Get rid of shutdown.cpp/shutdown.h, use SignalInterrupt directly

This change is mostly a refectoring that removes some code and gets rid of an
unnecessary layer of indirection after #27861

But it is not a pure refactoring since StartShutdown, AbortShutdown, and
WaitForShutdown functions used to abort on failure, and the replacement code
logs or returns errors instead.

src/init.cpp

@@ -66,7 +66,6 @@
 #include <rpc/util.h>
 #include <scheduler.h>
 #include <script/sigcache.h>
-#include <shutdown.h>
 #include <sync.h>
 #include <timedata.h>
 #include <torcontrol.h>
@@ -192,9 +191,15 @@ static void RemovePidFile(const ArgsManager& args)
     }
 }
 
+static std::optional<util::SignalInterrupt> g_shutdown;
+
 void InitContext(NodeContext& node)
 {
+    assert(!g_shutdown);
+    g_shutdown.emplace();
+
     node.args = &gArgs;
+    node.shutdown = &*g_shutdown;
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ -208,11 +213,9 @@ void InitContext(NodeContext& node)
 // The network-processing threads are all part of a thread group
 // created by AppInit() or the Qt main() function.
 //
-// A clean exit happens when StartShutdown() or the SIGTERM
-// signal handler sets ShutdownRequested(), which makes main thread's
-// WaitForShutdown() interrupts the thread group.
-// And then, WaitForShutdown() makes all other on-going threads
-// in the thread group join the main thread.
+// A clean exit happens when the SignalInterrupt object is triggered, which
+// makes the main thread's SignalInterrupt::wait() call return, and join all
+// other ongoing threads in the thread group to the main thread.
 // Shutdown() is then called to clean up database connections, and stop other
 // threads that should only be stopped after the main network-processing
 // threads have exited.
@@ -222,6 +225,11 @@ void InitContext(NodeContext& node)
 // shutdown thing.
 //
 
+bool ShutdownRequested(node::NodeContext& node)
+{
+    return bool{*Assert(node.shutdown)};
+}
+
 #if HAVE_SYSTEM
 static void ShutdownNotify(const ArgsManager& args)
 {
@@ -386,7 +394,9 @@ void Shutdown(NodeContext& node)
 #ifndef WIN32
 static void HandleSIGTERM(int)
 {
-    StartShutdown();
+    // Return value is intentionally ignored because there is not a better way
+    // of handling this failure in a signal handler.
+    (void)(*Assert(g_shutdown))();
 }
 
 static void HandleSIGHUP(int)
@@ -396,7 +406,10 @@ static void HandleSIGHUP(int)
 #else
 static BOOL WINAPI consoleCtrlHandler(DWORD dwCtrlType)
 {
-    StartShutdown();
+    if (!(*Assert(g_shutdown))()) {
+        LogPrintf("Error: failed to send shutdown signal on Ctrl-C\n");
+        return false;
+    }
     Sleep(INFINITE);
     return true;
 }
@@ -1145,11 +1158,13 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }, std::chrono::minutes{1});
 
     // Check disk space every 5 minutes to avoid db corruption.
-    node.scheduler->scheduleEvery([&args]{
+    node.scheduler->scheduleEvery([&args, &node]{
         constexpr uint64_t min_disk_space = 50 << 20; // 50 MB
         if (!CheckDiskSpace(args.GetBlocksDirPath(), min_disk_space)) {
             LogPrintf("Shutting down due to lack of disk space!\n");
-            StartShutdown();
+            if (!(*Assert(node.shutdown))()) {
+                LogPrintf("Error: failed to send shutdown signal after disk space check\n");
+            }
         }
     }, std::chrono::minutes{5});
 
@@ -1487,7 +1502,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }
     LogPrintf("* Using %.1f MiB for in-memory UTXO set (plus up to %.1f MiB of unused mempool space)\n", cache_sizes.coins * (1.0 / 1024 / 1024), mempool_opts.max_size_bytes * (1.0 / 1024 / 1024));
 
-    for (bool fLoaded = false; !fLoaded && !ShutdownRequested();) {
+    for (bool fLoaded = false; !fLoaded && !ShutdownRequested(node);) {
         node.mempool = std::make_unique<CTxMemPool>(mempool_opts);
 
         node.chainman = std::make_unique<ChainstateManager>(*Assert(node.shutdown), chainman_opts, blockman_opts);
@@ -1554,7 +1569,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
             return InitError(error);
         }
 
-        if (!fLoaded && !ShutdownRequested()) {
+        if (!fLoaded && !ShutdownRequested(node)) {
             // first suggest a reindex
             if (!options.reindex) {
                 bool fRet = uiInterface.ThreadSafeQuestion(
@@ -1563,7 +1578,9 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
                     "", CClientUIInterface::MSG_ERROR | CClientUIInterface::BTN_ABORT);
                 if (fRet) {
                     fReindex = true;
-                    AbortShutdown();
+                    if (!Assert(node.shutdown)->reset()) {
+                        LogPrintf("Internal error: failed to reset shutdown signal.\n");
+                    }
                 } else {
                     LogPrintf("Aborted block database rebuild. Exiting.\n");
                     return false;
@@ -1577,7 +1594,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // As LoadBlockIndex can take several minutes, it's possible the user
     // requested to kill the GUI during the last operation. If so, exit.
     // As the program has not fully started yet, Shutdown() is possibly overkill.
-    if (ShutdownRequested()) {
+    if (ShutdownRequested(node)) {
         LogPrintf("Shutdown requested. Exiting.\n");
         return false;
     }
@@ -1698,7 +1715,9 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         ImportBlocks(chainman, vImportFiles);
         if (args.GetBoolArg("-stopafterblockimport", DEFAULT_STOPAFTERBLOCKIMPORT)) {
             LogPrintf("Stopping after block import\n");
-            StartShutdown();
+            if (!(*Assert(node.shutdown))()) {
+                LogPrintf("Error: failed to send shutdown signal after finishing block import\n");
+            }
             return;
         }
 
@@ -1718,16 +1737,16 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // Wait for genesis block to be processed
     {
         WAIT_LOCK(g_genesis_wait_mutex, lock);
-        // We previously could hang here if StartShutdown() is called prior to
+        // We previously could hang here if shutdown was requested prior to
         // ImportBlocks getting started, so instead we just wait on a timer to
         // check ShutdownRequested() regularly.
-        while (!fHaveGenesis && !ShutdownRequested()) {
+        while (!fHaveGenesis && !ShutdownRequested(node)) {
             g_genesis_wait_cv.wait_for(lock, std::chrono::milliseconds(500));
         }
         block_notify_genesis_wait_connection.disconnect();
     }
 
-    if (ShutdownRequested()) {
+    if (ShutdownRequested(node)) {
         return false;
     }