What happen to Git tags pointing to a removed commit

Question

Say I do the following:

1. Create branch X
2. Create Tag t (to branch X)
3. Push
4. Remove branch X

What happen to tag t? is it just floating there? is it considered as garbage?

Should I remove all tags pointing at branch before removing the branch itself?

Reference

From Git Basics - Tagging:

Git uses two main types of tags: lightweight and annotated. A lightweight tag is very much like a branch that doesn’t change – it’s just a pointer to a specific commit.

Answer 1

What happen to tag t?

Let's say you created branch x from a commit E and then tagged that commit with tag t. E.g.

                           x (branch)
                           |
                           V             
A-----B------C------D------E
                           ^
                           |
                           t (tag)

If you remove branch x nothing happens to tag t.

git branch -D x

The tag still points to commit E.

A-----B------C------D------E
                           ^
                           |
                           t (tag)

is it considered as garbage?

No, because the commit is still referenced by tag t.

What if the commit is removed?

You do not remove commits. You remove pointers to commits and if commits are no longer referenced git will garbage collect them some day (depending on your configuration).

See git gc

Even if you removed all ordinary refs, like branches and tags, the commits will still be referenced in the reflog for some time and you can access them, e.g. re-create a branch, tag them or cherry-pick and so on.

You can see the reflog using git reflog. Also take a look at gc.reflogExpireUnreachable and gc.reflogExpire

---

EDIT

If somehow git's object database is corrupted. Either a file from .git/objects was deleted (e.g. you accidentially deleted it using your file explorer or a command-line command) or a ref points to a non-existent git object (like a commit, tree or blob object), you will get errors if git tries to access these objects.

Here is a list of errors that might occur when git tries to access an object that does not exist or if a non-existent object is referenced.

• commit

  fatal: Could not parse object '<ref-name>'.
  

  example:

  fatal: Could not parse object 'master'.
  

• tree

  fatal: unable to read tree <tree-sha1>
  

  example:

  fatal: unable to read tree 13a3e0908e4f6fc7526056377673a5987e753fc8
  

• blob

  error: unable to read sha1 file of <blob-name> (<blob-sha1>)
  

  example:

  error: unable to read sha1 file of test.txt (e69de29bb2d1d6434b8b29ae775ad8c2e48c5391)
  

Take a look at Git Internals for a deeper understanding.