Merge bitcoin/bitcoin#30659: wallet: fix UnloadWallet thread safety assumptions

f550a8e035b4603787273ea250f403f6f0be453f Rename ReleaseWallet to FlushAndDeleteWallet (furszy)
64e736d79efc7201768244fc297084f70c0bebc1 wallet: WaitForDeleteWallet, do not expect thread safety (Ryan Ofsky)
8872b4a6ca91a83bf8d5a118fb808c043b9e879d wallet: rename UnloadWallet to WaitForDeleteWallet (furszy)
5d15485aafefdc759ba97e039bb1b9ccac267358 wallet: unload, notify GUI as soon as possible (furszy)

Pull request description:

  Coming from #29073.

  Applied ryanofsky suggested changes on https://github.com/bitcoin/bitcoin/issues/29073#issuecomment-2274237242 with few modifications coming from https://github.com/bitcoin/bitcoin/pull/18338#issuecomment-605060348.

  The only point I did not tackle from https://github.com/bitcoin/bitcoin/pull/18338#issuecomment-605060348 is:

  > * Move log print and flush out of ReleaseWallet into CWallet destructor

  Because it would mean every `CWallet` object would flush data to disk during destruction. Which is not necessary for wallet tool utilities and unit tests.

ACKs for top commit:
  achow101:
    ACK f550a8e035b4603787273ea250f403f6f0be453f
  ryanofsky:
    Code review ACK f550a8e035b4603787273ea250f403f6f0be453f. Just a simple rename since last review
  ismaelsadeeq:
    Re-ACK f550a8e035b4603787273ea250f403f6f0be453f

Tree-SHA512: e2eb69bf36883c514f601f4838ae6a41113996b9559abf8dc2b46e16bbcdad401195ac0f2b9d1fb55a10e78bb8ea9953788a168c80474e3f101350d208cb3bd2

6 files changed, 27 insertions, 29 deletions

diff --git a/src/wallet/load.cpp b/src/wallet/load.cpp
index fe35f6b223..e26347d437 100644
--- a/src/wallet/load.cpp
+++ b/src/wallet/load.cpp
@@ -178,7 +178,7 @@ void UnloadWallets(WalletContext& context)
         wallets.pop_back();
         std::vector<bilingual_str> warnings;
         RemoveWallet(context, wallet, /* load_on_start= */ std::nullopt, warnings);
-        UnloadWallet(std::move(wallet));
+        WaitForDeleteWallet(std::move(wallet));
     }
 }
 } // namespace wallet
diff --git a/src/wallet/rpc/wallet.cpp b/src/wallet/rpc/wallet.cpp
index 7a0b0103c0..39582b3f6a 100644
--- a/src/wallet/rpc/wallet.cpp
+++ b/src/wallet/rpc/wallet.cpp
@@ -495,7 +495,7 @@ static RPCHelpMan unloadwallet()
         }
     }
 
-    UnloadWallet(std::move(wallet));
+    WaitForDeleteWallet(std::move(wallet));
 
     UniValue result(UniValue::VOBJ);
     PushWarnings(warnings, result);
diff --git a/src/wallet/test/util.cpp b/src/wallet/test/util.cpp
index b21a9a601d..ec6c9e6f3f 100644
--- a/src/wallet/test/util.cpp
+++ b/src/wallet/test/util.cpp
@@ -74,7 +74,7 @@ void TestUnloadWallet(std::shared_ptr<CWallet>&& wallet)
     // Calls SyncWithValidationInterfaceQueue
     wallet->chain().waitForNotificationsIfTipChanged({});
     wallet->m_chain_notifications_handler.reset();
-    UnloadWallet(std::move(wallet));
+    WaitForDeleteWallet(std::move(wallet));
 }
 
 std::unique_ptr<WalletDatabase> DuplicateMockDatabase(WalletDatabase& database)
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index 53f3bcc421..44ffddb168 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -889,7 +889,7 @@ BOOST_FIXTURE_TEST_CASE(CreateWalletWithoutChain, BasicTestingSetup)
     context.args = &m_args;
     auto wallet = TestLoadWallet(context);
     BOOST_CHECK(wallet);
-    UnloadWallet(std::move(wallet));
+    WaitForDeleteWallet(std::move(wallet));
 }
 
 BOOST_FIXTURE_TEST_CASE(RemoveTxs, TestChain100Setup)
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 8e31950beb..057fa729d7 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -162,10 +162,14 @@ bool RemoveWallet(WalletContext& context, const std::shared_ptr<CWallet>& wallet
 
