Add test to ensure delegated bin names are consistent with the hash
prefixes that are delegated to the role.

This is an implicit assumption of the current implementation, the
testing of which will enable us to modify the code with greater
confidence.

Signed-off-by: Joshua Lock <[email protected]>

As we are adding and removing items from the hashed bins and checking
for their presence/absence it's simplest if we being with the hashed
bins initially empty.

If we pass a list of targets when we call delgate_hashed_bins() the
delegated roles have an initial set of targets delegated to them,
which complicates testing of adding then removing a target to a
delegated bin.

Signed-off-by: Joshua Lock <[email protected]>

This helper will generate a hash of the passed target_filepath using the
algorithm defined by the repository_tool's HASH_FUNCTION variable.

Signed-off-by: Joshua Lock <[email protected]>

Add a helper function to determine the name of a bin that a hashed
targetfile will be delegated to.

Based sketches by Lukas Puehringer in issues theupdateframework#994 & theupdateframework#995

Signed-off-by: Joshua Lock <[email protected]>

Vastly simplify the implementation, using the _get_hash() and
_find_bin_for_hash() helpers added in earlier commits.

Furthermore, enable passing of the custom parameter to
add_target_to_bin() to better match add_target()

Signed-off-by: Joshua Lock <[email protected]>

Simplify the delegate_hashed_bins function based on the semi-pseudo-code
by Lukas Puehringer in theupdateframework#995.

Signed-off-by: Joshua Lock <[email protected]>

In delegate_to_hashed_bins() duplicate the code which determines the
target's file path for writing to the metadata in repository_lib's
generate_targets_metadata to ensure that the target path hashed by
delegate_hashed_bins() matches the target path in the metadata file.

Signed-off-by: Joshua Lock <[email protected]>

The file isn't strictly needed on-disk at the time add_target() and
add_targets() are called and this duplicates the check for the file's
presence in write[_all]()

By removing this check we allow extra versatility in adding targets.

Signed-off-by: Joshua Lock <[email protected]>

Match the pattern in add_target() where if the filepath already exists
in roleinfo['paths'] it is updated to replace the existing entry with
the new fileinfo.

Signed-off-by: Joshua Lock <[email protected]>

Add an additional optional parameter to add_target() and
add_target_to_bin() which is a fileinfo object matching
tuf.formats.FILEINFO_OBJECT

This parameter and the custom parameter are mutually exclusive and
thus cannot be passed at the same time.

Signed-off-by: Joshua Lock <[email protected]>

Previously, at the time of writing targets metadata, we accessed all
targets files and ensured they exist on disk before generating a
filinfo dict containing hashes of the target file.

This change enables targets metadata to be generated using the
fileinfo data that is already stored in the roledb.

Signed-off-by: Joshua Lock <[email protected]>

Merge the logger calls reporting information about the hashed bin
delegations into a single logger.info() call to ensure the messages
will be grouped together even when integrated into a logging system
with multiple parallel sources.

Signed-off-by: Joshua Lock <[email protected]>

Test the newly added functionality to:
* add a target to the repository without access to the target file on disk
* write targets metadata without access to target files on disk, by using
  the existing fileinfo data from the roledb

Signed-off-by: Joshua Lock <[email protected]>

When testing delegate_hashed_bins to ensure that hash_path_prefixes
map to the generated name of the bin, also check to ensure that at least
one of the delegations contains one or more path_hash_prefixes.

Signed-off-by: Joshua Lock <[email protected]>

We intend to deprecate the custom parameter of add_target() in favour
of using the fileinfo parameter with the custom value populated,
therefore it does not make sense to _add_ the custom parameter to
add_target_to_bin()

Signed-off-by: Joshua Lock <[email protected]>

Factor out code to calculate the prefix length, total number of prefixes
and number of prefixes to be allocated to each bin into a helper
function _get_bin_numbers() so that we a) do not duplicate logic,
b) can document the logic in one place and c) ensure the logic is
consistent.

Signed-off-by: Joshua Lock <[email protected]>

Add some additional checks to test_add_target_to_bin to ensure the code
to add a target passing a fileinfo is tested.

Signed-off-by: Joshua Lock <[email protected]>