tracing: drop GetHash().ToString() argument from the `validation:block_connected` tracepoint #23302

The tracepoint validation:block_connected was introduced in #22006. The first argument was the hash of the connected block as a pointer to a C-like String. The last argument passed the hash of the connected block as a pointer to 32 bytes. The hash was only passed as string to allow bpftrace scripts to print the hash. It was (incorrectly) assumed that bpftrace cannot hex-format and print the block hash given only the hash as bytes.

The block hash can be printed in bpftrace by calling printf("%02x") for each byte of the hash in an unroll () {...}. By starting from the last byte of the hash, it can be printed in big-endian (the block-explorer format).

  $p = $hash + 31;
  unroll(32) {
      $b = *(uint8*)$p;
      printf("%02x", $b);
      $p -= 1;
  }

See also: #22902 (comment)

This is a breaking change to the block_connected tracepoint API, however this tracepoint has not yet been included in a release.

Concept and code review ACK 53c9fa9e6253ea89ba1057b35e018ad1a25fb97e

Trace point invocations should be as light as possible to avoid overhead when they are disabled (the common case). Creating an intermediate string and deallocating it, as well as formatting overhead is something that should be avoided.

Concept ACK

ACK 53c9fa9e6253ea89ba1057b35e018ad1a25fb97e

this is the way.

If anyone is looking at the unroll and thinking that's a funny way to print something, it's because the BPF virtual machine inside the kernel requires guaranteed termination, you can't have infinite loops in BPF programs. unroll is a simple way that bpftrace implements fixed length looping via an integer literal. Printing data this way works great for fixed sized pieces of data, but you can't unroll on passed arguments.

I'm not familiar enough with BPF bytecode but I assume there is just no "jump" operation to implement loops, which is why you need to unroll them.

This is a breaking change to the block_connected tracepoint API, however this tracepoint has not yet been included in a release.

I would hate to think we are already at a point where we have to worry about backwards compatibility for tracepoints. Consumers should understand that they are still new/experimental and are very likely change over the next few releases, as improvements like this are made.