From 311f2234c549c7e8e86020693302a7f02a54a6d1 Mon Sep 17 00:00:00 2001
From: Matthew Parkinson <mattpark@microsoft.com>
Date: Wed, 10 Jun 2026 11:49:02 +0100
Subject: [PATCH] Move test_memory_stats to an integration test

test_memory_stats reads a process-global counter that every
allocation in the process feeds into. Placing it under tests/
runs it in its own binary, isolating its snapshots from any
other test thread.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 snmalloc-rs/src/lib.rs            | 36 -----------------------
 snmalloc-rs/tests/memory_stats.rs | 47 +++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 36 deletions(-)
 create mode 100644 snmalloc-rs/tests/memory_stats.rs

diff --git a/snmalloc-rs/src/lib.rs b/snmalloc-rs/src/lib.rs
index 467c4ef54..3a7a89cb1 100644
--- a/snmalloc-rs/src/lib.rs
+++ b/snmalloc-rs/src/lib.rs
@@ -233,40 +233,4 @@ mod tests {
             assert!(usz >= 8);
         }
     }
-
-    #[test]
-    fn test_memory_stats() {
-        let alloc = SnMalloc::new();
-        let before = SnMalloc::memory_stats();
-
-        let layout = Layout::from_size_align(1 << 20, 64).unwrap();
-        let ptr = unsafe { alloc.alloc(layout) };
-        assert!(!ptr.is_null());
-
-        let during = SnMalloc::memory_stats();
-        assert!(
-            during.current_memory_usage > 0,
-            "current usage should be non-zero after allocation"
-        );
-        assert!(
-            during.current_memory_usage >= before.current_memory_usage,
-            "current usage should not decrease after allocation"
-        );
-        assert!(
-            during.peak_memory_usage >= before.peak_memory_usage,
-            "peak usage should not decrease after allocation"
-        );
-
-        unsafe { alloc.dealloc(ptr, layout) };
-
-        let after = SnMalloc::memory_stats();
-        assert!(
-            after.current_memory_usage <= during.current_memory_usage,
-            "current usage should decrease after dealloc"
-        );
-        assert!(
-            after.peak_memory_usage >= during.peak_memory_usage,
-            "peak usage should never decrease"
-        );
-    }
 }
diff --git a/snmalloc-rs/tests/memory_stats.rs b/snmalloc-rs/tests/memory_stats.rs
new file mode 100644
index 000000000..e2b56986f
--- /dev/null
+++ b/snmalloc-rs/tests/memory_stats.rs
@@ -0,0 +1,47 @@
+//! `memory_stats` reads a process-global atomic counter that every
+//! allocation in the process feeds into. Running this assertion in the
+//! same binary as other allocating tests (which cargo runs in parallel
+//! threads by default) makes the `after <= during` check racy: another
+//! test thread can pull a fresh chunk from the backend between the two
+//! snapshots and push `current_memory_usage` up. Cargo runs each file
+//! under `tests/` as its own binary in its own process, giving us the
+//! isolation this test requires.
+
+use snmalloc_rs::SnMalloc;
+use std::alloc::{GlobalAlloc, Layout};
+
+#[test]
+fn test_memory_stats() {
+    let alloc = SnMalloc::new();
+    let before = SnMalloc::memory_stats();
+
+    let layout = Layout::from_size_align(1 << 20, 64).unwrap();
+    let ptr = unsafe { alloc.alloc(layout) };
+    assert!(!ptr.is_null());
+
+    let during = SnMalloc::memory_stats();
+    assert!(
+        during.current_memory_usage > 0,
+        "current usage should be non-zero after allocation"
+    );
+    assert!(
+        during.current_memory_usage >= before.current_memory_usage,
+        "current usage should not decrease after allocation"
+    );
+    assert!(
+        during.peak_memory_usage >= before.peak_memory_usage,
+        "peak usage should not decrease after allocation"
+    );
+
+    unsafe { alloc.dealloc(ptr, layout) };
+
+    let after = SnMalloc::memory_stats();
+    assert!(
+        after.current_memory_usage <= during.current_memory_usage,
+        "current usage should decrease after dealloc"
+    );
+    assert!(
+        after.peak_memory_usage >= during.peak_memory_usage,
+        "peak usage should never decrease"
+    );
+}