Merge #20605: init: Signal-safe instant shutdown

cd03513dc2fcccaa142e9632a28b38efd0056436 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 cd03513dc2fcccaa142e9632a28b38efd0056436 tested on Debian 5.9.11-1 (2020-11-27) x86_64 GNU/Linux

Tree-SHA512: ed2f532f69fec4855c17bf7b8f3d0eb96e78ee2a3c13d374dd2c6add06e3ad6a190da8ed8f9d7a76532cf998222d67f57e35b206aec29675e96437448ae7e13c

1 files changed, 17 insertions, 0 deletions

diff --git a/src/shutdown.h b/src/shutdown.h
index 3ed851c789..23f84179e9 100644
--- a/src/shutdown.h
+++ b/src/shutdown.h
@@ -6,8 +6,25 @@
 #ifndef BITCOIN_SHUTDOWN_H
 #define BITCOIN_SHUTDOWN_H
 
+/** Initialize shutdown state. This must be called before using either StartShutdown(),
+ * AbortShutdown() or WaitForShutdown(). Calling ShutdownRequested() is always safe.
+ */
+bool InitShutdownState();
+
+/** Request shutdown of the application. */
 void StartShutdown();
+
+/** Clear shutdown flag. Only use this during init (before calling WaitForShutdown in any
+ * thread), or in the unit tests. Calling it in other circumstances will cause a race condition.
+ */
 void AbortShutdown();
+
+/** Returns true if a shutdown is requested, false otherwise. */
 bool ShutdownRequested();
 
+/** Wait for StartShutdown to be called in any thread. This can only be used
+ * from a single thread.
+ */
+void WaitForShutdown();
+
 #endif