Go 1.17 includes a breaking change to both net.ParseIP and net.ParseCIDR
functions to reject IPv4 address octets written with leading zeros.
Our use of these functions as part of the various CIDR functions in the
Terraform language doesn't have the same security concerns that the Go
team had in evaluating this change to the standard library, and so we
can't justify an exception to our v1.0 compatibility promises on the same
sort of security grounds that the Go team used to justify their
compatibility exception.
For that reason, we'll now use our own fork of the Go library functions
which has the new check disabled in order to preserve the prior behavior.
We're taking this path, rather than pre-normalizing the IP address before
calling into the standard library, because an additional normalization
layer would be entirely new code and additional complexity, whereas this
fork is relatively minor in terms of code size and avoids any significant
changes to our own calls to these functions.
Thanks to the Kubernetes team for their prior work on carving out a subset
of the "net" package for their similar backward-compatibility concern.
Our "ipaddr" package here is a lightly-modified fork of their fork, with
only the comments changed to talk about Terraform instead of Kubernetes.
This fork is not intended for use in any other future feature
implementations, because they wouldn't be subject to the same
compatibility constraints as our existing functions. We will use these
forked implementations for new callers only if consistency with the
behavior of the existing functions is a key requirement.
Martin Atkins