Rework std::sys::windows::alloc

library/std/src/sys/windows/alloc.rs

@@ -1,7 +1,9 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 
 use crate::alloc::{GlobalAlloc, Layout, System};
+use crate::ffi::c_void;
 use crate::ptr;
+use crate::sync::atomic::{AtomicPtr, Ordering};
 use crate::sys::c;
 use crate::sys_common::alloc::{realloc_fallback, MIN_ALIGN};
 
@@ -17,6 +19,9 @@ const HEAP_ZERO_MEMORY: c::DWORD = 0x00000008;
 extern "system" {
  // Get a handle to the default heap of the current process, or null if the operation fails.
  //
+ // SAFETY: Successful calls to this function within the same process are assumed to
+ // always return the same handle, which remains valid for the entire lifetime of the process.
+ //
  // See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-getprocessheap
  fn GetProcessHeap() -> c::HANDLE;
 
@@ -66,29 +71,59 @@ extern "system" {
  // Returns a nonzero value if the operation is successful, and zero if the operation fails.
  //
  // SAFETY:
+ // - `hHeap` must be a non-null handle returned by `GetProcessHeap`.
  // - `dwFlags` must be set to zero.
  // - `lpMem` must be a pointer to an allocated block returned by `HeapAlloc` or `HeapReAlloc`,
  // that has not already been freed.
  // If the block was successfully freed, pointers pointing to the freed memory, such as `lpMem`,
  // must not be dereferenced ever again.
  //
- // Note that both `hHeap` is allowed to be any value, and `lpMem` is allowed to be null,
- // both of which will not cause the operation to fail.
+ // Note that `lpMem` is allowed to be null, which will not cause the operation to fail.
  //
  // See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heapfree
  fn HeapFree(hHeap: c::HANDLE, dwFlags: c::DWORD, lpMem: c::LPVOID) -> c::BOOL;
 }
 
+// Cached handle to the default heap of the current process.
+// Either a non-null handle returned by `GetProcessHeap`, or null when not yet initialized or `GetProcessHeap` failed.
+static HEAP: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());
+
+// Get a handle to the default heap of the current process, or null if the operation fails.
+// SAFETY: If this operation is successful, `HEAP` will be successfully initialized and contain
+// a non-null handle returned by `GetProcessHeap`.
+#[inline]
+unsafe fn init_or_get_process_heap() -> c::HANDLE {
+ let heap = HEAP.load(Ordering::Relaxed);
+ if heap.is_null() {
+ // `HEAP` has not yet been successfully initialized
+ let heap = unsafe { GetProcessHeap() };
+ if !heap.is_null() {
+ // SAFETY: No locking is needed because within the same process,
+ // successful calls to `GetProcessHeap` will always return the same value, even on different threads.
+ HEAP.store(heap, Ordering::Relaxed);
+
+ // SAFETY: `HEAP` contains a non-null handle returned by `GetProcessHeap`
+ heap
+ } else {
+ // Could not get the current process heap.
+ ptr::null_mut()
+ }
+ } else {
+ // SAFETY: `HEAP` contains a non-null handle returned by `GetProcessHeap`
+ heap
+ }
+}
+
 // Header containing a pointer to the start of an allocated block.
-// SAFETY: size and alignment must be <= `MIN_ALIGN`.
+// SAFETY: Size and alignment must be <= `MIN_ALIGN`.
 #[repr(C)]
 struct Header(*mut u8);
 
-// Allocates a block of optionally zeroed memory for a given `layout`.
-// Returns a pointer satisfying the guarantees of `System` about allocated pointers.
+// Allocate a block of optionally zeroed memory for a given `layout`.
+// SAFETY: Returns a pointer satisfying the guarantees of `System` about allocated pointers.
 #[inline]
 unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
- let heap = unsafe { GetProcessHeap() };
+ let heap = unsafe { init_or_get_process_heap() };
  if heap.is_null() {
  // Allocation has failed, could not get the current process heap.
  return ptr::null_mut();
@@ -147,14 +182,14 @@ unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
 unsafe impl GlobalAlloc for System {
  #[inline]
  unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
- // SAFETY: pointers returned by `allocate` satisfy the guarantees of `System`
+ // SAFETY: Pointers returned by `allocate` satisfy the guarantees of `System`
  let zeroed = false;
  unsafe { allocate(layout, zeroed) }
  }
 
  #[inline]
  unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
- // SAFETY: pointers returned by `allocate` satisfy the guarantees of `System`
+ // SAFETY: Pointers returned by `allocate` satisfy the guarantees of `System`
  let zeroed = true;
  unsafe { allocate(layout, zeroed) }
  }
@@ -173,24 +208,27 @@ unsafe impl GlobalAlloc for System {
  }
  };
 
+ // SAFETY: because `ptr` has been successfully allocated with this allocator,
+ // `HEAP` must have been successfully initialized and contain a non-null handle
+ // returned by `GetProcessHeap`.
+ let heap = HEAP.load(Ordering::Relaxed);
+
  // SAFETY: `block` is a pointer to the start of an allocated block.
  unsafe {
- let err = HeapFree(GetProcessHeap(), 0, block as c::LPVOID);
+ let err = HeapFree(heap, 0, block as c::LPVOID);
  debug_assert!(err != 0, "Failed to free heap memory: {}", c::GetLastError());
  }
  }
 
  #[inline]
  unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
  if layout.align() <= MIN_ALIGN {
- let heap = unsafe { GetProcessHeap() };
- if heap.is_null() {
- // Reallocation has failed, could not get the current process heap.
- return ptr::null_mut();
- }
+ // SAFETY: because `ptr` has been successfully allocated with this allocator,
+ // `HEAP` must have been successfully initialized and contain a non-null handle
+ // returned by `GetProcessHeap`.
+ let heap = HEAP.load(Ordering::Relaxed);
 
- // SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`,
- // `ptr` is a pointer to the start of an allocated block.
+ // SAFETY: `ptr` is a pointer to the start of an allocated block.
  // The returned pointer points to the start of an allocated block.
  unsafe { HeapReAlloc(heap, 0, ptr as c::LPVOID, new_size) as *mut u8 }
  } else {