util/backtrace: Optimize formatter to reduce memory allocation overhead

[tchaikov]

This commit addresses a critical memory allocation issue in backtrace formatting by directly specializing fmt::formatter for backtrace types, eliminating dependency on iostream-based formatting.

Problem:

When Seastar applications experience high memory pressure, logging backtrace information could fail due to additional memory allocation required by iostream formatting. This resulted in errors like:

ERROR 2024-12-10 01:59:16,905 [shard 0:main] seastar_memory - seastar/src/core/memory.cc:2126 @void seastar::memory::maybe_dump_memory_diagnostics(size_t, bool): failed to log message: fmt='Failed to allocate {} bytes at {}': std::__ios_failure (error iostream:1, basic_ios::clear: iostream error)"

Solution:

• Implement direct fmt::formatter specialization for backtrace
• Remove reliance on operator<< for string representation
• Reduce memory allocation pressure during out-of-memory scenarios
• Improve reliability of backtrace logging under extreme memory constraints

Additional Compatibility Note:

Due to changes in fmt library versioning, this implementation ensures fmt::formatter is always defined for backtrace types, even when using fmt versions earlier than 9.0. Previously, these formatters were conditionally defined based on the fmt version. Now, the formatters are consistently available, with operator<< implemented using the new fmt::formatter specialization.

This change ensures that backtrace logging can proceed even when the system is under significant memory pressure, providing more reliable post-mortem debugging information.

[tchaikov]

tested with scylla master HEAD. the new implementation is able to format the backtrace, like

DEBUG 2024-12-10 15:18:12,736 [shard 0:main] seastar_memory - Failed to allocate 131072 bytes at 0x130cf7e 0x130d2a0 0x130d598 0x122be82 0x122a60f 0x122a59f 0x122c6c5 0x5559036 0x13c9b3f 0x13c95c5 0x130ed96
   --------
   seastar::continuation<seastar::internal::promise_base_with_type<int>, seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}&&)::{lambda()#2}, seastar::future<void>::then_impl_nrvo<seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}&&)::{lambda()#2}, seastar::future<int> >(scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}&&)::{lambda(seastar::internal::promise_base_with_type<int>&&, seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, auto:1&&, (auto:2&&)...)::{lambda()#2}&, seastar::future_state<seastar::internal::monostate>&&)#1}, void>
   --------
   seastar::continuation<seastar::internal::promise_base_with_type<int>, seastar::future<int>::finally_body<seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}&&)::{lambda()#3}, false>, seastar::future<int>::then_wrapped_nrvo<seastar::future<int>, seastar::future<int>::finally_body<seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}&&)::{lambda()#3}, false> >(seastar::future<int>::finally_body<seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}&&)::{lambda()#3}, false>&&)::{lambda(seastar::internal::promise_base_with_type<int>&&, seastar::future<int>::finally_body<seastar::async<scylla_main(int, char**)::$_0::operator()() const::{lambda()#2}>(seastar::thread_attributes, auto:1&&, (auto:2&&)...)::{lambda()#3}, false>&, seastar::future_state<int>&&)#1}, int>
   --------
   seastar::continuation<seastar::internal::promise_base_with_type<int>, seastar::future<int>::finally_body<seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda()#2}, false>, seastar::future<int>::then_wrapped_nrvo<seastar::future<int>, seastar::future<int>::finally_body<seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda()#2}, false> >(seastar::future<int>::finally_body<seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda()#2}, false>&&)::{lambda(seastar::internal::promise_base_with_type<int>&&, seastar::future<int>::finally_body<seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda()#2}, false>&, seastar::future_state<int>&&)#1}, int>
   --------
   seastar::continuation<seastar::internal::promise_base_with_type<void>, seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda(int)#1}, seastar::future<int>::then_impl_nrvo<seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda(int)#1}, seastar::future<void> >(seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda(int)#1}&&)::{lambda(seastar::internal::promise_base_with_type<void>&&, seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&)::$_0::operator()()::{lambda(int)#1}&, seastar::future_state<int>&&)#1}, int>
   --------
   seastar::continuation<seastar::internal::promise_base_with_type<void>, seastar::future<void>::or_terminate()::{lambda(auto:1&&)#1}, seastar::future<void>::then_wrapped_nrvo<void, seastar::future<void>::or_terminate()::{lambda(auto:1&&)#1}>(seastar::future<void>::or_terminate()::{lambda(auto:1&&)#1}&&)::{lambda(seastar::internal::promise_base_with_type<void>&&, seastar::future<void>::or_terminate()::{lambda(auto:1&&)#1}&, seastar::future_state<seastar::internal::monostate>&&)#1}, void>

the format is identical to existing implementation.

[tchaikov]

v2:

• preserved the constructors accepting delimeter
• marked the constructors accepting delimeter parameter deprecated.

[tchaikov]

@scylladb/seastar-maint hello maintainers, could you help merge this change?