) where it can be imported to create a related watch-only wallet.
 
 ### Create the online `watch_only_wallet`
 
-1. On the online machine create a blank watch-only wallet which has private keys disabled and is named `watch_only_wallet`. This is achieved by using the `createwallet` options: `disable_private_keys=true, blank=true`.
-
+On the online machine import the watch-only wallet. This wallet will have the private keys disabled and is named `watch_only_wallet`. This is achieved by using the `restorewallet` rpc call.
 The `watch_only_wallet` wallet will be used to track and validate incoming transactions, create unsigned PSBTs when spending coins, and broadcast signed and finalized PSBTs.
 
-> [!NOTE]
-> `disable_private_keys` indicates that the wallet should refuse to import private keys, i.e. will be a dedicated watch-only wallet.
-
 ```sh
-[online]$ ./build/bin/bitcoin-cli -signet -named createwallet \
+[online]$ ./build/bin/bitcoin-cli -signet -named restorewallet \
               wallet_name="watch_only_wallet" \
-              disable_private_keys=true \
-              blank=true
+              backup_file=/path/to/watch_only_wallet.dat
 
 {
   "name": "watch_only_wallet"
 }
 ```
 
-2. Import the `offline_wallet`s public key descriptors to the online `watch_only_wallet` using the `descriptors.json` file created on the offline wallet.
-
-```sh
-[online]$ ./build/bin/bitcoin-cli -signet -rpcwallet="watch_only_wallet" importdescriptors "$(cat /path/to/descriptors.json)"
-
-[
-  {
-    "success": true
-  },
-  {
-    "success": true
-  },
-  {
-    "success": true
-  },
-  {
-    "success": true
-  },
-  {
-    "success": true
-  },
-  {
-    "success": true
-  },
-  {
-    "success": true
-  },
-  {
-    "success": true
-  }
-]
-```
-> [!NOTE]
-> Multiple success values indicate that multiple descriptors, for different address types, have been successfully imported. This allows generating different address types on the `watch_only_wallet`.
-
 ### Fund the `offline_wallet`
 
 At this point, it's important to understand that both the `offline_wallet` and online `watch_only_wallet` share the same public keys. As a result, they generate the same addresses. Transactions can be created using either wallet, but valid signatures can only be added by the `offline_wallet` as only it has the private keys.

src/script/descriptor.cpp

@@ -247,6 +247,9 @@ struct PubkeyProvider
 
     /** Get the count of keys known by this PubkeyProvider. Usually one, but may be more for key aggregation schemes */
     virtual size_t GetKeyCount() const { return 1; }
+
+    /** Whether this PubkeyProvider can always provide a public key without cache or private key arguments */
+    virtual bool CanSelfExpand() const = 0;
 };
 
 class OriginPubkeyProvider final : public PubkeyProvider
@@ -317,6 +320,7 @@ class OriginPubkeyProvider final : public PubkeyProvider
     {
         return std::make_unique<OriginPubkeyProvider>(m_expr_index, m_origin, m_provider->Clone(), m_apostrophe);
     }
+    bool CanSelfExpand() const override { return m_provider->CanSelfExpand(); }
 };
 
 /** An object representing a parsed constant public key in a descriptor. */
@@ -381,6 +385,7 @@ class ConstPubkeyProvider final : public PubkeyProvider
     {
         return std::make_unique<ConstPubkeyProvider>(m_expr_index, m_pubkey, m_xonly);
     }
+    bool CanSelfExpand() const final { return true; }
 };
 
 enum class DeriveType {
@@ -607,6 +612,7 @@ class BIP32PubkeyProvider final : public PubkeyProvider
     {
         return std::make_unique<BIP32PubkeyProvider>(m_expr_index, m_root_extkey, m_path, m_derive, m_apostrophe);
     }
+    bool CanSelfExpand() const override { return !IsHardened(); }
 };
 
 /** PubkeyProvider for a musig() expression */
@@ -810,6 +816,15 @@ class MuSigPubkeyProvider final : public PubkeyProvider
     {
         return 1 + m_participants.size();
     }
+    bool CanSelfExpand() const override
+    {
+        // Participants must be self expandable for all MuSig expressions to be self expandable; the aggregate pubkey cannot be stored
+        // in the descriptor cache, so even aggregate-then-derive still requires the self expansion of participants prior to aggregation.
+        for (const auto& key : m_participants) {
+            if (!key->CanSelfExpand()) return false;
+        }
+        return true;
+    }
 };
 
 /** Base class for all Descriptor implementations. */
@@ -824,7 +839,7 @@ class DescriptorImpl : public Descriptor
     std::vector<std::string> m_warnings;
 
     //! The sub-descriptor arguments (empty for everything but SH and WSH).
-    //! In doc/descriptors.m this is referred to as SCRIPT expressions sh(SCRIPT)
+    //! In doc/descriptors.md this is referred to as SCRIPT expressions sh(SCRIPT)
     //! and wsh(SCRIPT), and distinct from KEY expressions and ADDR expressions.
     //! Subdescriptors can only ever generate a single script.
     const std::vector<std::unique_ptr<DescriptorImpl>> m_subdescriptor_args;
@@ -1099,6 +1114,18 @@ class DescriptorImpl : public Descriptor
         }
         return count;
     }
+
+    // NOLINTNEXTLINE(misc-no-recursion)
+    bool CanSelfExpand() const override
+    {
+        for (const auto& key : m_pubkey_args) {
+            if (!key->CanSelfExpand()) return false;
+        }
+        for (const auto& sub : m_subdescriptor_args) {
+            if (!sub->CanSelfExpand()) return false;
+        }
+        return true;
+    }
 };
 
 /** A parsed addr(A) descriptor. */

src/script/descriptor.h

@@ -140,6 +140,9 @@ struct Descriptor {
     /** Convert the descriptor to a normalized string. Normalized descriptors have the xpub at the last hardened step. This fails if the provided provider does not have the private keys to derive that xpub. */
     virtual bool ToNormalizedString(const SigningProvider& provider, std::string& out, const DescriptorCache* cache = nullptr) const = 0;
 
+    /** Whether the descriptor can be used to produce its address(es) without needing a cache or private keys. */
+    virtual bool CanSelfExpand() const = 0;
+
     /** Expand a descriptor at a specified position.
      *
      * @param[in] pos The position at which to expand the descriptor. If IsRange() is false, this is ignored.