Merge #20605: init: Signal-safe instant shutdown

cd03513dc2 init: Signal-safe instant shutdown (Wladimir J. van der Laan)

Pull request description:

  Replace the 200ms polling loop with a faster and more efficient waiting operation. This should speed up short RPC tests.

  This change has been tried a few times before, but abandoned every time because solutions used a condition variable which is not safe for use in signals, as they need to be reentrant.

  On UNIX-ish OSes, use a safe way: a pipe. When shutdown is requested write a dummy byte to the pipe. Waiting for shutdown is a matter of a blocking read from the pipe.

  On Windows, there are no signals so using a condition variable is safe.

  This only affects bitcoind. The GUI is unaffected by this change, and keeps polling as before in `BitcoinGUI::detectShutdown()`. It might be possible to listen to a pipe there, too, but I'm not sure, and it's complicated by the GUI-node abstraction.

ACKs for top commit:
  jonatack:
    ACK cd03513dc2 tested on Debian 5.9.11-1 (2020-11-27) x86_64 GNU/Linux

Tree-SHA512: ed2f532f69fec4855c17bf7b8f3d0eb96e78ee2a3c13d374dd2c6add06e3ad6a190da8ed8f9d7a76532cf998222d67f57e35b206aec29675e96437448ae7e13c

src/init.cpp

@@ -917,6 +917,9 @@ bool AppInitBasicSetup(const ArgsManager& args)
     // Enable heap terminate-on-corruption
     HeapSetInformation(nullptr, HeapEnableTerminationOnCorruption, nullptr, 0);
 #endif
+    if (!InitShutdownState()) {
+        return InitError(Untranslated("Initializing wait-for-shutdown state failed."));
+    }
 
     if (!SetupNetworking()) {
         return InitError(Untranslated("Initializing networking failed."));