#+TITLE: Git stash, merge, rebase & co. #+STARTUP: overview hidestars inlineimages * discarding or stashing away local changes ** =git reset --hard= to discard uncommitted changes ** =git stash= to stash away uncommitted changes Run =git stash pop= to reinstate those changes in the working tree, uncommitted. * dealing with merge conflicts :PROPERTIES: :header-args:sh: :session *sh-for-merge-conflicts* :results output :END: ** let’s create a conflict! *** first, create our branch and commit something #+BEGIN_SRC sh cd /tmp git clone https://gitlab.inria.fr/guix-hpc/guix-hpc cd guix-hpc git checkout -b mes-trucs 16bf282dd3e53ca965921381a8a55f18f78970fb #+END_SRC #+RESULTS: #+begin_example Cloning into 'guix-hpc'... warning: redirecting to https://gitlab.inria.fr/guix-hpc/guix-hpc.git/ remote: Enumerating objects: 172, done. (1/172) remote: Counting objects: 1% (2/172) remote: Counting objects: 2% (4/172) remote: Counting objects: 3% (6/172) remote: Counting objects: 4% (7/172) remote: Counting objects: 5% (9/172) remote: Counting objects: 6% (11/172) remote: Counting objects: 7% (13/172) remote: Counting objects: 8% (14/172) remote: Counting objects: 9% (16/172) remote: Counting objects: 10% (18/172) remote: Counting objects: 11% (19/172) remote: Counting objects: 12% (21/172) remote: Counting objects: 13% (23/172) remote: Counting objects: 14% (25/172) remote: Counting objects: 15% (26/172) remote: Counting objects: 16% (28/172) remote: Counting objects: 17% (30/172) remote: Counting objects: 18% (31/172) remote: Counting objects: 19% (33/172) remote: Counting objects: 20% (35/172) remote: Counting objects: 21% (37/172) remote: Counting objects: 22% (38/172) remote: Counting objects: 23% (40/172) remote: Counting objects: 24% (42/172) remote: Counting objects: 25% (43/172) remote: Counting objects: 26% (45/172) remote: Counting objects: 27% (47/172) remote: Counting objects: 28% (49/172) remote: Counting objects: 29% (50/172) remote: Counting objects: 30% (52/172) remote: Counting objects: 31% (54/172) remote: Counting objects: 32% (56/172) remote: Counting objects: 33% (57/172) remote: Counting objects: 34% (59/172) remote: Counting objects: 35% (61/172) remote: Counting objects: 36% (62/172) remote: Counting objects: 37% (64/172) remote: Counting objects: 38% (66/172) remote: Counting objects: 39% (68/172) remote: Counting objects: 40% (69/172) remote: Counting objects: 41% (71/172) remote: Counting objects: 42% (73/172) remote: Counting objects: 43% (74/172) remote: Counting objects: 44% (76/172) remote: Counting objects: 45% (78/172) remote: Counting objects: 46% (80/172) remote: Counting objects: 47% (81/172) remote: Counting objects: 48% (83/172) remote: Counting objects: 49% (85/172) remote: Counting objects: 50% (86/172) remote: Counting objects: 51% (88/172) remote: Counting objects: 52% (90/172) remote: Counting objects: 53% (92/172) remote: Counting objects: 54% (93/172) remote: Counting objects: 55% (95/172) remote: Counting objects: 56% (97/172) remote: Counting objects: 57% (99/172) remote: Counting objects: 58% (100/172) remote: Counting objects: 59% (102/172) remote: Counting objects: 60% (104/172) remote: Counting objects: 61% (105/172) remote: Counting objects: 62% (107/172) remote: Counting objects: 63% (109/172) remote: Counting objects: 64% (111/172) remote: Counting objects: 65% (112/172) remote: Counting objects: 66% (114/172) remote: Counting objects: 67% (116/172) remote: Counting objects: 68% (117/172) remote: Counting objects: 69% (119/172) remote: Counting objects: 70% (121/172) remote: Counting objects: 71% (123/172) remote: Counting objects: 72% (124/172) remote: Counting objects: 73% (126/172) remote: Counting objects: 74% (128/172) remote: Counting objects: 75% (129/172) remote: Counting objects: 76% (131/172) remote: Counting objects: 77% (133/172) remote: Counting objects: 78% (135/172) remote: Counting objects: 79% (136/172) remote: Counting objects: 80% (138/172) remote: Counting objects: 81% (140/172) remote: Counting objects: 82% (142/172) remote: Counting objects: 83% (143/172) remote: Counting objects: 84% (145/172) remote: Counting