Merge bitcoin/bitcoin#25879: refactor: Make Join() util work with any container type

fa95315655fcd31a5482f5313faf04dbfa4de580 Use new Join() helper for ListBlockFilterTypes() (MacroFake)
fa1c71695593d5d3b264c2504baca318fc46d580 Make Join() util work with any container type (MacroFake)
faf8da3c8da275c104330ef2743c5a062bea2f4b Remove Join() helper only used in tests (MacroFake)

Pull request description:

  This allows to drop some code

ACKs for top commit:
  naumenkogs:
    ACK fa95315655fcd31a5482f5313faf04dbfa4de580
  stickies-v:
    ACK [fa95315](https://github.com/bitcoin/bitcoin/commit/fa95315655fcd31a5482f5313faf04dbfa4de580)

Tree-SHA512: efd65b65722f46b221bd53140ff22bd8e45adc83617980233f28f695be3108a6ab01affd751d715134ffcb9762228ba8952e9467e590cff022c83e0f5404cb74

3 files changed, 24 insertions, 40 deletions

diff --git a/src/blockfilter.cpp b/src/blockfilter.cpp
index 43d88df720..85929747be 100644
--- a/src/blockfilter.cpp
+++ b/src/blockfilter.cpp
@@ -3,7 +3,6 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <mutex>
-#include <sstream>
 #include <set>
 
 #include <blockfilter.h>
@@ -13,6 +12,7 @@
 #include <script/script.h>
 #include <streams.h>
 #include <util/golombrice.h>
+#include <util/string.h>
 
 /// SerType used to serialize parameters in GCS filter encoding.
 static constexpr int GCS_SER_TYPE = SER_NETWORK;
@@ -179,19 +179,7 @@ const std::set<BlockFilterType>& AllBlockFilterTypes()
 
 const std::string& ListBlockFilterTypes()
 {
-    static std::string type_list;
-
-    static std::once_flag flag;
-    std::call_once(flag, []() {
-            std::stringstream ret;
-            bool first = true;
-            for (const auto& entry : g_filter_types) {
-                if (!first) ret << ", ";
-                ret << entry.second;
-                first = false;
-            }
-            type_list = ret.str();
-        });
+    static std::string type_list{Join(g_filter_types, ", ", [](const auto& entry) { return entry.second; })};
 
     return type_list;
 }
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index 70e2f89e0b..921cd37327 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -238,15 +238,15 @@ BOOST_AUTO_TEST_CASE(span_write_bytes)
 BOOST_AUTO_TEST_CASE(util_Join)
 {
     // Normal version
-    BOOST_CHECK_EQUAL(Join({}, ", "), "");
-    BOOST_CHECK_EQUAL(Join({"foo"}, ", "), "foo");
-    BOOST_CHECK_EQUAL(Join({"foo", "bar"}, ", "), "foo, bar");
+    BOOST_CHECK_EQUAL(Join(std::vector<std::string>{}, ", "), "");
+    BOOST_CHECK_EQUAL(Join(std::vector<std::string>{"foo"}, ", "), "foo");
+    BOOST_CHECK_EQUAL(Join(std::vector<std::string>{"foo", "bar"}, ", "), "foo, bar");
 
     // Version with unary operator
     const auto op_upper = [](const std::string& s) { return ToUpper(s); };
-    BOOST_CHECK_EQUAL(Join<std::string>({}, ", ", op_upper), "");
-    BOOST_CHECK_EQUAL(Join<std::string>({"foo"}, ", ", op_upper), "FOO");
-    BOOST_CHECK_EQUAL(Join<std::string>({"foo", "bar"}, ", ", op_upper), "FOO, BAR");
+    BOOST_CHECK_EQUAL(Join(std::list<std::string>{}, ", ", op_upper), "");
+    BOOST_CHECK_EQUAL(Join(std::list<std::string>{"foo"}, ", ", op_upper), "FOO");
+    BOOST_CHECK_EQUAL(Join(std::list<std::string>{"foo", "bar"}, ", ", op_upper), "FOO, BAR");
 }
 
 BOOST_AUTO_TEST_CASE(util_ReplaceAll)
diff --git a/src/util/string.h b/src/util/string.h
index dd4de888bb..0bcf7d760f 100644
--- a/src/util/string.h
+++ b/src/util/string.h
@@ -58,34 +58,30 @@ void ReplaceAll(std::string& in_out, const std::string& search, const std::strin
 }
 
 /**
- * Join a list of items
+ * Join all container items. Typically used to concatenate strings but accepts
+ * containers with elements of any type.
  *
- * @param list       The list to join
- * @param separator  The separator
- * @param unary_op   Apply this operator to each item in the list
+ * @param container The items to join
+ * @param separator The separator
+ * @param unary_op  Apply this operator to each item
  */
-template <typename T, typename BaseType, typename UnaryOp>
-auto Join(const std::vector<T>& list, const BaseType& separator, UnaryOp unary_op)
-    -> decltype(unary_op(list.at(0)))
+template <typename C, typename S, typename UnaryOp>
+auto Join(const C& container, const S& separator, UnaryOp unary_op)
 {
-    decltype(unary_op(list.at(0))) ret;
-    for (size_t i = 0; i < list.size(); ++i) {
-        if (i > 0) ret += separator;
-        ret += unary_op(list.at(i));
+    decltype(unary_op(*container.begin())) ret;
+    bool first{true};
+    for (const auto& item : container) {
+        if (!first) ret += separator;
+        ret += unary_op(item);
+        first = false;
     }
     return ret;
 }
 
-template <typename T, typename T2>
-T Join(const std::vector<T>& list, const T2& separator)
+template <typename C, typename S>
+auto Join(const C& container, const S& separator)
 {
-    return Join(list, separator, [](const T& i) { return i; });
-}
-
-// Explicit overload needed for c_str arguments, which would otherwise cause a substitution failure in the template above.
-inline std::string Join(const std::vector<std::string>& list, std::string_view separator)
-{
-    return Join<std::string>(list, separator);
+    return Join(container, separator, [](const auto& i) { return i; });
 }
 
 /**