So maybe there is another correct way to say user that he must type passphrase?
It is need for QCA gnupg plugin. qca-gnupg plugins uses pipes to send/recieve
data with gpg. It was many time ago when I tried to fix problem. So now I can't
remember particularity problem. Seems it was gpg2 related.

I wrote this in my QCA TODO

• New plugin qca-gpgme to replace current qca-gnupg. qca-gnupg requires to have gpg binary which can be any 1.4.x or 2.x. Them behaviour is different. gpg2 requires gpg-agent to ask user for passphrase. No correct way to check that key requires passphrase.

Although the output timing of NEED_PASSPHRASE is different (than your

expectation), it is emitted after gpg reads passphrase string and it needs the
passphrase for signing.

It is nonsense. In this case status is such log file. Such behaviour is no
obviously and documentation says nothing about ths.
And user can't know must or no he provides passphrase.

I just now tested it on my Fedora 20 with gpg 1.4.19 and 2.0.27. I tried to use
--no-use-agent no password request again.