Store evicted cache entry so it can be restored

This makes recursively calling `resolve` safe even if the cache is filled.

Author: ChrisDenton

src/symbolize/gimli.rs

@@ -414,12 +414,19 @@ impl Cache {
             .next()
     }
 
-    fn mapping_for_lib<'a>(&'a mut self, lib: usize) -> Option<(&'a mut Context<'a>, &'a Stash)> {
+    fn mapping_for_lib<'a>(
+        &'a mut self,
+        lib: usize,
+    ) -> Option<(&'a mut Context<'a>, &'a Stash, Option<(usize, Mapping)>)> {
+        let mut evicted = None;
         let cache_idx = self.mappings.iter().position(|(lib_id, _)| *lib_id == lib);
 
         let cache_entry = if let Some(idx) = cache_idx {
             self.mappings.move_to_front(idx)
         } else {
+            if self.mappings.is_full() {
+                evicted = self.mappings.pop_back();
+            }
             // When the mapping is not in the cache, create a new mapping and insert it,
             // which will also evict the oldest entry.
             create_mapping(&self.libraries[lib])
@@ -434,6 +441,7 @@ impl Cache {
         Some((
             unsafe { mem::transmute::<&'a mut Context<'static>, &'a mut Context<'a>>(cx) },
             stash,
+            evicted,
         ))
     }
 }
@@ -456,10 +464,26 @@ pub unsafe fn resolve(what: ResolveWhat<'_>, cb: &mut dyn FnMut(&super::Symbol))
                 None => return,
             };
 
+            // If the cache needs to evict an entry to add a new one, we store
+            // the evicted entry so we can restore it in case of recursion.
+            struct CacheGuard<'a>(&'a mut Cache, Option<(usize, Mapping)>);
+            impl Drop for CacheGuard<'_> {
+                fn drop(&mut self) {
+                    if let Some(entry) = self.1.take() {
+                        self.0.mappings.push_back(entry);
+                    }
+                }
+            }
+            let mut guard = CacheGuard(cache, None);
+            let cache = &mut guard.0;
+
             // Finally, get a cached mapping or create a new mapping for this file, and
             // evaluate the DWARF info to find the file/line/name for this address.
             let (cx, stash) = match cache.mapping_for_lib(lib) {
-                Some((cx, stash)) => (cx, stash),
+                Some((cx, stash, evicted)) => {
+                    guard.1 = evicted;
+                    (cx, stash)
+                }
                 None => return,
             };
             let mut any_frames = false;

src/symbolize/gimli/lru.rs

@@ -34,6 +34,41 @@ impl<T, const N: usize> Lru<T, N> {
             .map(|init| unsafe { init.assume_init_ref() })
     }
 
+    #[inline]
+    pub fn is_full(&self) -> bool {
+        self.len == N
+    }
+
+    #[inline]
+    pub fn pop_back(&mut self) -> Option<T> {
+        if self.len == 0 {
+            None
+        } else {
+            self.len -= 1;
+            // SAFETY: we maintain len invariant and bail if len was equal to 0
+            Some(unsafe { mem::transmute_copy(&self.arr[self.len]) })
+        }
+    }
+
+    #[inline]
+    pub fn push_back(&mut self, value: T) {
+        if N == 0 {
+            return;
+        } else if self.len == N {
+            self.len = N - 1;
+            // SAFETY: we bail on N == 0 but the first entry will be invalid
+            // until the loop below rotates the array.
+            unsafe { ptr::drop_in_place(self.arr.as_mut_ptr().cast::<T>()) };
+        }
+        let len_to_init = self.len + 1;
+        let mut last = MaybeUninit::new(value);
+        for elem in self.arr[0..len_to_init].iter_mut().rev() {
+            // OPT(size): using `mem::swap` allows surprising size regressions
+            last = mem::replace(elem, last);
+        }
+        self.len = len_to_init;
+    }
+
     #[inline]
     pub fn push_front(&mut self, value: T) -> Option<&mut T> {
         if N == 0 {