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The "MoveDev/MoveDev" page has been changed by JulianFoad: https://wiki.apache.org/subversion/MoveDev/MoveDev?action=diff&rev1=4&rev2=5 - - = How to Add Moves to Svn = - == Summary == Subversion needs to handle moves and renames better than in version 1.8. This paper presents the rationale and a plan for doing so. @@ -29, +26 @@ Some previous ideas about how much we need to track moves explicitly: - * We need to track moves in the server so we can do forward history tracing. But does this really help unless the clients are aware and able to communicate these moves to and from the server? + * We need to track moves in the server so we can do forward history tracing. But does this really help unless the clients are aware and able to communicate these moves to and from the server? - * Track moves in the WC only – as implemented in Subversion 1.8. This helps with certain situations: it can apply incoming edits into a locally moved node, and it can prevent the accidental committing of just one half of a move. + * Track moves in the WC only – as implemented in Subversion 1.8. This helps with certain situations: it can apply incoming edits into a locally moved node, and it can prevent the accidental committing of just one half of a move. - * We don't need to track moves explicitly, as we can do everything we really need by recognizing copy-and-delete as a move, and that has the advantage of not changing the network protocols and so on. + * We don't need to track moves explicitly, as we can do everything we really need by recognizing copy-and-delete as a move, and that has the advantage of not changing the network protocols and so on. - * We don't need to track moves explicitly, as we can do everything we really need by always treating a copy in the same way as the copy half of a move. So, when merging, if there is a copy of the node being merged, then all changes destined for the copy-source node should go to the copy-dest node as well – or instead, if the copy-source node is deleted. In this way, the semantics of copy and move are unified. + * We don't need to track moves explicitly, as we can do everything we really need by always treating a copy in the same way as the copy half of a move. So, when merging, if there is a copy of the node being merged, then all changes destined for the copy-source node should go to the copy-dest node as well – or instead, if the copy-source node is deleted. In this way, the semantics of copy and move are unified. Arguments against treating any copy in the same way as the copy half of a move: - * Why should we treat a single copy (cp A A2; rm A) differently from the same situation plus an additional copy (cp A A2; cp A A3; rm A)? And if we decided to merge into all the copies alike, then why should we only do so when there is a delete? + * Why should we treat a single copy (cp A A2; rm A) differently from the same situation plus an additional copy (cp A A2; cp A A3; rm A)? And if we decided to merge into all the copies alike, then why should we only do so when there is a delete? * ... === Combining Changes === The problems with copy-and-delete boil down to various kinds of ambiguity, inconsistency or non-determinism. Many of these are related to the problem of representing a sequence of changes as a single change. It is fundamental in a version control system to be able to update, merge or diff between two widely separated revisions without having to step through all the intermediate revisions in sequence, and so it is necessary to have an unambiguous way of combining successive changes. If we attempt to interpret copy-and-delete as a move, that leads to ambiguous or context-dependent results when combining changes. - * Spatial ambiguity. When looking at a subtree that contains only one half of the move, we would see a copy or a delete, but if we then look at a wider subtree we would see a move. Look wider again, and we may see a second copy from the same source, which means there is no move because there is no unique copy. + * Spatial ambiguity. When looking at a subtree that contains only one half of the move, we would see a copy or a delete, but if we then look at a wider subtree we would see a move. Look wider again, and we may see a second copy from the same source, which means there is no move because there is no unique copy. - * Ambiguity when the copy-from is not the revision immediately before the copy-and-delete. If the deleted node was modified between the copy-from revision and the delete, then is it still a move? No, because it has a forked history. If we treat it as a move only if the delete side was not modified since the copy-from revision, then there is a race on commit because the change that gets committed will be seen as a move if nobody else modifies it in the meantime, or as a non-move if somebody gets in first. If we want the semantics of a move, we have to tell the server it is a move so it can avoid this. + * Ambiguity when the copy-from is not the revision immediately before the copy-and-delete. If the deleted node was modified between the copy-from revision and the delete, then is it still a move? No, because it has a forked history. If we treat it as a move only if the delete side was not modified since the copy-from revision, then there is a race on commit because the change that gets committed will be seen as a move if nobody