Basic update algorithm: XXX THIS IS ALMOST ENTIRELY WRONG - SEE "THEORY" XXX 1. recurse to all of our direct dependencies and update their bases and tips 2. Update our base. i. Compute our base's set of desired included deps: The set of desired included deps for a base is the union of the desired included deps for each branch named in the base's branch's direct deps, plus the name of every direct external dep. The set of desired included deps for a branch is the set of desired included deps for the branch's base plus the branch name itself. ii. For each source in the best order, do the following merge: (Our base has sources: - the branch for each direct dep - the remote base - the topgit base, if this is a topgit import) Find the (latest) common ancestor. Check for unwanted dependency removals. An unwanted dependency removal is A branch in the desired included deps Which exists in the common ancestor's actual included deps but which is missing in the source's actual included deps (NB that this definition excludes dependency removals which have already occurred on our base; these will be reverted later.) For each unwanted dependency removal (ie for each such branch), search as follows: * An "unwanted removal commit" is a non-merge commit in the history of the source, which removes the dep from the actual included deps. * But the search stops at any point where we would have to traverse a commit where .topbloke/deps is empty (which stops us looking into the hitory of non-topbloke-controlled branches). This can be done with git-rev-list --remove-empty. * It also stops at any point where we meet a commit which does have the dep in the actual included deps. We have to do this by hand. * The the relevant unwanted removal commit for that dep is the most recent unwanted removal commit, as defined. Select the unwantedly removed dep whose relevant unwanted removal commit is the earliest. Merge from the ancestor of that relevant unwanted removal commit. Merge from the relevant unwanted removal commit using -s ours. Now continue to the next unwanted dependency removal. (The purpose of this, and the result, is that the unwanted dependency removal has gone away. Doing things in this order tries to keep the unwanted dependency removal's reversions as close as possible to their originating points. The recursion, which processes dependencies before their clients, tries to keep the reversion churn localised: client patches of a patch affected by an unwanted removal will benefit from that client's resolution of the situation.) If there are no (more) unwanted dependency removals, merge from the source. iii. Check whether our list of dependencies has changed. If so we need to restart the whole base update. iv. Check for missing or unwanted dependency inclusions. Compare our base's desired included deps with our base's actual included deps. In exceptional conditions, they will not be identical. This can happen, for example, because a dependency removal was incorporated into our base branch but the removed branch was introduced as an explicit dependency. This will also happen if we remove a dependency ourselves. Do the unwanted inclusions first, and then the missing ones. (This is because that the missing ones cannot depend on the unwanted ones, as otherwise the unwanted ones would be in the desired inclusions. So removing the unwanted ones first reduces the chances of conflicts.) So, repeatedly: * Do the comparison between desired and actual included * Pick a missing inclusion, or failing that an unwanted one (call this the "relevant" branch) * Depth first search all of the dependencies of the relevant branch; if any of these is also a missing (resp. unwanted) inclusion, start again processing it instead. * Attempt to apply the appropriate diff to add (resp. remove) the contents of the relevant patch (adjusted appropriately for metadata, XXX??? particularly the actual inclusion list) XXX if we want to add a dep we need to update the dep first * Go round again looking for another discrepancy. 3. Update our branch. Our branch has sources: - our base - the remote for our branch - the topgit branch, if this is a topgit import For each source in the best order, do the merge. Double-check the actual dependency inclusions. In particular, if we just upgraded to actual dependency tracking we may need to explicitly add our branch name to the actual dependency inclusion list. The "best order" for merges is in order of recency of common ancestor, most recent first, and if that does not distinguish, merging from local branches first. "Recency" refers to the order from git-rev-list --date-order.