rust: do not assume realloc preserves alignment

Signed-off-by: Ferdinand Bachmann <theferdi265@gmail.com>

rust/src/dlmalloc.rs

@@ -7,7 +7,7 @@ use cty::*;
 
 extern "C" {
     pub fn malloc(size: size_t) -> *mut c_void;
-    pub fn realloc(p: *mut c_void, size: size_t) -> *mut c_void;
+    pub fn realloc_in_place(p: *mut c_void, size: size_t) -> *mut c_void;
     pub fn free(p: *mut c_void);
     pub fn posix_memalign(p: *mut *mut c_void, alignment: size_t, size: size_t) -> c_int;
 }
@@ -51,7 +51,29 @@ unsafe impl GlobalAlloc for DLMalloc {
     }
 
     #[inline]
-    unsafe fn realloc(&self, ptr: *mut u8, _layout: Layout, new_size: usize) -> *mut u8 {
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
-        unsafe { realloc(ptr as *mut c_void, new_size) as *mut u8 }
+        // Unfortunately, realloc doesn't make any alignment guarantees, so the memory
+        // has to be manually allocated as aligned memory if it cannot be resized
+        // in-place.
+        let mut new_ptr = unsafe { realloc_in_place(ptr as *mut c_void, new_size) as *mut u8 };
+
+        // return early if in-place resize succeeded
+        if !new_ptr.is_null() {
+            return new_ptr;
+        }
+
+        // allocate new aligned storage with correct layout
+        new_ptr =
+            unsafe { self.alloc(Layout::from_size_align_unchecked(new_size, layout.align())) };
+
+        // return early if allocation failed
+        if new_ptr.is_null() {
+            return ptr::null_mut();
+        }
+
+        // copy over the data and deallocate the old storage
+        unsafe { ptr::copy(ptr, new_ptr, layout.size().min(new_size)) };
+        unsafe { self.dealloc(ptr, layout) };
+        new_ptr
     }
 }