Merge bitcoin/bitcoin#34495: Replace boost signals with minimal compatible implementation

242b0ebb5c btcsignals: use a single shared_ptr for liveness and callback (Cory Fields)
b12f43a0a8 signals: remove boost::signals2 from depends and vcpkg (Cory Fields)
a4b1607983 signals: remove boost::signals2 mentions in linters and docs (Cory Fields)
375397ebd9 signals: remove boost includes where possible (Cory Fields)
091736a153 signals: re-add forward-declares to interface headers (Cory Fields)
9958f4fe49 Revert "signals: Temporarily add boost headers to bitcoind and bitcoin-node builds" (Cory Fields)
34eabd77a2 signals: remove boost compatibility guards (Cory Fields)
e60a0b9a22 signals: Add a simplified boost-compatible implementation (Cory Fields)
63c68e2a3f signals: add signals tests (Cory Fields)
edc2978058 signals: use an alias for the boost::signals2 namespace (Cory Fields)
9ade3929aa signals: remove forward-declare for signals (Cory Fields)
037e58b57b signals: use forwarding header for boost signals (Cory Fields)
2150153f37 signals: Temporarily add boost headers to bitcoind and bitcoin-node builds (Cory Fields)
fd5e9d9904 signals: Use a lambda to avoid connecting a signal to another signal (Cory Fields)

Pull request description:

  This drops our dependency on `boost::signals2`, leaving `boost::multi_index` as the only remaining boost dependency for bitcoind.

  `boost::signals2` is a complex beast, but we only use a small portion of it. Namely: it's a way for multiple subscribers to connect to the same event, and the ability to later disconnect individual subscribers from that event.

  `btcsignals` adheres to the subset of the `boost::signals2` API that we currently use, and thus is a drop-in replacement. Rather than implementing a complex `slot` tracking class that we never used anyway (and which was much more useful in the days before std::function existed), callbacks are simply wrapped directly in `std::function`s.

  The new tests work with either `boost::signals2` or the new `btcsignals` implementation. Reviewers can verify
  functional equivalency by running the tests in the commit that introduces them against `boost::signals2`, then again with `btcsignals`.

  The majority of the commits in this PR are preparation and cleanup. Once `boost::signals2` is no longer needed, it is removed from depends. Additionally, a few CMake targets no longer need boost includes as they were previously only required for signals.

  I think this is actually pretty straightforward to review. I kept things simple, including keeping types unmovable/uncopyable where possible rather than trying to define those semantics. In doing so, the new implementation has even fewer type requirements than boost, which I believe is due to a boost bug. I've opened a PR upstream for that to attempt to maintain parity between the implementations.

  See individual commits for more details.

  Closes #26442.

ACKs for top commit:
  fjahr:
    Code review ACK 242b0ebb5c
  maflcko:
    re-review ACK 242b0ebb5c 🎯
  w0xlt:
    reACK 242b0ebb5c

Tree-SHA512: 9a472afa4f655624fa44493774a63b57509ad30fb61bf1d89b6d0b52000cb9a1409a5b8d515a99c76e0b26b2437c30508206c29a7dd44ea96eb1979d572cd4d4

src/node/interface_ui.cpp

@@ -4,33 +4,31 @@
 
 #include <node/interface_ui.h>
 
+#include <btcsignals.h>
 #include <util/string.h>
 #include <util/translation.h>
 
-#include <boost/signals2/optional_last_value.hpp>
-#include <boost/signals2/signal.hpp>
-
 using util::MakeUnorderedList;
 
 CClientUIInterface uiInterface;
 
