From 6931e5718024c7148c1c4f9107a2716bbfa65aee Mon Sep 17 00:00:00 2001
From: Dzmitry Malyshau <kvark@fastmail.com>
Date: Mon, 7 Feb 2022 19:23:40 -0500
Subject: [PATCH] Don't recycle indices that reach EOL (#2462)

---
 wgpu-core/src/hub.rs | 22 ++++++++++++++++++++--
 wgpu-core/src/id.rs  |  2 +-
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/wgpu-core/src/hub.rs b/wgpu-core/src/hub.rs
index 63d3a12d20..c513a608a8 100644
--- a/wgpu-core/src/hub.rs
+++ b/wgpu-core/src/hub.rs
@@ -79,8 +79,12 @@ impl IdentityManager {
         let (index, epoch, _backend) = id.unzip();
         let pe = &mut self.epochs[index as usize];
         assert_eq!(*pe, epoch);
-        *pe += 1;
-        self.free.push(index);
+        // If the epoch reaches EOL, the index doesn't go
+        // into the free list, will never be reused again.
+        if epoch < id::EPOCH_MASK {
+            *pe = epoch + 1;
+            self.free.push(index);
+        }
     }
 }
 
@@ -1081,3 +1085,17 @@ fn _test_send_sync(global: &Global<IdentityManagerFactory>) {
     fn test_internal<T: Send + Sync>(_: T) {}
     test_internal(global)
 }
+
+#[test]
+fn test_epoch_end_of_life() {
+    use id::TypedId as _;
+    let mut man = IdentityManager::default();
+    man.epochs.push(id::EPOCH_MASK);
+    man.free.push(0);
+    let id1 = man.alloc::<id::BufferId>(Backend::Empty);
+    assert_eq!(id1.unzip().0, 0);
+    man.free(id1);
+    let id2 = man.alloc::<id::BufferId>(Backend::Empty);
+    // confirm that the index 0 is no longer re-used
+    assert_eq!(id2.unzip().0, 1);
+}
diff --git a/wgpu-core/src/id.rs b/wgpu-core/src/id.rs
index e75d5278c6..e646eb1d54 100644
--- a/wgpu-core/src/id.rs
+++ b/wgpu-core/src/id.rs
@@ -3,7 +3,7 @@ use std::{cmp::Ordering, fmt, marker::PhantomData, num::NonZeroU64};
 use wgt::Backend;
 
 const BACKEND_BITS: usize = 3;
-const EPOCH_MASK: u32 = (1 << (32 - BACKEND_BITS)) - 1;
+pub const EPOCH_MASK: u32 = (1 << (32 - BACKEND_BITS)) - 1;
 type Dummy = hal::api::Empty;
 
 #[repr(transparent)]