refactor, kernel: Add FlushStatus / FlushResult types

src/Makefile.am

@@ -195,6 +195,7 @@ BITCOIN_CORE_H = \
   kernel/mempool_removal_reason.h \
   kernel/messagestartchars.h \
   kernel/notifications_interface.h \
+  kernel/result.h \
   kernel/validation_cache_sizes.h \
   key.h \
   key_io.h \

src/kernel/result.h

@@ -0,0 +1,62 @@
+// Copyright (c) 2024 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_KERNEL_RESULT_H
+#define BITCOIN_KERNEL_RESULT_H
+
+#include <util/result.h>
+
+//! @file Kernel result types for use with util::Result.
+//!
+//! Currently, many kernel functions can trigger flushes and node shutdowns at
+//! low levels deep within the code. This can make it hard to reason about when
+//! a particular call might trigger a flush or shutdown. Ideally more
+//! information about conditions leading to flushes and shutdowns would be
+//! returned so higher level code could control when they happen, but in the
+//! meantime the special result types in this file are used to provide a
+//! reliable indication of when they have happened after the fact.
+
+namespace kernel {
+//! Result type for functions that might trigger flushes, indiciating whether
+//! flushes were skipped, performed successfully, or failed.
+enum class FlushStatus { SKIPPED, SUCCESS, FAILURE };
+
+//! Result type for functions that might trigger fatal errors and need to abort
+//! the node, indicating whether a fatal error did occur.
+struct AbortFailure {
+    bool fatal{false};
+};
+
+template <typename SuccessType = void, typename FailureType = void>
+using FlushResult = util::Result<SuccessType, FailureType, FlushStatus>;
+} // namespace kernel
+
+namespace util {
+template<>
+struct ResultTraits<kernel::FlushStatus>
+{
+    static void MergeInto(kernel::FlushStatus& dst, kernel::FlushStatus& src)
+    {
+        using kernel::FlushStatus;
+        if (dst == FlushStatus::FAILURE || src == FlushStatus::FAILURE) {
+            dst = FlushStatus::FAILURE;
+        } else if (dst == FlushStatus::SUCCESS || src == FlushStatus::SUCCESS) {
+           dst = FlushStatus::SUCCESS;
+        } else {
+           dst = FlushStatus::SKIPPED;
+        }
+    }
+};
+
+template<>
+struct ResultTraits<kernel::AbortFailure>
+{
+    static void MergeInto(kernel::AbortFailure& dst, kernel::AbortFailure& src)
+    {
+        dst.fatal |= src.fatal;
+    }
+};
+} // namespace util
+
+#endif // BITCOIN_KERNEL_RESULT_H