Merge #14941: rpc: Make unloadwallet wait for complete wallet unload

645e905 doc: Add release notes for unloadwallet change to synchronous call (João Barbosa)
c37851d rpc: Make unloadwallet wait for complete wallet unload (João Barbosa)

Pull request description:

  Currently the `unloadwallet` RPC is asynchronous, it only signals the intent to unload the wallet and then returns the response to the client. The actual unload can happen later and the client has no way to be notified of that.

  This PR makes the `unloadwallet` RPC synchronous, meaning that it blocks until the wallet is fully unloaded.

  Replaces #14919, fixes #14917.

Tree-SHA512: ad88b980e2f3652809a58f904afbfe020299f3aa6a517f495ba943b8d54d4520f6e70074d6749be8f5967065c0f476e0faedcde64c8b4899e5f99c70f0fd6534

Author: laanwj

doc/release-notes-14941.md

@@ -0,0 +1,5 @@
+Miscellaneous RPC changes
+------------
+
+- The `unloadwallet` RPC is now synchronous, meaning that it blocks until the
+  wallet is fully unloaded.

src/init.cpp

@@ -261,10 +261,10 @@ void Shutdown(InitInterfaces& interfaces)
         LogPrintf("%s: Unable to remove pidfile: %s\n", __func__, e.what());
     }
 #endif
+    interfaces.chain_clients.clear();
     UnregisterAllValidationInterfaces();
     GetMainSignals().UnregisterBackgroundSignalScheduler();
     GetMainSignals().UnregisterWithMempoolSignals(mempool);
-    interfaces.chain_clients.clear();
     globalVerifyHandle.reset();
     ECC_Stop();
     LogPrintf("%s: done\n", __func__);

src/wallet/init.cpp

@@ -242,7 +242,11 @@ void StopWallets()
 
 void UnloadWallets()
 {
-    for (const std::shared_ptr<CWallet>& pwallet : GetWallets()) {
-        RemoveWallet(pwallet);
+    auto wallets = GetWallets();
+    while (!wallets.empty()) {
+        auto wallet = wallets.back();
+        wallets.pop_back();
+        RemoveWallet(wallet);
+        UnloadWallet(std::move(wallet));
     }
 }

src/wallet/rpcwallet.cpp

@@ -2637,16 +2637,8 @@ static UniValue unloadwallet(const JSONRPCRequest& request)
     if (!RemoveWallet(wallet)) {
         throw JSONRPCError(RPC_MISC_ERROR, "Requested wallet already unloaded");
     }
-    UnregisterValidationInterface(wallet.get());
 
-    // The wallet can be in use so it's not possible to explicitly unload here.
-    // Just notify the unload intent so that all shared pointers are released.
-    // The wallet will be destroyed once the last shared pointer is released.
-    wallet->NotifyUnload();
-
-    // There's no point in waiting for the wallet to unload.
-    // At this point this method should never fail. The unloading could only
-    // fail due to an unexpected error which would cause a process termination.
+    UnloadWallet(std::move(wallet));
 
     return NullUniValue;
 }

src/wallet/wallet.cpp

@@ -82,13 +82,52 @@ std::shared_ptr<CWallet> GetWallet(const std::string& name)
     return nullptr;
 }
 
+static Mutex g_wallet_release_mutex;
+static std::condition_variable g_wallet_release_cv;
+static std::set<CWallet*> g_unloading_wallet_set;
+
 // Custom deleter for shared_ptr<CWallet>.
 static void ReleaseWallet(CWallet* wallet)
 {
+    // Unregister and delete the wallet right after BlockUntilSyncedToCurrentChain
+    // so that it's in sync with the current chainstate.
     wallet->WalletLogPrintf("Releasing wallet\n");
     wallet->BlockUntilSyncedToCurrentChain();
     wallet->Flush();
+    UnregisterValidationInterface(wallet);
     delete wallet;
+    // Wallet is now released, notify UnloadWallet, if any.
+    {
+        LOCK(g_wallet_release_mutex);
+        if (g_unloading_wallet_set.erase(wallet) == 0) {
+            // UnloadWallet was not called for this wallet, all done.
+            return;
+        }
+    }
+    g_wallet_release_cv.notify_all();
+}
+
+void UnloadWallet(std::shared_ptr<CWallet>&& wallet)
+{
+    // Mark wallet for unloading.
+    CWallet* pwallet = wallet.get();
+    {
+        LOCK(g_wallet_release_mutex);
+        auto it = g_unloading_wallet_set.insert(pwallet);
+        assert(it.second);
+    }
+    // The wallet can be in use so it's not possible to explicitly unload here.
+    // Notify the unload intent so that all remaining shared pointers are
+    // released.
+    pwallet->NotifyUnload();
+    // Time to ditch our shared_ptr and wait for ReleaseWallet call.
+    wallet.reset();
+    {
+        WAIT_LOCK(g_wallet_release_mutex, lock);
+        while (g_unloading_wallet_set.count(pwallet) == 1) {
+            g_wallet_release_cv.wait(lock);
+        }
+    }
 }
 
 const uint32_t BIP32_HARDENED_KEY_LIMIT = 0x80000000;

src/wallet/wallet.h

@@ -54,6 +54,13 @@ void StopWallets();
 //! Close all wallets.
 void UnloadWallets();
 
+//! Explicitly unload and delete the wallet.
+//  Blocks the current thread after signaling the unload intent so that all
+//  wallet clients release the wallet.
+//  Note that, when blocking is not required, the wallet is implicitly unloaded
+//  by the shared pointer deleter.
+void UnloadWallet(std::shared_ptr<CWallet>&& wallet);
+
 bool AddWallet(const std::shared_ptr<CWallet>& wallet);
 bool RemoveWallet(const std::shared_ptr<CWallet>& wallet);
 bool HasWallets();
@@ -770,6 +777,8 @@ class CWallet final : public CCryptoKeyStore, public CValidationInterface
 
     ~CWallet()
     {
+        // Should not have slots connected at this point.
+        assert(NotifyUnload.empty());
         delete encrypted_batch;
         encrypted_batch = nullptr;
     }