refactor: Add util::Result multiple error and warning messages

Add util::Result support for returning warning messages and multiple errors,
not just a single error string. This provides a way for functions to report
errors and warnings in a standard way, and simplifies interfaces.

The functionality is unit tested here, and put to use in followup PR
bitcoin#25722

src/Makefile.am

@@ -750,6 +750,7 @@ libbitcoin_util_a_SOURCES = \
   util/moneystr.cpp \
   util/rbf.cpp \
   util/readwritefile.cpp \
+  util/result.cpp \
   util/signalinterrupt.cpp \
   util/thread.cpp \
   util/threadinterrupt.cpp \
@@ -988,6 +989,7 @@ libbitcoinkernel_la_SOURCES = \
   util/hasher.cpp \
   util/moneystr.cpp \
   util/rbf.cpp \
+  util/result.cpp \
   util/serfloat.cpp \
   util/signalinterrupt.cpp \
   util/strencodings.cpp \

src/test/result_tests.cpp

@@ -2,10 +2,17 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <tinyformat.h>
 #include <util/check.h>
 #include <util/result.h>
+#include <util/translation.h>
 
+#include <algorithm>
 #include <boost/test/unit_test.hpp>
+#include <memory>
+#include <ostream>
+#include <string>
+#include <utility>
 
 inline bool operator==(const bilingual_str& a, const bilingual_str& b)
 {
@@ -90,15 +97,52 @@ enum FnError { ERR1, ERR2 };
 
 util::Result<int, FnError> IntFailFn(int i, bool success)
 {
-    if (success) return i;
+    if (success) return {util::Warning{Untranslated(strprintf("int %i warn.", i))}, i};
     return {util::Error{Untranslated(strprintf("int %i error.", i))}, i % 2 ? ERR1 : ERR2};
 }
 
+util::Result<std::string, FnError> StrFailFn(int i, bool success)
+{
+    auto result = IntFailFn(i, success);
+    if (!success) return {util::MoveFrom(result), util::Error{Untranslated("str error")}, result.GetFailure()};
+    return {util::MoveFrom(result), strprintf("%i", *result)};
+}
+
 util::Result<NoCopyNoMove, FnError> EnumFailFn(FnError ret)
 {
     return {util::Error{Untranslated("enum fail.")}, ret};
 }
 
+util::Result<void> WarnFn()
+{
+    return {util::Warning{Untranslated("warn.")}};
+}
+
+util::Result<int> MultiWarnFn(int ret)
+{
+    util::Result<void> result;
+    for (int i = 0; i < ret; ++i) {
+        result.AddWarning(strprintf(Untranslated("warn %i."), i));
+    }
+    return {util::MoveFrom(result), ret};
+}
+
+util::Result<void, int> ChainedFailFn(FnError arg, int ret)
+{
+    return {util::Error{Untranslated("chained fail.")}, util::MoveFrom(EnumFailFn(arg)), util::MoveFrom(WarnFn()), ret};
+}
+
+util::Result<int, FnError> AccumulateFn(bool success)
+{
+    util::Result<int, FnError> result;
+    util::Result<int> x = MultiWarnFn(1) >> result;
+    BOOST_REQUIRE(x);
+    util::Result<int> y = MultiWarnFn(2) >> result;
+    BOOST_REQUIRE(y);
+    result.Update(IntFailFn(*x + *y, success));
+    return result;
+}
+
 util::Result<int, int> TruthyFalsyFn(int i, bool success)
 {
     if (success) return i;
@@ -140,7 +184,13 @@ BOOST_AUTO_TEST_CASE(check_returned)
     ExpectFail(NoCopyNoMoveFn(5, false), Untranslated("nocopynomove 5 error."), 5);
     ExpectSuccess(StrFn(Untranslated("S"), true), {}, Untranslated("S"));
     ExpectResult(StrFn(Untranslated("S"), false), false, Untranslated("str S error."));
+    ExpectSuccess(StrFailFn(1, true), Untranslated("int 1 warn."), "1");
+    ExpectFail(StrFailFn(2, false), Untranslated("int 2 error. str error"), ERR2);
     ExpectFail(EnumFailFn(ERR2), Untranslated("enum fail."), ERR2);
+    ExpectFail(ChainedFailFn(ERR1, 5), Untranslated("chained fail. enum fail. warn."), 5);
+    ExpectSuccess(MultiWarnFn(3), Untranslated("warn 0. warn 1. warn 2."), 3);
+    ExpectSuccess(AccumulateFn(true), Untranslated("warn 0. warn 0. warn 1. int 3 warn."), 3);
+    ExpectFail(AccumulateFn(false), Untranslated("int 3 error. warn 0. warn 0. warn 1."), ERR1);
     ExpectSuccess(TruthyFalsyFn(0, true), {}, 0);
     ExpectFail(TruthyFalsyFn(0, false), Untranslated("failure value 0."), 0);
     ExpectSuccess(TruthyFalsyFn(1, true), {}, 1);
@@ -156,15 +206,15 @@ BOOST_AUTO_TEST_CASE(check_update)
     result.Update({util::Error{Untranslated("error")}, ERR1});
     ExpectFail(result, Untranslated("error"), ERR1);
     result.Update(2);
-    ExpectSuccess(result, Untranslated(""), 2);
+    ExpectSuccess(result, Untranslated("error"), 2);
 
