Make ADD using git context less noisy (`GIT_ADVICE=0` or `-c advice.detachedHead=false`)

[thaJeztah]

Description

I noticed this in CI runs, and initially thought the output originated from git commands executed within a RUN step, but a closer look shows that it's from git commands executed by BuildKit itself;

docker build -<<'EOF'
FROM alpine
ADD --keep-git-dir=true https://github.com/containers/dnsname.git#v1.3.1 .
EOF

docker buildx history logs
#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 124B done
#1 DONE 0.0s

# <cut> ....

#5 [2/2] ADD --keep-git-dir=true https://github.com/containers/dnsname.git#v1.3.1 .
#5 -0.001 512aea91bf1095fbcad0abceec38798df5c40c14	refs/tags/v1.3.1
#5 -0.001 18822f9a4fb35d1349eb256f4cd2bfd372474d84	refs/tags/v1.3.1^{}
#5 0.017 Initialized empty Git repository in /var/lib/desktop-containerd/daemon/io.containerd.snapshotter.v1.overlayfs/snapshots/40167/fs/
#5 1.656 From https://github.com/containers/dnsname
#5 1.656  * [new tag]         v1.3.1     -> v1.3.1
#5 1.658 512aea91bf1095fbcad0abceec38798df5c40c14
#5 1.667 Initialized empty Git repository in /var/lib/desktop-containerd/daemon/io.containerd.snapshotter.v1.overlayfs/snapshots/40168/fs/.git/
#5 1.668 tag
#5 1.723 From file:///var/lib/desktop-containerd/daemon/io.containerd.snapshotter.v1.overlayfs/snapshots/40167/fs
#5 1.723  * [new tag]         v1.3.1     -> v1.3.1
#5 1.758 Note: switching to 'FETCH_HEAD'.
#5 1.758
#5 1.758 You are in 'detached HEAD' state. You can look around, make experimental
#5 1.758 changes and commit them, and you can discard any commits you make in this
#5 1.758 state without impacting any branches by switching back to a branch.
#5 1.758
#5 1.758 If you want to create a new branch to retain commits you create, you may
#5 1.758 do so (now or later) by using -c with the switch command. Example:
#5 1.758
#5 1.758   git switch -c <new-branch-name>
#5 1.758
#5 1.758 Or undo this operation with:
#5 1.758
#5 1.758   git switch -
#5 1.758
#5 1.758 Turn off this advice by setting config variable advice.detachedHead to false
#5 1.758
#5 1.758 HEAD is now at 18822f9 Bump to v1.3.1
#5 DONE 2.5s

#6 [2/2] ADD --keep-git-dir=true https://github.com/containers/dnsname.git#v1.3.1 .
#6 DONE 0.1s

# <cut> ....

We can probably make the output less noisy; from a quick search, git allows setting these configs per invocation, so no need to persist config on disk.

git -c advice.detachedHead=false checkout FETCH_HEAD

or

git -c advice.detachedHead=false switch --detach FETCH_HEAD

There's apparently also a global --no-advice option and corresponding GIT_ADVICE=0 env-var to hide all "advice" output; https://git-scm.com/docs/git#Documentation/git.txt-GITADVICE

GIT_ADVICE

If set to 0, then disable all advice messages. These messages are intended to provide hints to human users that may help them get out of problematic situations or take advantage of new features. Users can disable individual messages using the advice.* config keys. These messages may be disruptive to tools that execute Git processes, so this variable is available to disable the messages. (The --no-advice global option is also available, but old Git versions may fail when this option is not understood. The environment variable will be ignored by Git versions that do not understand it.)

The --no-advice was introduced in Git 2.46.0, which is still relatively recent, so perhaps the GIT_ADVICE=0 would be a safer option to use.

