ipc: Add Ctrl-C handler for spawned subprocesses

This fixes an error reported by Antoine Poinsot <[email protected]> in
bitcoin-core/libmultiprocess#123 that does not happen
in master, but does happen with bitcoin/bitcoin#10102
applied, where if Ctrl-C is pressed when `bitcoin-node` is started, it is
handled by both `bitcoin-node` and `bitcoin-wallet` processes, causing the
wallet to shutdown abruptly instead of waiting for the node and shutting down
cleanly.

This change fixes the problem by having the wallet process print to stdout when
it receives a Ctrl-C signal but not otherwise react, letting the node shut
everything down cleanly.

Author: ryanofsky

src/ipc/interfaces.cpp

@@ -13,6 +13,7 @@
 #include <tinyformat.h>
 #include <util/fs.h>
 
+#include <csignal>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
@@ -26,6 +27,28 @@
 
 namespace ipc {
 namespace {
+#ifndef WIN32
+std::string g_ignore_ctrl_c;
+
+void HandleCtrlC(int)
+{
+    // (void)! needed to suppress -Wunused-result warning from GCC
+    (void)!write(STDOUT_FILENO, g_ignore_ctrl_c.data(), g_ignore_ctrl_c.size());
+}
+#endif
+
+void IgnoreCtrlC(std::string message)
+{
+#ifndef WIN32
+    g_ignore_ctrl_c = std::move(message);
+    struct sigaction sa{};
+    sa.sa_handler = HandleCtrlC;
+    sigemptyset(&sa.sa_mask);
+    sa.sa_flags = SA_RESTART;
+    sigaction(SIGINT, &sa, nullptr);
+#endif
+}
+
 class IpcImpl : public interfaces::Ipc
 {
 public:
@@ -53,6 +76,7 @@ class IpcImpl : public interfaces::Ipc
         if (!m_process->checkSpawned(argc, argv, fd)) {
             return false;
         }
+        IgnoreCtrlC(strprintf("[%s] SIGINT received — waiting for parent to shut down.\n", m_exe_name));
         m_protocol->serve(fd, m_exe_name, m_init);
         exit_status = EXIT_SUCCESS;
         return true;