From 7c842a3b483f8b41f08ba21584d9e8502ea26e4e Mon Sep 17 00:00:00 2001
From: Jim Blandy <jimb@red-bean.com>
Date: Wed, 1 May 2024 17:39:45 -0700
Subject: [PATCH] [core] Implement `downgrade` for the `lock` module's
 `RwLock`s.

Implement `RwLockWriteGuard::downgrade` for `lock::vanilla` and
`lock::ranked`, to downgrade a write lock to a read lock.
---
 wgpu-core/src/lock/ranked.rs  | 9 +++++++++
 wgpu-core/src/lock/vanilla.rs | 6 ++++++
 2 files changed, 15 insertions(+)

diff --git a/wgpu-core/src/lock/ranked.rs b/wgpu-core/src/lock/ranked.rs
index 4b5871d26f..4237116c2c 100644
--- a/wgpu-core/src/lock/ranked.rs
+++ b/wgpu-core/src/lock/ranked.rs
@@ -281,6 +281,15 @@ impl<T> RwLock<T> {
     }
 }
 
+impl<'a, T> RwLockWriteGuard<'a, T> {
+    pub fn downgrade(this: Self) -> RwLockReadGuard<'a, T> {
+        RwLockReadGuard {
+            inner: parking_lot::RwLockWriteGuard::downgrade(this.inner),
+            saved: this.saved,
+        }
+    }
+}
+
 impl<T: std::fmt::Debug> std::fmt::Debug for RwLock<T> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         self.inner.fmt(f)
diff --git a/wgpu-core/src/lock/vanilla.rs b/wgpu-core/src/lock/vanilla.rs
index 4fc419f12e..9a35b6d9d8 100644
--- a/wgpu-core/src/lock/vanilla.rs
+++ b/wgpu-core/src/lock/vanilla.rs
@@ -86,6 +86,12 @@ impl<T> RwLock<T> {
     }
 }
 
+impl<'a, T> RwLockWriteGuard<'a, T> {
+    pub fn downgrade(this: Self) -> RwLockReadGuard<'a, T> {
+        RwLockReadGuard(parking_lot::RwLockWriteGuard::downgrade(this.0))
+    }
+}
+
 impl<T: std::fmt::Debug> std::fmt::Debug for RwLock<T> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         self.0.fmt(f)