chsh corrupts /etc/pwd.db

[Alan Somers]

On Wed, Aug 21, 2019 at 7:22 PM Tom Samplonius <tom at samplonius.org> wrote:
>
>
>
> On Aug 21, 2019, at 2:55 PM, Alan Somers <asomers at freebsd.org> wrote:
>
> Today I tried to use chsh to change my shell from bash to fish.  The
> command completed successfully, but new logins continued to use bash!
> Investigating, I discovered that /etc/pwd.db and /etc/spwd.db seem to
> contain 3-4 entries per user.  One of those still refers to my old
>
>
>   Berkeley DB files can only have a single index, so users are stored three times, once by username, once by uid, and once by line number.  So that isn’t corruption.

But why was my entry stored four times?

>
>
> shell.  Worse, if I try using chsh again, it fails with an "entry
> inconsistent" error, and I have to restore the password files from
> backup.  Has anybody seen something like this before?  This is just a
> single system, with no NIS or LDAP.
>
>
>   You shouldn’t need to restore the files.  You should be able to just regenerate the *.db files from the master.passwd file:
>
> /usr/sbin/pwd_mkdb -p /etc/master.passwd

Ok, that worked.  Thanks!

>
>
> Unless, of course your master.passwd file was damaged.  But the *.db files are really just caches for faster access to user data.  The real master file is master.passwd.
>
>
> The ch* tools typically just change master.passwd, and then call pwd_mkdb to rebuild the *.db files.

The pwd.db file from before the snapshot only has three entries.  From
after, it has four, and one of them has the wrong shell.  So it does
seem that chsh is corrupting the file.  And fortunately the problem is
repeatable.  Any ideas about how to debug it?

-Alan

P.S. I failed to mention earlier that this is happening on 12.0-RELEASE-p10

-Alan