repository corruption when pushing commits to a repository running 'git gc --prune='

[Jan Smets <jan.smets@alcatel-lucent.com>]

Hi

I've recently expired my reflog to prune loose objects. On a live, bare, 
repository I ran 'git gc --prune=now'

All clients ended up having problems, they would report:
  error: refs/heads/master does not point to a valid object!
Running 'git log' on the bare repo gave : fatal: bad object HEAD



So I did a couple of tries reproducing the issue and with this little 
test I got a couple of interesting different/similar outcomes

A: mkdir -p /tmp/test; cd /tmp/test ; git init --bare bare ;
B: cd /tmp/test ; git clone bare 1 ; cd 1;  touch test; git add test; 
while true; do date >> test ; git commit -m "$(date)" test ; git push || 
break; done
A: cd bare; git gc --prune=now  (or run it in a loop)


1)

fatal: bad object 22f0351258fa0bb4cd28984b6473510957fbce69
fatal: bad object 22f0351258fa0bb4cd28984b6473510957fbce69
To /tmp/test/bare
  ! [remote rejected] master -> master (missing necessary objects)

2)

remote: error: unable to write sha1 filename 
objects/05/cdb51bb0ea3e229734a4b1bddd5ec70fbc65ed: No such file or directory
remote: fatal: failed to write object
error: unpack failed: unpack-objects abnormal exit
To /tmp/test/bare
  ! [remote rejected] master -> master (unpacker error)

3)

error: Ref refs/heads/master is at 
e992810e70949e797d33041bf6bc961c9fa4f3e5 but expected 
0000000000000000000000000000000000000000
remote: error: Cannot lock ref 'refs/heads/master':
To /tmp/test/bare
  ! [remote rejected] master -> master (failed to update ref)

4)

remote: error: cannot lock ref 'refs/heads/master': Unable to create 
'/tmp/test/bare/refs/heads/master.lock': File exists.
remote:
remote: If no other git process is currently running, this probably means a
remote: git process crashed in this repository earlier. Make sure no 
other git
remote: process is running and remove the file manually to continue.
To /tmp/test/bare
  ! [remote rejected] master -> master (failed to update ref)
error: failed to push some refs to '/tmp/test/bare'


And eventually running 'git gc --prune=now/all' on the bare repository 
could end with this message:

$ git gc --prune=all
error: refs/heads/master does not point to a valid object!
fatal: bad object refs/heads/master
error: failed to run repack
$ git log -1
fatal: bad object HEAD


This behaviour has been observed with version 2.4.4, 2.5.[12] and 2.6.1

Thank you

- Jan