libtraceevent + Rust

[Douglas Raillard <douglas.raillard@arm.com>]

Hi everyone,

The idea of having some Rust interop for libtraceevent was floated during the
Tracing Summit that took place this week.

The overall work has two scopes:
     1. libtraceevent: low-level helpers to decode event formats and binary buffer pages.
     2. high level API that e.g. can consume a trace.dat file and expose an iterator API etc.

The current discussion is only about #1. The 2nd part of the work has an
entirely different set of constraints as it would aim at integrating in the
Rust ecosystem as well as possible without trying to replace anything
pre-existent.


* Rust bindings for C code:
     Pros:
         * Faster to implement
         * By construction, any current covered use case stays covered.

     Cons:
         * Less straightforward to build and distribute on crates.io. Current
           best practice seems to simply expect the C lib to already be
           compiled/installed on the host system and find it with pkg-config.
           Otherwise, the lib sources can be bundled and a build recipe
           included. At any rate, it will bring back the daemons of C package
           management world where Rust typically only requires "cargo build" to
           obtain a working binary.

         * More unsafe code overall when combined with higher level Rust code.


* Rust implementation:
     Pros:
         * Occasion to fix unsatisfactory points in the current API:
             * Some inefficiencies due to looking up fields by name mentioned
               by Steven (correct that if I did not get it right).

             * Some unsatisfactory naming convention (Steven).

             * Event format is hard to parse as lots of event strongly
               depends on the format string to specify the actual type. E.g. The
               print event uses unsigned long for "ip" and then casts to (void*)
               in the format string. sched_switch or mm_page_alloc require even
               more advanced C parsing power to extract info necessary to
               correctly/usefully represent the state type in most languages.
               This might be fixable with more codified best practices and
               fixing existing trace events, but we would still need to deal
               with the existing situation for a few more years at best.  A new
               API could merge the type info from the parsed print format with
               the other "ABI info" to expose a richer representation to
               consumers.

         * Can be used by higher level Rust code, preserving soundness
           guarantees end to end.

         * Can open new use cases such as:
             * Likely easier to maintain Python bindings using e.g. PyO3
             * Good WASM support is something the Rust community cares about, so
               that might open an avenue for new trace visualization tools.

     Cons:
         * More work than just making bindings for existing C code.
         * Portability: is there any architecture not currenlty supported by
           Rust we care about ?
         * New toolchain required to build the code. rustup solves that issue
           very well though.

     Possible cons requiring a prototype to check:
         * If we aim at 100% API/ABI compatibility with current libtraceevent,
           this may force Rust code into very non-idiomatic code that may affect
           the benefits of the language.

         * While it can still be exposed to C code, some soundness
           guarantees will likely be lost in passing: Rust ref to C pointer
           conversion is statically lossy. Having a func implemented in Rust
           does not magically make consuming C compilers smarter and able to
           check for memory safety. The improvement would only be inside the
           implementation itself, but if we expose relatively simple low level
           helpers the advantage is decreased.


I'm sure I missed some things, so let's use this thread to get a better view of
where to head next.

Steven also made a twitter poll on the matter:
https://twitter.com/srostedt/status/1580589911706521600


Thanks,

Douglas

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