else modifies it in the meantime, or as a non-move if somebody gets in first. If we want the semantics of a move, we have to tell the server it is a move so it can avoid this. - * Ambiguity when the delete is not in the same revision as the copy. + * Ambiguity when the delete is not in the same revision as the copy. - * Temporal ambiguity. Difficulty in composing a series of changes (revisions) together. If we start with (cp A A2; rm A), that looks like a move, but if we then commit (cp A@orig-rev A3) and look at the overall combined change, we now see a multiple-copies scenario. Conversely, if we start with (cp A A2; cp A A3; rm A) and then commit (rm A2), we change a non-move into a move. + * Temporal ambiguity. Difficulty in composing a series of changes (revisions) together. If we start with (cp A A2; rm A), that looks like a move, but if we then commit (cp A@orig-rev A3) and look at the overall combined change, we now see a multiple-copies scenario. Conversely, if we start with (cp A A2; cp A A3; rm A) and then commit (rm A2), we change a non-move into a move. In one context, a certain copy and delete can be paired uniquely and thus interpreted as a move, while in another context the same copy and delete are not unique or are not both visible. === Move vs. Rename === We say “move” or “rename” interchangeably for most purposes. Their essential similarities include the concept of a preserved node identity. It can be useful sometimes to draw a distinction. When merging a rename-only (A/foo → A/bar) with a move-only (A/foo → B/foo) we can suggest that the most likely merge resolution would be to apply both the move and the rename (→ B/bar). - == H2 { margin-bottom: 0.21cm; }H2.western { }H2.ctl { font-family: "FreeSans"; }P { margin-bottom: 0.21cm; }P.western { }A:link { } == == System Overview == Move support can be added in phases. The “core components”, outlined in yellow in the following diagrams. must be upgraded to get a basic level of support in which commits and updates support moves. The other components, including merge, can be supported later. Client side: - - Server side: - - == Core Components == === Client ↔ RA ↔ Repos === @@ -89, +81 @@ When not being driven by a move-aware editor: - * Insert the heuristic move-detector, if desired. + * Insert the heuristic move-detector, if desired. * Otherwise, only copies and deletes are seen. - * Lose any move heuristics currently built in to copy & delete. I think this only affects the conflict resolution. + * Lose any move heuristics currently built in to copy & delete. I think this only affects the conflict resolution. === Repos ↔ FS ↔ FSFS === TODO... @@ -104, +96 @@ ==== In existing FSFS (format 6) ==== Alter the node id and copy-id assignment rules. - * A moved node gets the same copy-id as its copy-from node. + * A moved node gets the same copy-id as its copy-from node. - * All children of a moved node get the same new copy-id as their parent. + * All children of a moved node get the same new copy-id as their parent. Adjust implementation of existing APIs to see those moves as copies (for back-compat). @@ -117, +109 @@ ==== New FS APIs ==== Provide new APIs that see moves as moves: - * Find “the same” node in another revision. This query can be shaped in various ways, such as: + * Find “the same” node in another revision. This query can be shaped in various ways, such as: - - * For a given set of nodes in revision X, find where the “same” nodes exist in revision Y. + * For a given set of nodes in revision X, find where the “same” nodes exist in revision Y. - * Compare directories PATH1@REV2 and PATH2@REV2, and return a list of matching name-pairs between them. + * Compare directories PATH1@REV2 and PATH2@REV2, and return a list of matching name-pairs between them. TODO... @@ -130, +121 @@ === Delta-Editor === Options: - * Transmit moves transparently over the old svn_delta_editor_t. A move-aware producer will drive the existing editor interface in a way that is (more or less) backward compatible with existing consumers. + * Transmit moves transparently over the old svn_delta_editor_t. A move-aware producer will drive the existing editor interface in a way that is (more or less) backward compatible with existing consumers. - * Use Ev2. Supposed to have support for moves. Untested and unknown. + * Use Ev2. Supposed to have support for moves. Untested and unknown. It may be better to start by adding support to a well known editor first, if that is possible (which it seems to be). Then see if there are any functionality or efficiency issues that could be improved by use of Ev2 (or something like it). @@ -184, +175 @@ == Backward Compatibility == We can and will preserve backward compatibility between move-aware clients and move-unaware repositories, and between move-aware repositories and move-unaware clients. There are two complementary parts to this: - * when sending a move to an old client or server, we shall convert to copy + delete; + * when sending a move to an old client or server, we shall convert to copy + delete; - * when receiving copy + delete from an old client or server, we could heuristically convert some cases to a move. + * when receiving copy + delete from an old client or server, we could heuristically convert some cases to a move. === Heuristic Detection of Moves === The server could perform heuristic detection of moves when an old client is committing.
