From 94b6df699b566bcbff909e24f08b17416f8e1e52 Mon Sep 17 00:00:00 2001
From: Martin Grigorov <martin-g@users.noreply.github.com>
Date: Wed, 10 Sep 2025 11:41:20 +0300
Subject: [PATCH] fs: fill the destination buffer with 0s for MockFile::read()
 (#7596)

---
 tokio/src/fs/mocks.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tokio/src/fs/mocks.rs b/tokio/src/fs/mocks.rs
index 5343fb90b..3a8ac2bdd 100644
--- a/tokio/src/fs/mocks.rs
+++ b/tokio/src/fs/mocks.rs
@@ -56,6 +56,13 @@ mock! {
 
 impl Read for MockFile {
     fn read(&mut self, dst: &mut [u8]) -> io::Result<usize> {
+        // Placate Miri.  Tokio will call this method with an uninitialized
+        // buffer, which is ok because std::io::Read::read implementations don't usually read
+        // from their input buffers.  But Mockall 0.12-0.13 will try to Debug::fmt the
+        // buffer, even if there is no failure, triggering an uninitialized data access alert from
+        // Miri.  Initialize the data here just to prevent those Miri alerts.
+        // This can be removed after upgrading to Mockall 0.14.
+        dst.fill(0);
         self.inner_read(dst)
     }
 }