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==============================
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Computing last_modified values
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==============================
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Bazaar (through at least 0.19) computes a ``last_modified``
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attribute for all inventory entries and stores it at commit time.
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This is the ``revision_id`` that last changed or merged the file. It is
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used in knit and weave repositories to look up the file text, and to index
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into the file graph. It's also used to determine which revisions of the
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file text to pull during ``fetch``.
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This data is not natively stored by most other systems so we need to
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synthesize it during conversion.
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This is a case of non-core data that we might wish to treat as cached,
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rather than always stored.
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Take the set of all "heads": all the versions of these files in parent
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Reduce the heads by eliminating any whose last_modified is an ancestor of
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the last_modified of any other head.
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If there is still more than one head, a new last_modified is assigned.
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This points to the merge point in the file graph.
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If the file text and properties are the same as the sole remaining head,
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its last_modified is inherited. Property changes include executable bit,
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filename, and containing directory.
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Otherwise, a new last_modified is used.
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(This is meant to be the simplest statement, but it may not be the most
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efficient algorithm; anything that gives equivalent results can be used.)
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Commit and converters both need this when writing into Bazaar native
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This is an O(tree) operation because it needs to check for files with
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multiple heads. It could be reduced to O(changed_or_merged_files) if that
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was faster to determine. So it needs to be fast.
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For the single-parent commit case, we just need to determine which files have
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changed compared to the parent. If the file was changed, it gets the
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revision id of the new revision; otherwise it inherits the value from the
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In the multi-parent commit case (commit of a merge), it can take the value
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from any of the parent trees, or of the new revision.
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Commit in a dirstate tree should be able to do this more easily by looking
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at a row of the dirstate to get the per-file parents. It still needs to
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look at the revision or file graph information to work out whether heads can be
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eliminated as previously merged. At the moment ``find_previous_heads`` works on
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inventories, so needs to spend considerable effort building whole
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inventories, including files that are not modified or merged. (Called
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from ``record_entry_contents``.) It might be better to have the commit
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builder pass in the per-entry parents so that dirstate can generate just
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those that are necessary. (See also the spec for
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``iter_changes_multiple_parents``.)
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If merge used a per-file graph then it would know when one version fully
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supersedes another, and it could emit only a single parent. Merge could
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in fact do this even when not using per-file graphs. In the current
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dirstate format we need to store the full data for all trees because they
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can be extracted from the dirstate, but it could mark some parents as
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Alternatively, we could change the dirstate to include
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only the base and current trees, and cache the merged-in parents
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(Offtopic other dirstate changes: we could also omit the working-copy
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hash, and just have a stat-fingerprint of when it was last known equal to
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the basis revision. That reduces the amount of data stored and possibly
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makes it simpler to update, and shouldn't penalize common cases.)
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Generation during conversion
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----------------------------
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Accessing a foreign branch requires synthesizing this information.
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If last_modified is removed from a future bzr version, we will also need
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to synthesize it to pull back to earlier formats.
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Because last_modified is not natively stored in the foreign branches, we
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want to take advantage of any conversion we've already done, so that we
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don't need to recursively generate them on every access. We'd
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prefer to find a revision that's already converted to a Bazaar inventory
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within another related repository, such as the target of a conversion.
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Avoiding last_modified
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----------------------
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last_modified is potentially expensive to determine and we may not want to
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store it in inventories in future. Therefore we should use it only when
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* When writing out an inventory format that includes it.
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* In Bazaar formats that use it as a key for the file text or file
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ancestry. This should be hidden behind the Repository/RevisionTree
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* When a user operation specifically requires the last_modified (e.g.
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hypothetical annotate directory).
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We already do this in most cases.
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1. Single parent, unmodified file
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2. Single parent, modified file
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3. Two parents, one descended from the other, modified in one parent only
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4. Two parents, one descended from the other, modified in one parent only,
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but also modified locally.
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5. Two parents, not descended from each other, modified in one parent only.
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6. Two parents, not descended from each other, modified in one parent only,
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but also modified locally.
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7. Two parents, modified in both to different values.
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8. Two parents, modified in both to the same value.
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9. Two parents, modified in both, and reverted in both back to the
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10. Three parents, modified in only one
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11. Three parents, modified in only one, also modified locally.
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12. Three parents, modified in 2
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13. Three parents, modified in 2, and locally.
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14. Three parents, modified in 2, but one is a descendant of the other.
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Performance considerations
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--------------------------
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Often we'll want the last_modified information for multiple files, perhaps
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everything in a directory or in a whole tree. It may be more efficient
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for the api to accommodate this. Often the last_modified will be similar
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for multiple files, and if we process them all at once we can avoid some
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repeated work in calculating their heads.
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* How does caching ``find_heads`` interact with cherry-picks?
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For a single file, if I am different from all parents, 'new'. (Do not need
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to evaluate last modified).
added
removed
doc/developers/last-modified.txt
