mem: Fix several issues in Scratch_Allocator

1. The size was being adjusted for the alignment which does not make any
   sense without the context of the base pointer. Now we just add the
   `alignment - 1` to the size if needed then adjust the pointer.

2. The root pointer of the last allocation is now stored in order to
   make the free operation more useful (and to cover the right memory
   region for ASan).

3. Resizing now only works on the last allocation instead of any address
   in a valid range, which resulted in overwriting allocations that had
   just been made.

4. `old_memory` is now re-poisoned entirely before the resized range is
   returned with the new range unpoisoned. This will guarantee that
   there are no unpoisoned gaps.

Fixes #2694

core/mem/allocators.odin

@@ -328,11 +328,12 @@ scratch_allocator_destroy :: scratch_destroy
 Scratch allocator data.
 */
 Scratch :: struct {
-	data:               []byte,
-	curr_offset:        int,
-	prev_allocation:    rawptr,
-	backup_allocator:   Allocator,
-	leaked_allocations: [dynamic][]byte,
+	data:                 []byte,
+	curr_offset:          int,
+	prev_allocation:      rawptr,
+	prev_allocation_root: rawptr,
+	backup_allocator:     Allocator,
+	leaked_allocations:   [dynamic][]byte,
 }
 
 /*
@@ -350,6 +351,10 @@ into the backing buffer as a whole, it will be allocated using a backing
 allocator, and pointer to the allocated memory region will be put into the
 `leaked_allocations` array.
 
+Allocations which are resized will be resized in-place if they were the last
+allocation. Otherwise, they are re-allocated to avoid overwriting previous
+allocations.
+
 The `leaked_allocations` array is managed by the `context` allocator.
 */
 @(require_results)
@@ -367,6 +372,7 @@ scratch_init :: proc(s: ^Scratch, size: int, backup_allocator := context.allocat
 	s.data = make_aligned([]byte, size, 2*align_of(rawptr), backup_allocator) or_return
 	s.curr_offset = 0
 	s.prev_allocation = nil
+	s.prev_allocation_root = nil
 	s.backup_allocator = backup_allocator
 	s.leaked_allocations.allocator = backup_allocator
 	sanitizer.address_poison(s.data)
@@ -467,23 +473,37 @@ scratch_alloc_bytes_non_zeroed :: proc(
 		}
 		scratch_init(s, DEFAULT_BACKING_SIZE)
 	}
-	size := size
-	size = align_forward_int(size, alignment)
-	if size <= len(s.data) {
+	aligned_size := size
+	if alignment > 1 {
+		// It is possible to do this with less bytes, but this is the
+		// mathematically simpler solution, and this being a Scratch allocator,
+		// we don't need to be so strict about every byte.
+		aligned_size += alignment - 1
+	}
+	if aligned_size <= len(s.data) {
 		offset := uintptr(0)
-		if s.curr_offset+size <= len(s.data) {
+		if s.curr_offset+aligned_size <= len(s.data) {
 			offset = uintptr(s.curr_offset)
 		} else {
+			// The allocation will cause an overflow past the boundary of the
+			// space available, so reset to the starting offset.
 			offset = 0
 		}
 		start := uintptr(raw_data(s.data))
-		ptr   := align_forward_uintptr(offset+start, uintptr(alignment))
-		s.prev_allocation = rawptr(ptr)
-		s.curr_offset = int(offset) + size
-		result := byte_slice(rawptr(ptr), size)
+		ptr := rawptr(offset+start)
+		// We keep track of the original base pointer without extra alignment
+		// in order to later allow the free operation to work from that point.
+		s.prev_allocation_root = ptr
+		if !is_aligned(ptr, alignment) {
+			ptr = align_forward(ptr, uintptr(alignment))
+		}
+		s.prev_allocation = ptr
+		s.curr_offset = int(offset) + aligned_size
+		result := byte_slice(ptr, size)
 		sanitizer.address_unpoison(result)
 		return result, nil
 	} else {
+		// NOTE: No need to use `aligned_size` here, as the backup allocator will handle alignment for us.
 		a := s.backup_allocator
 		if a.procedure == nil {
 			a = context.allocator
@@ -525,9 +545,10 @@ scratch_free :: proc(s: ^Scratch, ptr: rawptr, loc := #caller_location) -> Alloc
 	end := start + uintptr(len(s.data))
 	old_ptr := uintptr(ptr)
 	if s.prev_allocation == ptr {
-		s.curr_offset = int(uintptr(s.prev_allocation) - start)
+		s.curr_offset = int(uintptr(s.prev_allocation_root) - start)
 		sanitizer.address_poison(s.data[s.curr_offset:])
 		s.prev_allocation = nil
+		s.prev_allocation_root = nil
 		return nil
 	}
 	if start <= old_ptr && old_ptr < end {
@@ -685,7 +706,13 @@ scratch_resize_bytes_non_zeroed :: proc(
 	begin   := uintptr(raw_data(s.data))
 	end     := begin + uintptr(len(s.data))
 	old_ptr := uintptr(old_memory)
-	if begin <= old_ptr && old_ptr < end && old_ptr+uintptr(size) < end {
+	// We can only sanely resize the last allocation; to do otherwise may
+	// overwrite memory that could very well just have been allocated.
+	//
+	// Also, the alignments must match, otherwise we must re-allocate to
+	// guarantee the user's request.
+	if s.prev_allocation == old_memory && is_aligned(old_memory, alignment) && old_ptr+uintptr(size) < end {
+		sanitizer.address_poison(old_memory)
 		s.curr_offset = int(old_ptr-begin)+size
 		result := byte_slice(old_memory, size)
 		sanitizer.address_unpoison(result)