     // Unregister with the validation interface which also drops shared pointers.
     wallet->m_chain_notifications_handler.reset();
-    LOCK(context.wallets_mutex);
-    std::vector<std::shared_ptr<CWallet>>::iterator i = std::find(context.wallets.begin(), context.wallets.end(), wallet);
-    if (i == context.wallets.end()) return false;
-    context.wallets.erase(i);
+    {
+        LOCK(context.wallets_mutex);
+        std::vector<std::shared_ptr<CWallet>>::iterator i = std::find(context.wallets.begin(), context.wallets.end(), wallet);
+        if (i == context.wallets.end()) return false;
+        context.wallets.erase(i);
+    }
+    // Notify unload so that upper layers release the shared pointer.
+    wallet->NotifyUnload();
 
     // Write the wallet setting
     UpdateWalletSetting(chain, name, load_on_start, warnings);
@@ -223,38 +227,35 @@ static std::set<std::string> g_loading_wallet_set GUARDED_BY(g_loading_wallet_mu
 static std::set<std::string> g_unloading_wallet_set GUARDED_BY(g_wallet_release_mutex);
 
 // Custom deleter for shared_ptr<CWallet>.
-static void ReleaseWallet(CWallet* wallet)
+static void FlushAndDeleteWallet(CWallet* wallet)
 {
     const std::string name = wallet->GetName();
-    wallet->WalletLogPrintf("Releasing wallet\n");
+    wallet->WalletLogPrintf("Releasing wallet %s..\n", name);
     wallet->Flush();
     delete wallet;
-    // Wallet is now released, notify UnloadWallet, if any.
+    // Wallet is now released, notify WaitForDeleteWallet, if any.
     {
         LOCK(g_wallet_release_mutex);
         if (g_unloading_wallet_set.erase(name) == 0) {
-            // UnloadWallet was not called for this wallet, all done.
+            // WaitForDeleteWallet was not called for this wallet, all done.
             return;
         }
     }
     g_wallet_release_cv.notify_all();
 }
 
-void UnloadWallet(std::shared_ptr<CWallet>&& wallet)
+void WaitForDeleteWallet(std::shared_ptr<CWallet>&& wallet)
 {
     // Mark wallet for unloading.
     const std::string name = wallet->GetName();
     {
         LOCK(g_wallet_release_mutex);
-        auto it = g_unloading_wallet_set.insert(name);
-        assert(it.second);
+        g_unloading_wallet_set.insert(name);
+        // Do not expect to be the only one removing this wallet.
+        // Multiple threads could simultaneously be waiting for deletion.
     }
-    // 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.
-    wallet->NotifyUnload();
 
-    // Time to ditch our shared_ptr and wait for ReleaseWallet call.
+    // Time to ditch our shared_ptr and wait for FlushAndDeleteWallet call.
     wallet.reset();
     {
         WAIT_LOCK(g_wallet_release_mutex, lock);
@@ -2971,7 +2972,7 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
     const auto start{SteadyClock::now()};
     // TODO: Can't use std::make_shared because we need a custom deleter but
     // should be possible to use std::allocate_shared.
-    std::shared_ptr<CWallet> walletInstance(new CWallet(chain, name, std::move(database)), ReleaseWallet);
+    std::shared_ptr<CWallet> walletInstance(new CWallet(chain, name, std::move(database)), FlushAndDeleteWallet);
     walletInstance->m_keypool_size = std::max(args.GetIntArg("-keypool", DEFAULT_KEYPOOL_SIZE), int64_t{1});
     walletInstance->m_notify_tx_changed_script = args.GetArg("-walletnotify", "");
 
@@ -4387,7 +4388,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
         if (!RemoveWallet(context, wallet, /*load_on_start=*/std::nullopt, warnings)) {
             return util::Error{_("Unable to unload the wallet before migrating")};
         }
-        UnloadWallet(std::move(wallet));
+        WaitForDeleteWallet(std::move(wallet));
         was_loaded = true;
     } else {
         // Check if the wallet is BDB
@@ -4531,7 +4532,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
                     error += _("\nUnable to cleanup failed migration");
                     return util::Error{error};
                 }
-                UnloadWallet(std::move(w));
+                WaitForDeleteWallet(std::move(w));
             } else {
                 // Unloading for wallets in local context
                 assert(w.use_count() == 1);
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 984a2d9c48..3ea1cf48b2 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -83,12 +83,9 @@ struct bilingual_str;
 namespace wallet {
 struct WalletContext;
 
-//! Explicitly unload and delete the wallet.
-//! Blocks the current thread after signaling the unload intent so that all
-//! wallet pointer owners 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);
+//! Explicitly delete the wallet.
+//! Blocks the current thread until the wallet is destructed.
+void WaitForDeleteWallet(std::shared_ptr<CWallet>&& wallet);
 
 bool AddWallet(WalletContext& context, const std::shared_ptr<CWallet>& wallet);
 bool RemoveWallet(WalletContext& context, const std::shared_ptr<CWallet>& wallet, std::optional<bool> load_on_start, std::vector<bilingual_str>& warnings);