Documentation/howto/keep-canonical-history-correct.txt

   1 From: Junio C Hamano <gitster@pobox.com>
   2 Date: Wed, 07 May 2014 13:15:39 -0700
   3 Subject: Beginner question on "Pull is mostly evil"
   4 Abstract: This how-to explains a method for keeping a
   5  project's history correct when using git pull.
   6 Content-type: text/asciidoc
   7
   8 Keep authoritative canonical history correct with git pull
   9 ==========================================================
  10
  11 Sometimes a new project integrator will end up with project history
  12 that appears to be "backwards" from what other project developers
  13 expect. This howto presents a suggested integration workflow for
  14 maintaining a central repository.
  15
  16 Suppose that that central repository has this history:
  17
  18 ------------
  19     ---o---o---A
  20 ------------
  21
  22 which ends at commit `A` (time flows from left to right and each node
  23 in the graph is a commit, lines between them indicating parent-child
  24 relationship).
  25
  26 Then you clone it and work on your own commits, which leads you to
  27 have this history in *your* repository:
  28
  29 ------------
  30     ---o---o---A---B---C
  31 ------------
  32
  33 Imagine your coworker did the same and built on top of `A` in *his*
  34 repository in the meantime, and then pushed it to the
  35 central repository:
  36
  37 ------------
  38     ---o---o---A---X---Y---Z
  39 ------------
  40
  41 Now, if you `git push` at this point, because your history that leads
  42 to `C` lacks `X`, `Y` and `Z`, it will fail.  You need to somehow make
  43 the tip of your history a descendant of `Z`.
  44
  45 One suggested way to solve the problem is "fetch and then merge", aka
  46 `git pull`. When you fetch, your repository will have a history like
  47 this:
  48
  49 ------------
  50     ---o---o---A---B---C
  51                 \
  52                  X---Y---Z
  53 ------------
  54
  55 Once you run merge after that, while still on *your* branch, i.e. `C`,
  56 you will create a merge `M` and make the history look like this:
  57
  58 ------------
  59     ---o---o---A---B---C---M
  60                 \         /
  61                  X---Y---Z
  62 ------------
  63
  64 `M` is a descendant of `Z`, so you can push to update the central
  65 repository.  Such a merge `M` does not lose any commit in both
  66 histories, so in that sense it may not be wrong, but when people want
  67 to talk about "the authoritative canonical history that is shared
  68 among the project participants", i.e. "the trunk", they often view
  69 it as "commits you see by following the first-parent chain", and use
  70 this command to view it:
  71
  72 ------------
  73     $ git log --first-parent
  74 ------------
  75
  76 For all other people who observed the central repository after your
  77 coworker pushed `Z` but before you pushed `M`, the commit on the trunk
  78 used to be `o-o-A-X-Y-Z`.  But because you made `M` while you were on
  79 `C`, `M`'s first parent is `C`, so by pushing `M` to advance the
  80 central repository, you made `X-Y-Z` a side branch, not on the trunk.
  81
  82 You would rather want to have a history of this shape:
  83
  84 ------------
  85     ---o---o---A---X---Y---Z---M'
  86                 \             /
  87                  B-----------C
  88 ------------
  89
  90 so that in the first-parent chain, it is clear that the project first
  91 did `X` and then `Y` and then `Z` and merged a change that consists of
  92 two commits `B` and `C` that achieves a single goal.  You may have
  93 worked on fixing the bug #12345 with these two patches, and the merge
  94 `M'` with swapped parents can say in its log message "Merge
  95 fix-bug-12345". Having a way to tell `git pull` to create a merge
  96 but record the parents in reverse order may be a way to do so.
  97
  98 Note that I said "achieves a single goal" above, because this is
  99 important.  "Swapping the merge order" only covers a special case
 100 where the project does not care too much about having unrelated
 101 things done on a single merge but cares a lot about first-parent
 102 chain.
 103
 104 There are multiple schools of thought about the "trunk" management.
 105
 106  1. Some projects want to keep a completely linear history without any
 107     merges.  Obviously, swapping the merge order would not match their
 108     taste.  You would need to flatten your history on top of the
 109     updated upstream to result in a history of this shape instead:
 110 +
 111 ------------
 112     ---o---o---A---X---Y---Z---B---C
 113 ------------
 114 +
 115 with `git pull --rebase` or something.
 116
 117  2. Some projects tolerate merges in their history, but do not worry
 118     too much about the first-parent order, and allow fast-forward
 119     merges.  To them, swapping the merge order does not hurt, but
 120     it is unnecessary.
 121
 122  3. Some projects want each commit on the "trunk" to do one single
 123     thing.  The output of `git log --first-parent` in such a project
 124     would show either a merge of a side branch that completes a single
 125     theme, or a single commit that completes a single theme by itself.
 126     If your two commits `B` and `C` (or they may even be two groups of
 127     commits) were solving two independent issues, then the merge `M'`
 128     we made in the earlier example by swapping the merge order is
 129     still not up to the project standard.  It merges two unrelated
 130     efforts `B` and `C` at the same time.
 131
 132 For projects in the last category (Git itself is one of them),
 133 individual developers would want to prepare a history more like
 134 this:
 135
 136 ------------
 137                  C0--C1--C2     topic-c
 138                 /
 139     ---o---o---A                master
 140                 \
 141                  B0--B1--B2     topic-b
 142 ------------
 143
 144 That is, keeping separate topics on separate branches, perhaps like
 145 so:
 146
 147 ------------
 148     $ git clone $URL work && cd work
 149     $ git checkout -b topic-b master
 150     $ ... work to create B0, B1 and B2 to complete one theme
 151     $ git checkout -b topic-c master
 152     $ ... same for the theme of topic-c
 153 ------------
 154
 155 And then
 156
 157 ------------
 158     $ git checkout master
 159     $ git pull --ff-only
 160 ------------
 161
 162 would grab `X`, `Y` and `Z` from the upstream and advance your master
 163 branch:
 164
 165 ------------
 166                  C0--C1--C2     topic-c
 167                 /
 168     ---o---o---A---X---Y---Z    master
 169                 \
 170                  B0--B1--B2     topic-b
 171 ------------
 172
 173 And then you would merge these two branches separately:
 174
 175 ------------
 176     $ git merge topic-b
 177     $ git merge topic-c
 178 ------------
 179
 180 to result in
 181
 182 ------------
 183                  C0--C1---------C2
 184                 /                 \
 185     ---o---o---A---X---Y---Z---M---N
 186                 \             /
 187                  B0--B1-----B2
 188 ------------
 189
 190 and push it back to the central repository.
 191
 192 It is very much possible that while you are merging topic-b and
 193 topic-c, somebody again advanced the history in the central repository
 194 to put `W` on top of `Z`, and make your `git push` fail.
 195
 196 In such a case, you would rewind to discard `M` and `N`, update the
 197 tip of your 'master' again and redo the two merges:
 198
 199 ------------
 200     $ git reset --hard origin/master
 201     $ git pull --ff-only
 202     $ git merge topic-b
 203     $ git merge topic-c
 204 ------------
 205
 206 The procedure will result in a history that looks like this:
 207
 208 ------------
 209                  C0--C1--------------C2
 210                 /                     \
 211     ---o---o---A---X---Y---Z---W---M'--N'
 212                 \                 /
 213                  B0--B1---------B2
 214 ------------
 215
 216 See also http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html