notes.md

Notes

Git and Horrible Terrible No Good Very Bad Day

Git Bisect

git bisect to find bad commits

git bisect start
git bisect good <good-commit-hash>
git bisect bad <bad-commit-hash>  # usually HEAD

# While git iterates through commits, you should mark commits good or bad
git bisect good # or git bisect bad

# Do not forget to cleanup and return to where you started
git reset

Git Revert

git revert to safely undo changes

git revert will create a new commit and will not affect repo history

Git Reset

You can use git reset to safely move commits to other branch

# You accidentally comiited to master instead of feature-branch
git branch feature-branch
git reset HEAD~1  # or to commit which should be latest one on master

Git Reflog

The whole history of HEAD movements can be obtained from git reflog

You can reset to commit from reflog or even cherry-pick one from it

Git Filter-Branch

The common usage of it is to remove an accidentally committed file from the history completely

Difference between .. and ...

• using .. will show the diff between commits
• using ... will show the difference at the merge base

Specify the specific parent

If HEAD has several parents you can use HEAD^2 to specify second parent or HEAD^1 to specify the first one

Jedi Mind Tricks for Git

Working with hooks

• pre-commit

  • first thing executed after git commit
  • can abandon commit
  • no arguments

  IDEA: abandon commits on the master branch

  #!/bin/bash
  # Check where the HEAD is
  if [ "$(git symbolic-ref --short HEAD)" == "master" ]; then
      echo "This is not the branch you are looking for"
      exit 1
  fi
  exit 0

• commit-msg

  • executes before showing commit message editor
  • accepts one argument -- temporary file

  IDEA reference issue from the commit-message

Branch-based delivery

• work on feature branch (prevent work on master)
• reference issues
• run CI setup
• deliver through "ready" branches
• deploy if CI succeeds

Handling Binary

Working with binaries in git is annoying. No useful messages, nothing

• git attributes with them you can assign behaviour per file or path
• custom drivers specify the behaviour scenario

Git Attributes

Format: pattern attribute1 attribute2 ...

Git Attributes are commonly used for:

• changing CRLF behaviour
• text/binary behaviour

In order to handle different file types you can create custom drivers

Scaling Git at Microsoft

Preamble

• Windows repo is huge
• Splitting repo is no choice, thus submodules and subtrees won't work here
• IDEA virtual zing FS underneath Git

GVFS

• File system driver
• read object hook
• protocol to request a single blob/object was developed. This allows to download files on demand
• Cache server

The whole idea is to download only files that user need, and thus do not fetch a lot of unnecessary files.

Scaling Mercurial at Facebook

Current state

• use monorepos
• not using feature branches
• rebasing instead of merging
• single commit per push
• team is mostly online
• use bookmarks instead of branches

Problems

• size of .hg directory
• push rate is huge
• large files in repo

Toolbelt

• Scale

  • pushrebase extension
  • remote file log
  • infinite push redirects branch push to blobstore. This allows unlimited branches
    • push every commit
    • sync between machines

• Better UX for hg log (smartlog)

  Something similar to git log --oneline --graph

• Hg absorb

  Identifies where the pending changes are

  (something like git commit -p)

  • hg fold
  • hg split

• Sparse checkouts

External links

• Github's recap
• Tim Petterson's recap
• Git rev
• SSI: @rgaiacs writes in detail about Git Merge 2017 organised by @github
• Edward Thomson: The Git Contributor Summit
• Carmen Bremen: Review – Git Merge 2017