Here we introduce a new idea of a "configuration snapshot", which is an
in-memory copy of the source code of each of the files that make up
the configuration. The primary intended purpose for this is as an
intermediate step before writing the configuration files into a plan file,
and then reading them out when that plan file is later applied.
During earlier configs package development we expected to use an afero vfs
implementation to read directly from the zip file, but that doesn't work
in practice because we need to preserve module paths from the source file
system that might include parent directory traversals (../) while
retaining the original path for use in error messages.
The result, for now, is a bit of an abstraction inversion: we implement
a specialized afero vfs implementation that makes the sparse filesystem
representation from a snapshot appear like a normal filesystem just well
enough that the config loader and parser can work with it.
In future we may wish to rework the internals here so that the main
abstraction is at a similar level to the snapshot and then that API is
mapped to the native filesystem in the normal case, removing afero. For
now though, this approach avoids the need for a significant redesign
of the parser/loader internals, at the expense of some trickiness in the
case where we're reading from a snapshot.
This commit does not yet include the reading and writing of snapshots into
plan files. That will follow in a subsequent commit.
We can only do this when modules are loaded with Parser.LoadConfigDir,
but in practice this is the common case anyway.
This is important to support the path.module and path.root expressions in
configuration.
Some of the fields in our config structs are either mandatory in primary
files or there is a default value that we apply if absent.
Unfortunately override files impose the additional constraint that we
be allowed to omit required fields (which have presumably already been
set in the primary files) and that we are able to distinguish between a
default value and omitting a value entirely.
Since most of our fields were already acceptable for override files, here
we just add some new fields to deal with the few cases where special
handling is required and a helper function to disable the "Required" flag
on attributes in a given schema.
This method wraps LoadConfigFile to load all of the .tf and .tf.json files
in a given directory and then bundle them together into a Module object.
This function also deals with the distinction between primary and override
files, first appending together the primary files in lexicographic order
by filename, and then merging in override files in the same order.
The merging behavior is not fully implemented as of this commit, and so
will be expanded in future commits.
Alex Pilon