Update backtrace with source locations of resume points

Add resume point locations to `tasktrace` object.
Update `formatter::format` to print source location of next resume alone
with task type.

Author: Radosław Cybulski

include/seastar/util/backtrace.hh

@@ -139,14 +139,17 @@ class tasktrace {
 public:
     using entry = std::variant<shared_backtrace, task_entry>;
     using vector_type = boost::container::static_vector<entry, 16>;
+    using vector_resume_points_type = boost::container::static_vector<std::source_location, 16>;
 private:
     simple_backtrace _main;
     vector_type _prev;
+    vector_resume_points_type _prev_resume_points;
     scheduling_group _sg;
     size_t _hash;
 public:
     tasktrace() = default;
     tasktrace(simple_backtrace main, vector_type prev, size_t prev_hash, scheduling_group sg) noexcept;
+    tasktrace(simple_backtrace main, vector_type prev, vector_resume_points_type prev_resume_points, size_t prev_hash, scheduling_group sg) noexcept;
     tasktrace(const tasktrace&) = default;
     tasktrace& operator=(const tasktrace&) = default;
     ~tasktrace();

src/util/backtrace.cc

@@ -135,6 +135,7 @@ tasktrace current_tasktrace() noexcept {
     auto main = current_backtrace_tasklocal();
 
     tasktrace::vector_type prev;
+    tasktrace::vector_resume_points_type prev_resume_points;
     size_t hash = 0;
     if (local_engine && g_current_context) {
         task* tsk = nullptr;
@@ -148,6 +149,7 @@ tasktrace current_tasktrace() noexcept {
 
         while (tsk && prev.size() < prev.max_size()) {
             shared_backtrace bt = tsk->get_backtrace();
+            prev_resume_points.push_back(tsk->get_resume_point());
             hash *= 31;
             if (bt) {
                 hash ^= bt->hash();
@@ -161,7 +163,7 @@ tasktrace current_tasktrace() noexcept {
         }
     }
 
-    return tasktrace(std::move(main), std::move(prev), hash, current_scheduling_group());
+    return tasktrace(std::move(main), std::move(prev), std::move(prev_resume_points), hash, current_scheduling_group());
 }
 
 saved_backtrace current_backtrace() noexcept {
@@ -175,6 +177,14 @@ tasktrace::tasktrace(simple_backtrace main, tasktrace::vector_type prev, size_t
     , _hash(_main.hash() * 31 ^ prev_hash)
 { }
 
+tasktrace::tasktrace(simple_backtrace main, tasktrace::vector_type prev, vector_resume_points_type prev_resume_points, size_t prev_hash, scheduling_group sg) noexcept
+    : _main(std::move(main))
+    , _prev(std::move(prev))
+    , _prev_resume_points(std::move(prev_resume_points))
+    , _sg(sg)
+    , _hash(_main.hash() * 31 ^ prev_hash)
+{ }
+
 bool tasktrace::operator==(const tasktrace& o) const noexcept {
     return _hash == o._hash && _main == o._main && _prev == o._prev;
 }
@@ -203,8 +213,15 @@ auto formatter<seastar::tasktrace>::format(const seastar::tasktrace& b, format_c
     -> decltype(ctx.out()) {
     auto out = ctx.out();
     out = fmt::format_to(out, "{}", b._main);
-    for (auto&& e : b._prev) {
+    for(auto i = 0u; i < b._prev.size(); ++i) {
+        const auto &e = b._prev[i];
+        auto resume_loc = i < b._prev_resume_points.size() ? b._prev_resume_points[i] : std::source_location{};
+
         out = fmt::format_to(out,  "\n   --------");
+
+        if (resume_loc.file_name()[0] != 0) {
+            out = fmt::format_to(out, "\n   {}:{}:{}", resume_loc.file_name(), resume_loc.line(), resume_loc.column());
+        }
         out = std::visit(seastar::make_visitor(
             [&] (const seastar::shared_backtrace& sb) {
                 return fmt::format_to(out,  "\n{}", sb);