 struct UISignals {
-    boost::signals2::signal<CClientUIInterface::ThreadSafeMessageBoxSig, boost::signals2::optional_last_value<bool>> ThreadSafeMessageBox;
-    boost::signals2::signal<CClientUIInterface::ThreadSafeQuestionSig, boost::signals2::optional_last_value<bool>> ThreadSafeQuestion;
-    boost::signals2::signal<CClientUIInterface::InitMessageSig> InitMessage;
-    boost::signals2::signal<CClientUIInterface::InitWalletSig> InitWallet;
-    boost::signals2::signal<CClientUIInterface::NotifyNumConnectionsChangedSig> NotifyNumConnectionsChanged;
-    boost::signals2::signal<CClientUIInterface::NotifyNetworkActiveChangedSig> NotifyNetworkActiveChanged;
-    boost::signals2::signal<CClientUIInterface::NotifyAlertChangedSig> NotifyAlertChanged;
-    boost::signals2::signal<CClientUIInterface::ShowProgressSig> ShowProgress;
-    boost::signals2::signal<CClientUIInterface::NotifyBlockTipSig> NotifyBlockTip;
-    boost::signals2::signal<CClientUIInterface::NotifyHeaderTipSig> NotifyHeaderTip;
-    boost::signals2::signal<CClientUIInterface::BannedListChangedSig> BannedListChanged;
+    btcsignals::signal<CClientUIInterface::ThreadSafeMessageBoxSig, btcsignals::optional_last_value<bool>> ThreadSafeMessageBox;
+    btcsignals::signal<CClientUIInterface::ThreadSafeQuestionSig, btcsignals::optional_last_value<bool>> ThreadSafeQuestion;
+    btcsignals::signal<CClientUIInterface::InitMessageSig> InitMessage;
+    btcsignals::signal<CClientUIInterface::InitWalletSig> InitWallet;
+    btcsignals::signal<CClientUIInterface::NotifyNumConnectionsChangedSig> NotifyNumConnectionsChanged;
+    btcsignals::signal<CClientUIInterface::NotifyNetworkActiveChangedSig> NotifyNetworkActiveChanged;
+    btcsignals::signal<CClientUIInterface::NotifyAlertChangedSig> NotifyAlertChanged;
+    btcsignals::signal<CClientUIInterface::ShowProgressSig> ShowProgress;
+    btcsignals::signal<CClientUIInterface::NotifyBlockTipSig> NotifyBlockTip;
+    btcsignals::signal<CClientUIInterface::NotifyHeaderTipSig> NotifyHeaderTip;
+    btcsignals::signal<CClientUIInterface::BannedListChangedSig> BannedListChanged;
 };
 static UISignals g_ui_signals;
 
 #define ADD_SIGNALS_IMPL_WRAPPER(signal_name)                                                                 \
-    boost::signals2::connection CClientUIInterface::signal_name##_connect(std::function<signal_name##Sig> fn) \
+    btcsignals::connection CClientUIInterface::signal_name##_connect(std::function<signal_name##Sig> fn) \
     {                                                                                                         \
         return g_ui_signals.signal_name.connect(fn);                                                          \
     }

src/node/interface_ui.h

@@ -15,11 +15,9 @@ class CBlockIndex;
 enum class SynchronizationState;
 struct bilingual_str;
 
-namespace boost {
-namespace signals2 {
-class connection;
-}
-} // namespace boost
+namespace btcsignals {
+    class connection;
+} // namespace btcsignals
 
 /** Signals for UI communication. */
 class CClientUIInterface
@@ -71,7 +69,7 @@ public:
 #define ADD_SIGNALS_DECL_WRAPPER(signal_name, rtype, ...)                                  \
     rtype signal_name(__VA_ARGS__);                                                        \
     using signal_name##Sig = rtype(__VA_ARGS__);                                           \
-    boost::signals2::connection signal_name##_connect(std::function<signal_name##Sig> fn)
+    btcsignals::connection signal_name##_connect(std::function<signal_name##Sig> fn)
 
     /** Show message box. */
     ADD_SIGNALS_DECL_WRAPPER(ThreadSafeMessageBox, bool, const bilingual_str& message, unsigned int style);

src/node/interfaces.cpp

@@ -5,6 +5,7 @@
 #include <addrdb.h>
 #include <banman.h>
 #include <blockfilter.h>
+#include <btcsignals.h>
 #include <chain.h>
 #include <chainparams.h>
 #include <common/args.h>
@@ -72,8 +73,6 @@
 #include <stdexcept>
 #include <utility>
 
-#include <boost/signals2/signal.hpp>
-
 using interfaces::BlockRef;
 using interfaces::BlockTemplate;
 using interfaces::BlockTip;