objects: 85% (147/172) remote: Counting objects: 86% (148/172) remote: Counting objects: 87% (150/172) remote: Counting objects: 88% (152/172) remote: Counting objects: 89% (154/172) remote: Counting objects: 90% (155/172) remote: Counting objects: 91% (157/172) remote: Counting objects: 92% (159/172) remote: Counting objects: 93% (160/172) remote: Counting objects: 94% (162/172) remote: Counting objects: 95% (164/172) remote: Counting objects: 96% (166/172) remote: Counting objects: 97% (167/172) remote: Counting objects: 98% (169/172) remote: Counting objects: 99% (171/172) remote: Counting objects: 100% (172/172) remote: Counting objects: 100% (172/172), done. (1/72) remote: Compressing objects: 2% (2/72) remote: Compressing objects: 4% (3/72) remote: Compressing objects: 5% (4/72) remote: Compressing objects: 6% (5/72) remote: Compressing objects: 8% (6/72) remote: Compressing objects: 9% (7/72) remote: Compressing objects: 11% (8/72) remote: Compressing objects: 12% (9/72) remote: Compressing objects: 13% (10/72) remote: Compressing objects: 15% (11/72) remote: Compressing objects: 16% (12/72) remote: Compressing objects: 18% (13/72) remote: Compressing objects: 19% (14/72) remote: Compressing objects: 20% (15/72) remote: Compressing objects: 22% (16/72) remote: Compressing objects: 23% (17/72) remote: Compressing objects: 25% (18/72) remote: Compressing objects: 26% (19/72) remote: Compressing objects: 27% (20/72) remote: Compressing objects: 29% (21/72) remote: Compressing objects: 30% (22/72) remote: Compressing objects: 31% (23/72) remote: Compressing objects: 33% (24/72) remote: Compressing objects: 34% (25/72) remote: Compressing objects: 36% (26/72) remote: Compressing objects: 37% (27/72) remote: Compressing objects: 38% (28/72) remote: Compressing objects: 40% (29/72) remote: Compressing objects: 41% (30/72) remote: Compressing objects: 43% (31/72) remote: Compressing objects: 44% (32/72) remote: Compressing objects: 45% (33/72) remote: Compressing objects: 47% (34/72) remote: Compressing objects: 48% (35/72) remote: Compressing objects: 50% (36/72) remote: Compressing objects: 51% (37/72) remote: Compressing objects: 52% (38/72) remote: Compressing objects: 54% (39/72) remote: Compressing objects: 55% (40/72) remote: Compressing objects: 56% (41/72) remote: Compressing objects: 58% (42/72) remote: Compressing objects: 59% (43/72) remote: Compressing objects: 61% (44/72) remote: Compressing objects: 62% (45/72) remote: Compressing objects: 63% (46/72) remote: Compressing objects: 65% (47/72) remote: Compressing objects: 66% (48/72) remote: Compressing objects: 68% (49/72) remote: Compressing objects: 69% (50/72) remote: Compressing objects: 70% (51/72) remote: Compressing objects: 72% (52/72) remote: Compressing objects: 73% (53/72) remote: Compressing objects: 75% (54/72) remote: Compressing objects: 76% (55/72) remote: Compressing objects: 77% (56/72) remote: Compressing objects: 79% (57/72) remote: Compressing objects: 80% (58/72) remote: Compressing objects: 81% (59/72) remote: Compressing objects: 83% (60/72) remote: Compressing objects: 84% (61/72) remote: Compressing objects: 86% (62/72) remote: Compressing objects: 87% (63/72) remote: Compressing objects: 88% (64/72) remote: Compressing objects: 90% (65/72) remote: Compressing objects: 91% (66/72) remote: Compressing objects: 93% (67/72) remote: Compressing objects: 94% (68/72) remote: Compressing objects: 95% (69/72) remote: Compressing objects: 97% (70/72) remote: Compressing objects: 98% (71/72) remote: Compressing objects: 100% (72/72) remote: Compressing objects: 100% (72/72), done. (1/172) Receiving objects: 1% (2/172) Receiving objects: 2% (4/172) Receiving objects: 3% (6/172) Receiving objects: 4% (7/172) Receiving objects: 5% (9/172) Receiving objects: 6% (11/172) Receiving objects: 7% (13/172) Receiving objects: 8% (14/172) Receiving objects: 9% (16/172) Receiving objects: 10% (18/172) Receiving objects: 11% (19/172) Receiving objects: 12% (21/172) Receiving objects: 13% (23/172) Receiving objects: 14% (25/172) Receiving objects: 15% (26/172) Receiving objects: 16% (28/172) Receiving objects: 