util: Add SignalInterrupt class and use in shutdown.cpp

This change helps generalize shutdown code so an interrupt can be
provided to libbitcoinkernel callers. This may also be useful to
eventually de-globalize all of the shutdown code.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/kernel/context.cpp

@@ -14,9 +14,12 @@
 
 
 namespace kernel {
+Context* g_context;
 
 Context::Context()
 {
+    assert(!g_context);
+    g_context = this;
     std::string sha256_algo = SHA256AutoDetect();
     LogPrintf("Using the '%s' SHA256 implementation\n", sha256_algo);
     RandomInit();
@@ -26,6 +29,8 @@ Context::Context()
 Context::~Context()
 {
     ECC_Stop();
+    assert(g_context);
+    g_context = nullptr;
 }
 
 } // namespace kernel

src/kernel/context.h

@@ -5,6 +5,8 @@
 #ifndef BITCOIN_KERNEL_CONTEXT_H
 #define BITCOIN_KERNEL_CONTEXT_H
 
+#include <util/signalinterrupt.h>
+
 #include <memory>
 
 namespace kernel {
@@ -16,12 +18,24 @@ namespace kernel {
 //! State stored directly in this struct should be simple. More complex state
 //! should be stored to std::unique_ptr members pointing to opaque types.
 struct Context {
+    //! Interrupt object that can be used to stop long-running kernel operations.
+    util::SignalInterrupt interrupt;
+
     //! Declare default constructor and destructor that are not inline, so code
     //! instantiating the kernel::Context struct doesn't need to #include class
     //! definitions for all the unique_ptr members.
     Context();
     ~Context();
 };
+
+//! Global pointer to kernel::Context for legacy code. New code should avoid
+//! using this, and require state it needs to be passed to it directly.
+//!
+//! Having this pointer is useful because it allows state be moved out of global
+//! variables into the kernel::Context struct before all global references to
+//! that state are removed. This allows the global references to be removed
+//! incrementally, instead of all at once.
+extern Context* g_context;
 } // namespace kernel
 
 #endif // BITCOIN_KERNEL_CONTEXT_H