To-and-Fro Merging

To-and-Fro MergingJulian FoadWANdisco, plc

Contents➔What's wrong with sync and reintegrate?Why symmetric?Continue after reintegrateHow do sync and reintegrate work?SurpriseImplementationLimitationsResults & Plans

30merging with v1.7

Graph notationPPQPQRPQRSTediteditreintegratePSPQSPQSTPQRSTcp + editsynceditsync

40symmetric merging

furtherdevelopment

ContentsWhat's wrong with sync and reintegrate?➔ Why symmetric?Continue after reintegrateHow do sync and reintegrate work?ImplementationLimitationsResults

46 – Any common ancestor

ContentsWhat's wrong with sync and reintegrate?Why symmetric?➔ Continue after reintegrateHow do sync and reintegrate work?ImplementationResults

55.1

55.2

We can use the same merge command

We can continue after reintegrate

ContentsWhat's wrong with sync and reintegrate?➔How do sync and reintegrate work?ImplementationResults

Graph notation

Sync- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil

Sync- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil- eligible changes: A1, A2- 3-way merge,base: pred(A1)right: A2- record mergeinfo: “A:1-2”

Sync- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil- eligible changes: A1, A2- 3-way merge,base: pred(A1)right: A2- record mergeinfo: “A:1-2”- Youngest Common Ancestor: O- all source changes: A1, A2, A3, A4- target's mergeinfo about source: “A:1-2”

Sync- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil- eligible changes: A1, A2- 3-way merge,base: pred(A1)right: A2- record mergeinfo: “A:1-2”- Youngest Common Ancestor: O- all source changes: A1, A2, A3, A4- target's mergeinfo about source: “A:1-2”- eligible changes: A3, A4- 3-way merge,base: pred(A3)right: A4- record mergeinfo: “A:3-4”

Reintegrate- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil- eligible changes: A1, A2- 3-way merge,base: pred(A1)right: A2- record mergeinfo: “A:1-2”- Youngest Common Ancestor: O- all target changes: A1, A2, A3, A4- source's mergeinfo about target: “A:1-2”- last target change merged to source: A2

Reintegrate- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil- eligible changes: A1, A2- 3-way merge,base: pred(A1)right: A2- record mergeinfo: “A:1-2”- Youngest Common Ancestor: O- all target changes: A1, A2, A3, A4- source's mergeinfo about target: “A:1-2”- last target change merged to source: A2- eligible change: diff(A2, B4)- 3-way merge,base: A2right: B4

Reintegrate- Youngest Common Ancestor: O- all changes on source: A1, A2- target's mergeinfo about source: nil- eligible changes: A1, A2- 3-way merge,base: pred(A1)right: A2- record mergeinfo: “A:1-2”- Youngest Common Ancestor: O- all target changes: A1, A2, A3, A4- source's mergeinfo about target: “A:1-2”- last target change merged to source: A2- eligible change: diff(A2, B4)- 3-way merge,base: A2right: B4- record mergeinfo: “B:1-4”

DifferencesBase node issync: on source branchreintegrate: on target branchSync:can skip cherry-picked revscan fill in subtree mergescan handle local mods in the WCReintegrate:can't

Surprisesvn 1.7 merges just work

Surprisesvn 1.7 merges just work

ContentsWhat's wrong with sync and reintegrate?How do sync and reintegrate work?➔ImplementationLimitationsResults & Plans

Symmetric algorithmFind the best base●Find the latest rev of A synced to B and of B synced to A.●Choose the more recent base.Then, ideally...●Identify cherry-picks.●Break into 3-way merges, skipping the cherry-picks.●Perform the 3-way merges and mergeinfo addition.but currently...– Run “sync” if base on source– Run “reintegrate” if base on target

Symmetric algorithmFind the best base●Find the latest rev of A synced to B and of B synced to A.* Find 'BASE', such that* all changes on SOURCE_BRANCH after YCA up to and including BASE* are recorded in TARGET_MERGEINFO.** BASE* o­­­­­­­o­­­­­­­­­­­­­­­ SOURCE_BRANCH* / \* ­­­­­o prev. \* YCA \ merges \* o­­­­­­­­­­­o­­­­­­­­­­­

Symmetric algorithmFind the best base●Find the latest rev of A synced to B and of B synced to A.* In terms of mergeinfo:** Source a­­... o=change, ­=no­op revision* branch / \* YCA ­­> o a­­­o­­­o­­­o­­­o­­­ d=delete, a=add­as­a­copy** Eligible ­.eee.eeeeeeeeeeeeeeeeeeee .=not a source branch location** Tgt­mi ­.mmm.mmmmm­­­­­­­m­­­­­­­ m=merged, ­=not merged** Eligible ­.­­­.­­­­­eeeeeee­eeeeeee** Next ­­­­­­­­­­­^­­­­­­­­­­­­­­ BASE is just before here.

Symmetric algorithmLimitations●●Merge code is not symmetric insideOnly same-direction-again merges can handle...● cherry-picks, subtrees, local mods, sparse WC, ...

A foot-noteMore stable branchLess stable branch

Results

Results+ svn merge ­­reintegrate '^/B' A­­­ Merging differences between repository URLsinto 'A':AA/pickle­­­ Recording mergeinfo for merge betweenrepository URLs into 'A':UA+ svn ci ­m ''SendingAddingAA/pickleCommitted revision 6.+ svn up ­q+ svn merge '^/B' A­­­ Merging differences between repository URLsinto 'A':AA/pickle­­­ Recording mergeinfo for merge betweenrepository URLs into 'A':UA+ svn ci ­m ''SendingAddingAA/pickleCommitted revision 6.+ svn up ­q+ svn merge '^/A' B­­­ Merging r5 through r6 into 'B':C B/pickle­­­ Recording mergeinfo for merge of r5 throughr6 into 'B':UBSummary of conflicts:Tree conflicts: 1+ svn merge '^/A' B­­­ Recording mergeinfo for merge betweenrepository URLs into 'B':UB

ResultsIn the current state:✔✔Never need to specify “--reintegrate” optionKeep using a dev branch after reintegrating it

PlansBefore releasable:●●●●Support sub-tree mergeinfo●to the extent that 1.7 doesRemove “--symmetric” optionInvoke symmetric code automaticallyDeprecate “--reintegrate” option

PlansOther:●●●Rewrite merge code to be symmetric inside?●not for user benefit, but for developmentUsability tweaks●error if target unrelated to sourceTackle renames