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, 6 deletions

diff --git a/contrib/tracing/README.md b/contrib/tracing/README.md
index 047354cda1..1f93474fa0 100644
--- a/contrib/tracing/README.md
+++ b/contrib/tracing/README.md
@@ -176,17 +176,12 @@ third acts as a duration threshold in milliseconds. When the `ConnectBlock()`
 function takes longer than the threshold, information about the block, is
 printed. For more details, see the header comment in the script.
 
-By default, `bpftrace` limits strings to 64 bytes due to the limited stack size
-in the kernel VM. Block hashes as zero-terminated hex strings are 65 bytes which
-exceed the string limit. The string size limit can be set to 65 bytes with the
-environment variable `BPFTRACE_STRLEN`.
-
 The following command can be used to benchmark, for example, `ConnectBlock()`
 between height 20000 and 38000 on SigNet while logging all blocks that take
 longer than 25ms to connect.
 
 ```
-$ BPFTRACE_STRLEN=65 bpftrace contrib/tracing/connectblock_benchmark.bt 20000 38000 25
+$ bpftrace contrib/tracing/connectblock_benchmark.bt 20000 38000 25
 ```
 
 In a different terminal, starting Bitcoin Core in SigNet mode and with