gpa: Fix GeneralPurposeAllocator double free stack traces

[Frojdholm]

Fixes #19978

I wrote the following program to test different allocation sizes

const std = @import("std");

const page_size = std.mem.page_size;

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{
        .safety = true,
        .never_unmap = true,
        .retain_metadata = true,
    }){};
    defer std.debug.assert(gpa.deinit() == .ok);
    const allocator = gpa.allocator();

    var size_class: usize = 1;
    while (size_class <= page_size) {
        std.debug.print("allocating {}\n", .{size_class * 8});
        const alloc = try allocator.alloc(u8, size_class);
        allocator.free(alloc);
        allocator.free(alloc);
        size_class *= 2;
    }
}

[squeek502]

Nice work!

[Frojdholm]

Not sure if this test should be included in this PR, but I found it not obvious that this was the way to extract the size_class from an empty bucket. It's hidden away in a branch of the free function and if this invariant changes the metadata tracking system will break.

[squeek502]

How about adding a function to BucketHeader, something like:

/// Only valid for buckets within `empty_buckets`, and relies on the `alloc_cursor`
/// of empty buckets being set to `slot_count` when they are added to `empty_buckets`
fn sizeClassOfEmptyBucket(bucket: *BucketHeader) usize {
    // alloc_cursor was set to slot count when bucket added to empty_buckets
    // so we can get the size_class
    return @divExact(page_size, bucket.alloc_cursor);
}

and then using that everywhere this @divExact(page_size, bucket.alloc_cursor) calculation is done?

[Frojdholm]

Could be *const

[andrewrk]

Thanks!