From fe3c0c66e73e2e5ed61066246d65d109167f9dc7 Mon Sep 17 00:00:00 2001
From: not-matthias <matthias@codspeed.io>
Date: Mon, 26 Oct 2026 17:32:40 +0100
Subject: [PATCH] feat: seed Go's global math/rand deterministically

Set GODEBUG=randautoseed=0 on benchmark processes in every mode, so the
global math/rand source starts from Seed(1) instead of a random seed and
code that draws from it behaves the same on every run.

The setting is prepended to the user's GODEBUG. Go uses the last
occurrence of a setting, so the user's other settings are kept and an
explicit randautoseed from the user still wins.

Refs COD-4013
---
 crates/runner-shared/src/runtime_env/go.rs  | 34 +++++++++++++++++++++
 crates/runner-shared/src/runtime_env/mod.rs |  2 ++
 2 files changed, 36 insertions(+)
 create mode 100644 crates/runner-shared/src/runtime_env/go.rs

diff --git a/crates/runner-shared/src/runtime_env/go.rs b/crates/runner-shared/src/runtime_env/go.rs
new file mode 100644
index 000000000..ba00d8ce9
--- /dev/null
+++ b/crates/runner-shared/src/runtime_env/go.rs
@@ -0,0 +1,34 @@
+const GODEBUG: &str = "GODEBUG";
+/// Starts the global `math/rand` source from `Seed(1)` instead of a random seed.
+const RANDAUTOSEED_OFF: &str = "randautoseed=0";
+
+/// Go env:
+/// - `GODEBUG=randautoseed=0`: deterministic global `math/rand` in every mode.
+pub(super) fn env() -> Vec<(&'static str, String)> {
+    let user_godebug = std::env::var(GODEBUG).ok();
+    vec![(GODEBUG, godebug(user_godebug.as_deref()))]
+}
+
+/// Prepends our settings to the user's `GODEBUG`. Go uses the last occurrence
+/// of a setting, so an explicit `randautoseed` from the user still wins.
+fn godebug(user_godebug: Option<&str>) -> String {
+    match user_godebug {
+        Some(user) if !user.is_empty() => format!("{RANDAUTOSEED_OFF},{user}"),
+        _ => RANDAUTOSEED_OFF.to_owned(),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn keeps_user_settings_after_ours() {
+        assert_eq!(godebug(None), "randautoseed=0");
+        assert_eq!(godebug(Some("")), "randautoseed=0");
+        assert_eq!(
+            godebug(Some("gctrace=1,randautoseed=1")),
+            "randautoseed=0,gctrace=1,randautoseed=1"
+        );
+    }
+}
diff --git a/crates/runner-shared/src/runtime_env/mod.rs b/crates/runner-shared/src/runtime_env/mod.rs
index 8b70f1b16..fc3b2631f 100644
--- a/crates/runner-shared/src/runtime_env/mod.rs
+++ b/crates/runner-shared/src/runtime_env/mod.rs
@@ -1,5 +1,6 @@
 use crate::measurement_mode::MeasurementMode;
 
+mod go;
 mod java;
 mod python;
 
@@ -10,5 +11,6 @@ pub fn env(mode: MeasurementMode) -> Vec<(&'static str, String)> {
     let mut env = vec![(RUNNER_MODE_ENV, mode.runner_mode_env_value().to_owned())];
     env.extend(python::env(mode));
     env.extend(java::env(mode));
+    env.extend(go::env());
     env
 }