wallet: Add CWallet::ExportWatchOnly

ExportWatchOnly produces a watchonly wallet file from a CWallet. This
can be restored onto another instance of Bitcoin Core to allow that
instance to watch the same descriptors, and also have all of the same
initial address book and transactions.

Author: achow101

src/wallet/wallet.cpp

@@ -4511,4 +4511,169 @@ util::Result<std::vector<WalletDescInfo>> CWallet::ExportDescriptors(bool export
     }
     return wallet_descriptors;
 }
+
+util::Result<std::string> CWallet::ExportWatchOnlyWallet(const fs::path& destination, WalletContext& context) const
+{
+    AssertLockHeld(cs_wallet);
+
+    if (destination.empty()) {
+        return util::Error{_("Error: Export destination cannot be empty")};
+    }
+    if (fs::exists(destination)) {
+        return util::Error{strprintf(_("Error: Export destination '%s' already exists"), fs::PathToString(destination))};
+    }
+    fs::path canonical_dest = fs::canonical(destination.parent_path());
+    canonical_dest /= destination.filename();
+
+    // Get the descriptors from this wallet
+    util::Result<std::vector<WalletDescInfo>> exported = ExportDescriptors(/*export_private=*/false);
+    if (!exported) {
+        return util::Error{util::ErrorString(exported)};
+    }
+
+    // Setup DatabaseOptions to create a new sqlite database
+    DatabaseOptions options;
+    options.require_existing = false;
+    options.require_create = true;
+    options.require_format = DatabaseFormat::SQLITE;
+
+    // Make the wallet with the same flags as this wallet, but without private keys
+    options.create_flags = GetWalletFlags() | WALLET_FLAG_DISABLE_PRIVATE_KEYS;
+
+    // Make the watchonly wallet
+    DatabaseStatus status;
+    std::vector<bilingual_str> warnings;
+    std::string wallet_name = GetName() + "_watchonly";
+    bilingual_str error;
+    std::unique_ptr<WalletDatabase> database = MakeWalletDatabase(wallet_name, options, status, error);
+    if (!database) {
+        return util::Error{strprintf(_("Wallet file creation failed: %s"), error)};
+    }
+    WalletContext empty_context;
+    empty_context.args = context.args;
+    std::shared_ptr<CWallet> watchonly_wallet = CWallet::Create(empty_context, wallet_name, std::move(database), options.create_flags, error, warnings);
+    if (!watchonly_wallet) {
+        return util::Error{_("Error: Failed to create new watchonly wallet")};
+    }
+
+    {
+        LOCK(watchonly_wallet->cs_wallet);
+
+        // Parse the descriptors and add them to the new wallet
+        for (const WalletDescInfo& desc_info : *exported) {
+            // Parse the descriptor
+            FlatSigningProvider keys;
+            std::string parse_err;
+            std::vector<std::unique_ptr<Descriptor>> descs = Parse(desc_info.descriptor, keys, parse_err, /*require_checksum=*/true);
+            assert(descs.size() == 1); // All of our descriptors should be valid, and not multipath
+
+            // Get the range if there is one
+            int32_t range_start = 0;
+            int32_t range_end = 0;
+            if (desc_info.range) {
+                range_start = desc_info.range->first;
+                range_end = desc_info.range->second;
+            }
+
+            WalletDescriptor w_desc(std::move(descs.at(0)), desc_info.creation_time, range_start, range_end, desc_info.next_index);
+
+            // For descriptors that cannot self expand (i.e. needs private keys or cache), retrieve the cache
+            uint256 desc_id = w_desc.id;
+            if (!w_desc.descriptor->CanSelfExpand()) {
+                DescriptorScriptPubKeyMan* desc_spkm = dynamic_cast<DescriptorScriptPubKeyMan*>(GetScriptPubKeyMan(desc_id));
+                w_desc.cache = WITH_LOCK(desc_spkm->cs_desc_man, return desc_spkm->GetWalletDescriptor().cache);
+            }
+
+            // Add to the watchonly wallet
+            if (auto spkm_res = watchonly_wallet->AddWalletDescriptor(w_desc, keys, "", false); !spkm_res) {
+                return util::Error{util::ErrorString(spkm_res)};
+            }
+
+            // Set active spkms as active
+            if (desc_info.active) {
+                // Determine whether this descriptor is internal
+                // This is only set for active spkms
+                bool internal = false;
+                if (desc_info.internal) {
+                    internal = *desc_info.internal;
+                }
+                watchonly_wallet->AddActiveScriptPubKeyMan(desc_id, *w_desc.descriptor->GetOutputType(), internal);
