loop: add Sentinel iterator support to parallel_for_each()

before this change, we only support containers, whose `begin()` and
`end()` are of the same type, but ranges do not have this requirement.

in order to support range views, let's relax this requirement. so that,
we can pass two iterators denoting a range to `parallel_for_each()`.
see also b1b0bff.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

include/seastar/core/loop.hh

@@ -381,15 +381,15 @@ future<> keep_doing(AsyncAction action) noexcept {
 }
 
 namespace internal {
-template <typename Iterator, typename AsyncAction>
+template <typename Iterator, class Sentinel, typename AsyncAction>
 class do_for_each_state final : public continuation_base<> {
     Iterator _begin;
-    Iterator _end;
+    Sentinel _end;
     AsyncAction _action;
     promise<> _pr;
 
 public:
-    do_for_each_state(Iterator begin, Iterator end, AsyncAction action, future<>&& first_unavailable)
+    do_for_each_state(Iterator begin, Sentinel end, AsyncAction action, future<>&& first_unavailable)
         : _begin(std::move(begin)), _end(std::move(end)), _action(std::move(action)) {
         internal::set_callback(std::move(first_unavailable), this);
     }
@@ -422,16 +422,16 @@ public:
     }
 };
 
-template<typename Iterator, typename AsyncAction>
+template<typename Iterator, typename Sentinel, typename AsyncAction>
 inline
-future<> do_for_each_impl(Iterator begin, Iterator end, AsyncAction action) {
+future<> do_for_each_impl(Iterator begin, Sentinel end, AsyncAction action) {
     while (begin != end) {
         auto f = futurize_invoke(action, *begin++);
         if (f.failed()) {
             return f;
         }
         if (!f.available() || need_preempt()) {
-            auto* s = new internal::do_for_each_state<Iterator, AsyncAction>{
+            auto* s = new internal::do_for_each_state<Iterator, Sentinel, AsyncAction>{
                 std::move(begin), std::move(end), std::move(action), std::move(f)};
             return s->get_future();
         }
@@ -454,12 +454,15 @@ future<> do_for_each_impl(Iterator begin, Iterator end, AsyncAction action) {
 ///               when it is acceptable to process the next item.
 /// \return a ready future on success, or the first failed future if
 ///         \c action failed.
-template<typename Iterator, typename AsyncAction>
-requires requires (Iterator i, AsyncAction aa) {
-    { futurize_invoke(aa, *i) } -> std::same_as<future<>>;
-}
+template<typename Iterator, typename Sentinel, typename AsyncAction>
+requires (
+    requires (Iterator i, AsyncAction aa) {
+        { futurize_invoke(aa, *i) } -> std::same_as<future<>>;
+    } &&
+    (std::same_as<Sentinel, Iterator> || std::sentinel_for<Sentinel, Iterator>)
+)
 inline
-future<> do_for_each(Iterator begin, Iterator end, AsyncAction action) noexcept {
+future<> do_for_each(Iterator begin, Sentinel end, AsyncAction action) noexcept {
     try {
         return internal::do_for_each_impl(std::move(begin), std::move(end), std::move(action));
     } catch (...) {
@@ -472,19 +475,19 @@ future<> do_for_each(Iterator begin, Iterator end, AsyncAction action) noexcept
 /// For each item in a range, call a function, waiting for the previous
 /// invocation to complete before calling the next one.
 ///
-/// \param c an \c Container object designating input range
+/// \param c an \c Range object designating input range
 /// \param action a callable, taking a reference to objects from the range
 ///               as a parameter, and returning a \c future<> that resolves
 ///               when it is acceptable to process the next item.
 /// \return a ready future on success, or the first failed future if
 ///         \c action failed.
-template<typename Container, typename AsyncAction>
-requires requires (Container c, AsyncAction aa) {
+template<typename Range, typename AsyncAction>
+requires requires (Range c, AsyncAction aa) {
     { futurize_invoke(aa, *std::begin(c)) } -> std::same_as<future<>>;
     std::end(c);
 }
 inline
-future<> do_for_each(Container& c, AsyncAction action) noexcept {
+future<> do_for_each(Range& c, AsyncAction action) noexcept {
     try {
         return internal::do_for_each_impl(std::begin(c), std::end(c), std::move(action));
     } catch (...) {