Fix initializing the global hash map with a small stack

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "string_cache"
-version = "0.7.3"  # Also update README.md when making a semver-breaking change
+version = "0.7.4"  # Also update README.md when making a semver-breaking change
 authors = [ "The Servo Project Developers" ]
 description = "A string interning library for Rust, developed as part of the Servo project."
 license = "MIT / Apache-2.0"
@@ -41,3 +41,7 @@ string_cache_codegen = { version = "0.4", path = "./string-cache-codegen" }
 
 [build-dependencies]
 string_cache_codegen = { version = "0.4", path = "./string-cache-codegen" }
+
+[[test]]
+name = "small-stack"
+harness = false

src/atom.rs

@@ -39,7 +39,7 @@ const NB_BUCKETS: usize = 1 << 12;  // 4096
 const BUCKET_MASK: u64 = (1 << 12) - 1;
 
 struct StringCache {
-    buckets: [Option<Box<StringCacheEntry>>; NB_BUCKETS],
+    buckets: Box<[Option<Box<StringCacheEntry>>; NB_BUCKETS]>,
 }
 
 lazy_static! {
@@ -67,8 +67,13 @@ impl StringCacheEntry {
 
 impl StringCache {
     fn new() -> StringCache {
+        type T = Option<Box<StringCacheEntry>>;
+        let _static_assert_size_eq = std::mem::transmute::<T, usize>;
+        let vec = std::mem::ManuallyDrop::new(vec![0_usize; NB_BUCKETS]);
         StringCache {
-            buckets: unsafe { mem::zeroed() },
+            buckets: unsafe {
+                Box::from_raw(vec.as_ptr() as *mut [T; NB_BUCKETS])
+            },
         }
     }
 

tests/small-stack.rs

@@ -0,0 +1,12 @@
+// Regression test for https://github.com/servo/html5ever/issues/393
+//
+// Create a dynamic atom − causing initialization of the golbal hash map −
+// in a thread that has a small stack.
+//
+// This is a separate test program rather than a `#[test] fn` among others
+// to make sure that nothing else has already initialized the map in this process.
+fn main() {
+    std::thread::Builder::new().stack_size(50_000).spawn(|| {
+        string_cache::DefaultAtom::from("12345678");
+    }).unwrap().join().unwrap()
+}