The metadata fetcher scripts run each time an instance starts, and it
is not safe to assume that responses from the instance metadata
service (IMDS) will be as they were on first boot.
Example: an EC2 instance can have its user data changed while
the instance is stopped. When the instance is restarted, we want to
see the new user data applied.
According to Freenode's ##AWS, the metadata server can sometimes
take a few moments to get its shoes on, and the very first boot
of a machine can see failed requests for a few moments.
AWS's metadata service has two versions. Version 1 allowed plain HTTP
requests to get metadata. However, this was frequently abused when a
user could trick an AWS-hosted server in to proxying requests to the
metadata service. Since the metadata service is frequently used to
generate AWS access keys, this is pretty gnarly. Version two is
identical except it requires the caller to request a token and provide
it on each request.
Today, starting a NixOS AMI in EC2 where the metadata service is
configured to only allow v2 requests fails: the user's SSH key is not
placed, and configuration provided by the user-data is not applied.
The server is useless. This patch addresses that.
Note the dependency on curl is not a joyful one, and it expand the
initrd by 30M. However, see the added comment for more information
about why this is needed. Note the idea of using `echo` and `nc` are
laughable. Don't do that.