17% (30/172) Receiving objects: 18% (31/172) Receiving objects: 19% (33/172) Receiving objects: 20% (35/172) Receiving objects: 21% (37/172) Receiving objects: 22% (38/172) Receiving objects: 23% (40/172) Receiving objects: 24% (42/172) remote: Total 172 (delta 87), reused 142 (delta 71) (43/172) Receiving objects: 26% (45/172) Receiving objects: 27% (47/172) Receiving objects: 28% (49/172) Receiving objects: 29% (50/172) Receiving objects: 30% (52/172) Receiving objects: 31% (54/172) Receiving objects: 32% (56/172) Receiving objects: 33% (57/172) Receiving objects: 34% (59/172) Receiving objects: 35% (61/172) Receiving objects: 36% (62/172) Receiving objects: 37% (64/172) Receiving objects: 38% (66/172) Receiving objects: 39% (68/172) Receiving objects: 40% (69/172) Receiving objects: 41% (71/172) Receiving objects: 42% (73/172) Receiving objects: 43% (74/172) Receiving objects: 44% (76/172) Receiving objects: 45% (78/172) Receiving objects: 46% (80/172) Receiving objects: 47% (81/172) Receiving objects: 48% (83/172) Receiving objects: 49% (85/172) Receiving objects: 50% (86/172) Receiving objects: 51% (88/172) Receiving objects: 52% (90/172) Receiving objects: 53% (92/172) Receiving objects: 54% (93/172) Receiving objects: 55% (95/172) Receiving objects: 56% (97/172) Receiving objects: 57% (99/172) Receiving objects: 58% (100/172) Receiving objects: 59% (102/172) Receiving objects: 60% (104/172) Receiving objects: 61% (105/172) Receiving objects: 62% (107/172) Receiving objects: 63% (109/172) Receiving objects: 64% (111/172) Receiving objects: 65% (112/172) Receiving objects: 66% (114/172) Receiving objects: 67% (116/172) Receiving objects: 68% (117/172) Receiving objects: 69% (119/172) Receiving objects: 70% (121/172) Receiving objects: 71% (123/172) Receiving objects: 72% (124/172) Receiving objects: 73% (126/172) Receiving objects: 74% (128/172) Receiving objects: 75% (129/172) Receiving objects: 76% (131/172) Receiving objects: 77% (133/172) Receiving objects: 78% (135/172) Receiving objects: 79% (136/172) Receiving objects: 80% (138/172) Receiving objects: 81% (140/172) Receiving objects: 82% (142/172) Receiving objects: 83% (143/172) Receiving objects: 84% (145/172) Receiving objects: 85% (147/172) Receiving objects: 86% (148/172) Receiving objects: 87% (150/172) Receiving objects: 88% (152/172) Receiving objects: 89% (154/172) Receiving objects: 90% (155/172) Receiving objects: 91% (157/172) Receiving objects: 92% (159/172) Receiving objects: 93% (160/172) Receiving objects: 94% (162/172) Receiving objects: 95% (164/172) Receiving objects: 96% (166/172) Receiving objects: 97% (167/172) Receiving objects: 98% (169/172) Receiving objects: 99% (171/172) Receiving objects: 100% (172/172) Receiving objects: 100% (172/172), 37.29 KiB | 2.87 MiB/s, done. (0/87) Resolving deltas: 2% (2/87) Resolving deltas: 3% (3/87) Resolving deltas: 24% (21/87) Resolving deltas: 27% (24/87) Resolving deltas: 48% (42/87) Resolving deltas: 63% (55/87) Resolving deltas: 68% (60/87) Resolving deltas: 70% (61/87) Resolving deltas: 71% (62/87) Resolving deltas: 80% (70/87) Resolving deltas: 81% (71/87) Resolving deltas: 86% (75/87) Resolving deltas: 100% (87/87) Resolving deltas: 100% (87/87), done. Switched to a new branch 'mes-trucs' #+end_example *** second, commit something **** a non-conflicting change #+BEGIN_SRC sh pwd sed -i -es/multicore/manymanycore/g inria/storm.scm git add inria/storm.scm git commit -m 'Exascale, yay!' #+END_SRC #+RESULTS: : /tmp/guix-hpc : sh-4.4$ sh-4.4$ [mes-trucs 96c852d] Exascale, yay! : 1 file changed, 1 insertion(+), 1 deletion(-) **** and now a conflicting change #+BEGIN_SRC sh sed -i -e's/1.2.4/42.3/g' inria/storm.scm git add inria/storm.scm git commit -m 'Update StarPU to 42.3, yay!' #+END_SRC #+RESULTS: : : sh-4.4$ [mes-trucs 55a2d56] Update StarPU to 42.3, yay! : 1 file changed, 1 insertion(+), 1 deletion(-) *** second, try to merge =master=, fasten your seat belts! #+BEGIN_SRC sh git branch git merge master #+END_SRC #+RESULTS: : master : * mes-trucs : Auto-merging inria/storm.scm : CONFLICT (content): Merge conflict in inria/storm.scm : Automatic merge failed; fix