+            }
+        }
+
+        // Copy locked coins that are persisted
+        for (const auto& [coin, persisted] : m_locked_coins) {
+            if (!persisted) continue;
+            watchonly_wallet->LockCoin(coin, persisted);
+        }
+
+        {
+            // Make a WalletBatch for the watchonly_wallet so that everything else can be written atomically
+            WalletBatch watchonly_batch(watchonly_wallet->GetDatabase());
+            if (!watchonly_batch.TxnBegin()) {
+                return util::Error{strprintf(_("Error: database transaction cannot be executed for new watchonly wallet %s"), watchonly_wallet->GetName())};
+            }
+
+            // Copy minversion
+            // Don't use SetMinVersion to account for the newly created wallet having FEATURE_LATEST
+            // while the source wallet doesn't.
+            watchonly_wallet->LoadMinVersion(GetVersion());
+            watchonly_batch.WriteMinVersion(watchonly_wallet->GetVersion());
+
+            // Copy orderPosNext
+            watchonly_batch.WriteOrderPosNext(watchonly_wallet->nOrderPosNext);
+
+            // Write the best block locator to avoid rescanning on reload
+            CBlockLocator best_block_locator;
+            {
+                WalletBatch local_wallet_batch(GetDatabase());
+                if (!local_wallet_batch.ReadBestBlock(best_block_locator)) {
+                    return util::Error{_("Error: Unable to read wallet's best block locator record")};
+                }
+            }
+            if (!watchonly_batch.WriteBestBlock(best_block_locator)) {
+                return util::Error{_("Error: Unable to write watchonly wallet best block locator record")};
+            }
+
+            // Copy the transactions
+            for (const auto& [txid, wtx] : mapWallet) {
+                const CWalletTx& to_copy_wtx = wtx;
+                if (!watchonly_wallet->LoadToWallet(txid, [&](CWalletTx& ins_wtx, bool new_tx) EXCLUSIVE_LOCKS_REQUIRED(watchonly_wallet->cs_wallet) {
+                    if (!new_tx) return false;
+                    ins_wtx.SetTx(to_copy_wtx.tx);
+                    ins_wtx.CopyFrom(to_copy_wtx);
+                    return true;
+                })) {
+                    return util::Error{strprintf(_("Error: Could not add tx %s to watchonly wallet"), txid.GetHex())};
+                }
+                watchonly_batch.WriteTx(watchonly_wallet->mapWallet.at(txid));
+            }
+
+            // Copy address book
+            for (const auto& [dest, entry] : m_address_book) {
+                auto address{EncodeDestination(dest)};
+                if (entry.purpose) watchonly_batch.WritePurpose(address, PurposeToString(*entry.purpose));
+                if (entry.label) watchonly_batch.WriteName(address, *entry.label);
+                for (const auto& [id, request] : entry.receive_requests) {
+                    watchonly_batch.WriteAddressReceiveRequest(dest, id, request);
+                }
+                if (entry.previously_spent) watchonly_batch.WriteAddressPreviouslySpent(dest, true);
+            }
+
+            if (!watchonly_batch.TxnCommit()) {
+                return util::Error{_("Error: cannot commit db transaction for watchonly wallet export")};
+            }
+        }
+
+        // Make a backup of this wallet at the specified destination directory
+        watchonly_wallet->BackupWallet(fs::PathToString(canonical_dest));
+    }
+
+    // Delete the watchonly wallet now that it has been exported to the desired location
+    fs::path watchonly_path = fs::PathFromString(watchonly_wallet->GetDatabase().Filename()).parent_path();
+    watchonly_wallet.reset();
+    fs::remove_all(watchonly_path);
+
+    return fs::PathToString(canonical_dest);
+}
 } // namespace wallet

src/wallet/wallet.h

@@ -1078,6 +1078,10 @@ class CWallet final : public WalletStorage, public interfaces::Chain::Notificati
 
     //! Export the descriptors from this wallet so that they can be imported elsewhere
     util::Result<std::vector<WalletDescInfo>> ExportDescriptors(bool export_private) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
+
+    //! Make a new watchonly wallet file containing the public descriptors from this wallet
+    //! The exported watchonly wallet file will be named and placed at the path specified in 'destination'
+    util::Result<std::string> ExportWatchOnlyWallet(const fs::path& destination, WalletContext& context) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
 };
 
 /**