feature: Added ability for users to add a startup command

doc/release-notes-15367.md

@@ -0,0 +1,6 @@
+Configuration option changes
+----------------------------
+
+- The `startupnotify` option is used to specify a command to
+  execute when Bitcoin Core has finished with its startup
+  sequence. (#15367)

src/init.cpp

@@ -422,6 +422,9 @@ void SetupServerArgs(NodeContext& node)
     argsman.AddArg("-reindex", "Rebuild chain state and block index from the blk*.dat files on disk", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-reindex-chainstate", "Rebuild chain state from the currently indexed blocks. When in pruning mode or if blocks on disk might be corrupted, use full -reindex instead.", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-settings=<file>", strprintf("Specify path to dynamic settings data file. Can be disabled with -nosettings. File is written at runtime and not meant to be edited by users (use %s instead for custom settings). Relative paths will be prefixed by datadir location. (default: %s)", BITCOIN_CONF_FILENAME, BITCOIN_SETTINGS_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+#if HAVE_SYSTEM
+    argsman.AddArg("-startupnotify=<cmd>", "Execute command on startup.", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+#endif
 #ifndef WIN32
     argsman.AddArg("-sysperms", "Create new files with system default permissions, instead of umask 077 (only effective with disabled wallet functionality)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
 #else
@@ -670,6 +673,17 @@ static void CleanupBlockRevFiles()
     }
 }
 
+#if HAVE_SYSTEM
+static void StartupNotify(const ArgsManager& args)
+{
+    std::string cmd = args.GetArg("-startupnotify", "");
+    if (!cmd.empty()) {
+        std::thread t(runCommand, cmd);
+        t.detach(); // thread runs free
+    }
+}
+#endif
+
 static void ThreadImport(ChainstateManager& chainman, std::vector<fs::path> vImportFiles, const ArgsManager& args)
 {
     const CChainParams& chainparams = Params();
@@ -1969,5 +1983,9 @@ bool AppInitMain(const util::Ref& context, NodeContext& node, interfaces::BlockA
         banman->DumpBanlist();
     }, DUMP_BANS_INTERVAL);
 
+#if HAVE_SYSTEM
+    StartupNotify(args);
+#endif
+
     return true;
 }