createPosixBlock

pub fn createPosixBlock(
    existing: Environ,
    gpa: Allocator,
    options: CreatePosixBlockOptions,
) Allocator.Error!PosixBlock {
    const contains_zig_progress = for (existing.block.view().slice) |entry| {
        if (mem.eql(u8, mem.sliceTo(entry, '='), "ZIG_PROGRESS")) break true;
    } else false;

    const ZigProgressAction = enum { nothing, edit, delete, add };
    const zig_progress_action: ZigProgressAction = action: {
        const fd = options.zig_progress_fd orelse break :action .nothing;
        if (fd >= 0) {
            break :action if (contains_zig_progress) .edit else .add;
        } else {
            if (contains_zig_progress) break :action .delete;
        }
        break :action .nothing;
    };

    const envp = try gpa.allocSentinel(?[*:0]u8, len: {
        var len: usize = existing.block.slice.len;
        switch (zig_progress_action) {
            .add => len += 1,
            .delete => len -= 1,
            .nothing, .edit => {},
        }
        break :len len;
    }, null);
    var envp_len: usize = 0;
    errdefer {
        envp[envp_len] = null;
        PosixBlock.deinit(.{ .slice = envp[0..envp_len :null] }, gpa);
    }
    if (zig_progress_action == .add) {
        envp[envp_len] = try std.fmt.allocPrintSentinel(gpa, "ZIG_PROGRESS={d}", .{options.zig_progress_fd.?}, 0);
        envp_len += 1;
    }

    var existing_index: usize = 0;
    while (existing.block.slice[existing_index]) |entry| : (existing_index += 1) {
        if (mem.eql(u8, mem.sliceTo(entry, '='), "ZIG_PROGRESS")) switch (zig_progress_action) {
            .add => unreachable,
            .delete => continue,
            .edit => {
                envp[envp_len] = try std.fmt.allocPrintSentinel(gpa, "ZIG_PROGRESS={d}", .{options.zig_progress_fd.?}, 0);
                envp_len += 1;
                continue;
            },
            .nothing => {},
        };
        envp[envp_len] = try gpa.dupeSentinel(u8, mem.span(entry), 0);
        envp_len += 1;
    }

    assert(envp_len == envp.len);
    return .{ .slice = envp };
}