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docs: internals: feature flags set theory
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@ -321,10 +321,8 @@ or modified. It looks like this:
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The *version* field can be either 1 or 2. The versions differ in the
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way feature flags are handled, described below.
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The *timestamp* field was used to avoid a certain class of replay attack.
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It is still used for that purpose, however, the newer replay protection
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introduced in Borg 1.1 includes all reply attacks. Thus it is not strictly
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necessary any more.
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The *timestamp* field is used to avoid logical replay attacks where
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the server just resets the repository to a previous state.
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*item_keys* is a list containing all Item_ keys that may be encountered in
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the repository. It is used by *borg check*, which verifies that all keys
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@ -337,7 +335,9 @@ the manifest, since an ID check is not possible.
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*config* is a general-purpose location for additional metadata. All versions
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of Borg preserve its contents (it may have been a better place for *item_keys*,
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which is not preserved by unaware Borg versions).
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which is not preserved by unaware Borg versions, releases predating 1.0.4).
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.. This was implemented in PR#1149, 78121a8 and a7b5165
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Feature flags
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+++++++++++++
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@ -362,8 +362,9 @@ but is not expected to be a major hurdle; it doesn't have to be handled efficien
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needs to be handled.
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Lastly, cache_ invalidation is handled by noting which feature
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flags were and were not used to manipulate a cache. This allows to detect whether
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the cache needs to be invalidated, i.e. rebuilt from scratch. See `Cache feature flags`_ below.
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flags were and which were not understood while manipulating a cache.
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This allows to detect whether the cache needs to be invalidated,
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i.e. rebuilt from scratch. See `Cache feature flags`_ below.
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The *config* key stores the feature flags enabled on a repository:
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@ -403,7 +404,7 @@ Upon reading the manifest, the Borg client has already determined which operatio
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is to be performed. If feature flags are found in the manifest, the set
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of feature flags supported by the client is compared to the mandatory set
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found in the manifest. If any unsupported flags are found (i.e. the mandatory set is
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a superset of the features supported by the Borg client used), the operation
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not a subset of the features supported by the Borg client used), the operation
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is aborted with a *MandatoryFeatureUnsupported* error:
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Unsupported repository feature(s) {'some_feature'}. A newer version of borg is required to access this repository.
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@ -428,8 +429,8 @@ Then, two sets of features are computed;
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- those features that are supported by the client and mandated by the manifest
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are added to the *mandatory_features* set,
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- the complement to *mandatory_features*, *ignored_features* comprised
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of those features mandated by the manifest, but not supported by the client.
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- the *ignored_features* set comprised of those features mandated by the manifest,
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but not supported by the client.
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Because the client previously checked compliance with the mandatory set of features
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required for the particular operation it is executing, the *mandatory_features* set
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@ -439,7 +440,7 @@ Conversely, the *ignored_features* set contains only those features which were n
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relevant to operating the cache. Otherwise, the client would not pass the feature
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set test against the manifest.
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When opening a cache and the *mandatory_features* set is a superset of the features
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When opening a cache and the *mandatory_features* set is a not a subset of the features
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supported by the client, the cache is wiped out and rebuilt,
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since a client not supporting a mandatory feature that the cache was built with
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would be unable to update it correctly.
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