conflicts and then commit the result. Oh la la! *** what’s the status? #+BEGIN_SRC sh git status #+END_SRC #+BEGIN_SRC emacs-lisp results: none (magit-status "/tmp/guix-hpc") #+END_SRC #+RESULTS: *** let’s edit [[/tmp/guix-hpc/inria/storm.scm][the conflict]] *** once you’re done #+BEGIN_SRC sh git add inria/storm.scm # add change to the index git merge --continue # resume merge operation #+END_SRC *** view the result * context: code review/development iteration ** the process: (1) submit, (2) review, (3) goto 1 ** iteration history *** squash intermediate commits because they don’t matter *** adjust commits and commit logs according to review * keeping the history linear ** what ‘git merge’ and ‘git pull’ (and ‘git push’ sometimes) do *** creates a “merge commit” see the awesome [[http://eagain.net/articles/git-for-computer-scientists/][Git for Computer Scientists]] [[./git-merge-commit.svg]] *** show example of linear and non-linear history - linear: [[https://github.com/open-mpi/hwloc/commits/master][hwloc]], [[https://git.savannah.gnu.org/cgit/guix.git/log][Guix]] - non-linear: [[https://gitlab.inria.fr/solverstack/ScalFMM][ScalFMM]], [[https://github.com/libgit2/libgit2/][libgit2]], [[https://github.com/NixOS/nixpkgs/][NixOS]] ** why should i care? *** helps understand code evolution *** helps bisection (‘git bisect’) *** possibly helps continuous integration ** visualizing the history - using =gitk= (which is part of Git) - using Magit (in Emacs) - using a Web interface (GitLab, cgit, etc.) ** pull/merge requests *** by default, they introduce merge commits! ** note: sometimes you do need parallel dev branches, that’s fine! * rebasing ** the Git commit graph Do not miss [[http://eagain.net/articles/git-for-computer-scientists/][Git for Computer Scientists]]! [[./git-rebase.svg]] ** rebasing your changes before pushing For example, assuming you are working on the ‘master’ branch and have a bunch of local commits ready to be pushed, you would run these commands: #+BEGIN_SRC sh $ git pull --rebase # fetch remote changes, and rebase on top of them $ git push #+END_SRC Sometimes ‘git pull --rebase’ will result in conflicts that need to be resolved. Assuming ‘foo.cpp’ has a conflict, you would edit it to fix those conflicts (look for “>>>”), and then: #+BEGIN_SRC sh $ git add foo.cpp $ git rebase --continue #+END_SRC ** make ‘git pull’ rebase by default #+BEGIN_SRC sh git config branch.master.rebase true #+END_SRC From now on ‘git pull’ will always rebase. \o/ ** tell GitLab to rebase when you accept merge requests [[./gitlab-merge-request-rebase.png]] * rebase, “history rewriting” ** Caveat: don’t rebase a published branch! ** tools **** [[http://github.com/magit/magit/][Magit]] (Emacs) [[./magit.png]] **** [[https://github.com/tpope/vim-fugitive][Fugitive]] (Vim) **** command-line: ‘git rebase -i’ etc. ** amending the last commit of a branch #+BEGIN_SRC sh git add foo # stage changes git commit --amend # amend the last commit on the branch #+END_SRC ** rebasing a branch on top of another one This will simplify the reviewers life by make sure the changes apply on the latest ‘master’, for example. #+BEGIN_SRC sh git checkout master git pull # update my copy of 'master' git checkout my-feature-branch git rebase master # rebase 'my-feature-branch' on 'master' #+END_SRC ** squashing, dropping, reordering, rewriting commits All with ‘git rebase -i’ (or similar in your favorite UI). Glossary: - squashing two or more commits: turning them into a single commit - dropping: get rid of commits you don’t like, after all - reodering: you can change the order of commits in the history - rewriting: change the commit log and/or what the commit does ** pro tip: autosquash (thanks bgoglin!) - Write “squash! the summary line” in your commit log and then ‘git rebase n--autosquash’ will automatically flag it as candidate for squashing with the commit that has “the summary line”. See git-rebase(1). Also works with “fixup!” (difference is that the fixup commit log is discarded.) - =git config branch.THE-BRANCH.autoSquash true= - Actually, ‘git commit --fixup ID’ creates the right summary line!