For a list of advices; https://git-scm.com/docs/git-config/2.51.1#Documentation/git-config.txt-advice

Proposal

I think in most cases these messages are just noise, BUT maybe some advices could be useful for debugging, but given that many of these won't be actionable, it may not be useful to most users; as the docs mentions:

intended to provide hints to human users that may help them get out of problematic situations or take advantage of new features

• -c advice.detachedHead=false should be a safe option to set; I don't think that advice provides much value
• We can probably set GIT_ADVICE=0 by default; possibly with debug enabled or similar, we could omit it?
• If we want to apply granular, we should evaluate https://git-scm.com/docs/git-config/2.51.1#Documentation/git-config.txt-advice and create a curated list of advices that are not useful, even for debugging.

[thaJeztah]

Quick generated table of options available for git 2.51.1; https://git-scm.com/docs/git-config/2.51.1#Documentation/git-config.txt-advice

Advice	What it indicates	BuildKit relevance	Proposed policy
addEmbeddedRepo	Adding a Git repo inside another	Low	Suppress
addEmptyPathspec	git add without pathspec	None	Suppress
addIgnoredFile	Adding ignored file	Low	Suppress
amWorkDir	git am patch failure location	None	Suppress
ambiguousFetchRefspec	Fetch refspec ambiguity causing tracking setup failure	Medium	Keep in debug only
checkoutAmbiguousRemoteBranchName	Ambiguous remote branch checkout	Medium	Keep in debug only
commitBeforeMerge	Merge refused due to local changes	None/Low	Suppress
detachedHead	Checkout/switch detached HEAD	Low	Suppress always
diverging	Fast-forward not possible	Medium	Keep in debug only
fetchShowForcedUpdates	Forced-update calculation slow/disabled	Low	Suppress by default
forceDeleteBranch	Deleting unmerged branch without force	None	Suppress
ignoredHook	Hook ignored because not executable	Low	Suppress by default
implicitIdentity	Git guessed user identity	Low	Suppress
mergeConflict	Command stopped because of conflicts	Medium	Keep in debug only
nestedTag	Recursive tag of tag object	Low	Suppress
pushAlreadyExists	Push rejected, non-FF/tag exists	None	Suppress
pushFetchFirst	Push rejected, missing remote object locally	None	Suppress
pushNeedsForce	Push rejected; would need force	None	Suppress
pushNonFFCurrent	Push non-fast-forward current branch	None	Suppress
pushNonFFMatching	Push non-fast-forward matching refs	None	Suppress
pushRefNeedsUpdate	Forced push rejected due to stale tracking ref	None	Suppress
pushUnqualifiedRefname	Git cannot infer push destination namespace	None	Suppress
pushUpdateRejected	Umbrella for several push rejection advices	None	Suppress
rebaseTodoError	Rebase todo edit problem	None	Suppress
refSyntax	Illegal ref name	Medium	Keep in debug only
resetNoRefresh	Reset refresh took >2s	Low	Suppress
resolveConflict	Conflict prevents operation	Medium	Keep in debug only
rmHints	git rm failure hints	None	Suppress
sequencerInUse	Sequencer operation already active	None	Suppress
skippedCherryPicks	Rebase skipped duplicate commits	None	Suppress
sparseIndexExpanded	Sparse index expanded unexpectedly	Low/Medium	Keep in debug only
statusAheadBehind	Ahead/behind calculation slow	None	Suppress
statusHints	Human status/commit/switch directions	Low	Suppress
statusUoption	Untracked enumeration slow	None	Suppress
submoduleAlternateErrorStrategyDie	Submodule alternate error strategy fatal	Medium	Keep in debug only
submoduleMergeConflict	Non-trivial submodule merge conflict	Medium	Keep in debug only
submodulesNotUpdated	Submodule update not run	Medium	Keep in debug only
suggestDetachingHead	Switch refused detach without --detach	Low	Suppress
updateSparsePath	Sparse checkout path update issue	Medium	Keep in debug only
waitingForEditor	Git waiting for editor	None	Suppress
worktreeAddOrphan	Invalid worktree ref; suggests unborn branch	None	Suppress