From 2c9528232d597ee3395bd2f68cb20ba495482f69 Mon Sep 17 00:00:00 2001
From: Pratik Mankawde <3397372+pratikmankawde@users.noreply.github.com>
Date: Mon, 27 Apr 2026 14:34:47 +0100
Subject: [PATCH] fix(telemetry): use thread_local PRNG for span IDs and update
 class diagram

Replace per-call std::random_device with thread_local std::mt19937 in
txSpan() for span ID generation. random_device is ~423x slower due to
/dev/urandom syscalls on each construction; mt19937 is seeded once per
thread and reused for all subsequent span IDs.

Update the SpanGuard class ASCII diagram to include txSpan factory
methods that were added in the hash-derived trace ID commit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 include/xrpl/telemetry/SpanGuard.h  | 36 +++++++++++++++--------------
 src/libxrpl/telemetry/SpanGuard.cpp |  4 ++--
 2 files changed, 21 insertions(+), 19 deletions(-)

diff --git a/include/xrpl/telemetry/SpanGuard.h b/include/xrpl/telemetry/SpanGuard.h
index 691e6ce8dc..11afe36c92 100644
--- a/include/xrpl/telemetry/SpanGuard.h
+++ b/include/xrpl/telemetry/SpanGuard.h
@@ -9,23 +9,25 @@
 
     Dependency diagram:
 
-        +-------------------------------------------+
-        |              SpanGuard                     |
-        +-------------------------------------------+
-        | - impl_ : unique_ptr<Impl>  (pimpl)       |
-        +-------------------------------------------+
-        | + span(name) : SpanGuard          [static] |
-        | + span(cat, prefix, name)         [static] |
-        | + childSpan(name) : SpanGuard              |
-        | + linkedSpan(name) : SpanGuard             |
-        | + captureContext() : SpanContext            |
-        | + setAttribute(key, value)                 |
-        | + setOk() / setError(desc)                 |
-        | + addEvent(name)                           |
-        | + recordException(e)                       |
-        | + discard()                                |
-        | + operator bool()                          |
-        +-------------------------------------------+
+        +------------------------------------------------+
+        |                 SpanGuard                      |
+        +------------------------------------------------+
+        | - impl_ : unique_ptr<Impl>  (pimpl)            |
+        +------------------------------------------------+
+        | + span(name) : SpanGuard               [static] |
+        | + span(cat, prefix, name)              [static] |
+        | + childSpan(name) : SpanGuard                   |
+        | + linkedSpan(name) : SpanGuard                  |
+        | + txSpan(prefix, name, hash)           [static] |
+        | + txSpan(prefix, name, hash, parent)   [static] |
+        | + captureContext() : SpanContext                 |
+        | + setAttribute(key, value)                      |
+        | + setOk() / setError(desc)                      |
+        | + addEvent(name)                                |
+        | + recordException(e)                            |
+        | + discard()                                     |
+        | + operator bool()                               |
+        +------------------------------------------------+
                         |  hides (pimpl)
                 +-------+-------+
                 |               |
diff --git a/src/libxrpl/telemetry/SpanGuard.cpp b/src/libxrpl/telemetry/SpanGuard.cpp
index 9facee670f..71f157d2b2 100644
--- a/src/libxrpl/telemetry/SpanGuard.cpp
+++ b/src/libxrpl/telemetry/SpanGuard.cpp
@@ -258,9 +258,9 @@ SpanGuard::txSpan(
     otel_trace::TraceId traceId(opentelemetry::nostd::span<std::uint8_t const, 16>(hashData, 16));
 
     std::uint8_t spanIdBytes[8];
-    std::random_device rd;
+    thread_local std::mt19937 prng{std::random_device{}()};
     for (auto& b : spanIdBytes)
-        b = static_cast<std::uint8_t>(rd());
+        b = static_cast<std::uint8_t>(prng());
     otel_trace::SpanId spanId(opentelemetry::nostd::span<std::uint8_t const, 8>(spanIdBytes, 8));
 
     otel_trace::SpanContext syntheticCtx(