tracing: drop block_connected hash.toString() arg

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).

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

See also: https://github.com/bitcoin/bitcoin/pull/22902#discussion_r705176691

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

1 files changed, 1 insertions, 8 deletions

diff --git a/doc/tracing.md b/doc/tracing.md
index 87fc9603fe..57104c43a0 100644
--- a/doc/tracing.md
+++ b/doc/tracing.md
@@ -101,19 +101,12 @@ Is called *after* a block is connected to the chain. Can, for example, be used
 to benchmark block connections together with `-reindex`.
 
 Arguments passed:
-1. Block Header Hash as `pointer to C-style String` (64 characters)
+1. Block Header Hash as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
 2. Block Height as `int32`
 3. Transactions in the Block as `uint64`
 4. Inputs spend in the Block as `int32`
 5. SigOps in the Block (excluding coinbase SigOps) `uint64`
 6. Time it took to connect the Block in microseconds (µs) as `uint64`
-7. Block Header Hash as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
-
-Note: The 7th argument can't be accessed by bpftrace and is purposefully chosen
-to be the block header hash as bytes. See [bpftrace argument limit] for more
-details.
-
-[bpftrace argument limit]: #bpftrace-argument-limit
 
 ## Adding tracepoints to Bitcoin Core