     // Test the same thing but with non-copyable success and failure types.
     util::Result<NoCopy, NoCopy> result2{0};
     ExpectSuccess(result2, {}, 0);
     result2.Update({util::Error{Untranslated("error")}, 3});
     ExpectFail(result2, Untranslated("error"), 3);
     result2.Update(4);
-    ExpectSuccess(result2, Untranslated(""), 4);
+    ExpectSuccess(result2, Untranslated("error"), 4);
 }
 
 BOOST_AUTO_TEST_CASE(check_dereference_operators)
@@ -188,6 +238,16 @@ BOOST_AUTO_TEST_CASE(check_value_or)
     BOOST_CHECK_EQUAL(StrFn(Untranslated("A"), false).value_or(Untranslated("B")), Untranslated("B"));
 }
 
+BOOST_AUTO_TEST_CASE(check_message_accessors)
+{
+    util::Result<void> result{util::Error{Untranslated("Error.")}, util::Warning{Untranslated("Warning.")}};
+    BOOST_CHECK_EQUAL(Assert(result.GetMessages())->errors.size(), 1);
+    BOOST_CHECK_EQUAL(Assert(result.GetMessages())->errors[0], Untranslated("Error."));
+    BOOST_CHECK_EQUAL(Assert(result.GetMessages())->warnings.size(), 1);
+    BOOST_CHECK_EQUAL(Assert(result.GetMessages())->warnings[0], Untranslated("Warning."));
+    BOOST_CHECK_EQUAL(util::ErrorString(result), Untranslated("Error. Warning."));
+}
+
 struct Derived : NoCopyNoMove {
     using NoCopyNoMove::NoCopyNoMove;
 };

src/util/result.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) 2024 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#include <util/result.h>
+
+#include <algorithm>
+#include <initializer_list>
+#include <iterator>
+#include <util/translation.h>
+
+namespace util {
+namespace detail {
+bilingual_str JoinMessages(const Messages& messages)
+{
+    bilingual_str result;
+    for (const auto& messages : {messages.errors, messages.warnings}) {
+        for (const auto& message : messages) {
+            if (!result.empty()) result += Untranslated(" ");
+            result += message;
+        }
+    }
+    return result;
+}
+} // namespace detail
+
+void ResultTraits<Messages>::MergeInto(Messages& dst, Messages& src) {
+    dst.errors.insert(dst.errors.end(), std::make_move_iterator(src.errors.begin()), std::make_move_iterator(src.errors.end()));
+    dst.warnings.insert(dst.warnings.end(), std::make_move_iterator(src.warnings.begin()), std::make_move_iterator(src.warnings.end()));
+    src.errors.clear();
+    src.warnings.clear();
+}
+} // namespace util