init: Signal-safe instant shutdown

Replace the 200ms polling loop with a faster and more efficient waiting
operation.

This was tried a few times before, but given up every time because
solutions use 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.

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