To reproduce:

unzip the attachment bug_report.zip to /tmp/gpg_bug

#run the following command, you will see the output, and then notice that
#it hangs until you hit CTRL-D

GNUPGHOME=/tmp/gpg_bug/gnupg gpg --decrypt --batch --yes /tmp/gpg_bug/corrupt.gpg
gpg: encrypted with 768-bit RSA key, ID 49BB32A3, created 2008-07-14

"CryptFactory Test Key (Old RSA Nopass 2) <cryptfactory@fxfn.com>"

testing, encrypt this!
How are you?

Type CTRL-D here

gpg: can't handle this ambiguous signature data
gpg: WARNING: message was not integrity protected

I am sorry, but the sample file is not useful for me. It requires the use of
the IDEA algorithm. I can't use this.

What might be going on here is that after decryption the signature looks like a
detached signature and gpg tries to read the data belonging to the signature
from stdin. The solution is to redirect stdin to /dev/null. You should do this
always in scripts to catch such situations.

Redirect stdin from /dev/null seems to work as a workaround and I will change
my application to call it like that. But it was my understanding that the --
batch flag explicitly means that the program should never expect input on
stdin. Allowing the program to hang waiting for stdin seems to go against the
point of that option.

If you agree this point is valid, then please consider reviewing the places
where the code reads from stdin based on the decrypted file information and
erroring out if the --batch parameter was used.