gpg_sign: detach_sign: fix gpg > 2.1 STDIN file descriptor

Starting from v2.1 passing passwords directly to gpg does not work
anymore [1], instead a loopback interface must be used otherwise
gpg >2.1 will error out with:
"gpg: signing failed: Inappropriate ioctl for device"

gpg <2.1 does not work with the new --pinentry-mode arg and gives an
invalid option error, so we detect what is the running version of gpg
and pass it accordingly.

[1] https://wiki.archlinux.org/index.php/GnuPG#Unattended_passphrase

(From OE-Core rev: 0413bd8e294ca8ac972ac68662b43a981952f5ae)

Signed-off-by: Ioan-Adrian Ratiu <adrian.ratiu@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/gpg_sign.py

@@ -66,6 +66,13 @@ class LocalSigner(object):
         if armor:
             cmd += ['--armor']
 
+        #gpg > 2.1 supports password pipes only through the loopback interface
+        #gpg < 2.1 errors out if given unknown parameters
+        dots = self.get_gpg_version().split('.')
+        assert len(dots) >= 2
+        if int(dots[0]) >= 2 and int(dots[1]) >= 1:
+            cmd += ['--pinentry-mode', 'loopback']
+
         cmd += [input_file]
 
         try:
@@ -89,6 +96,15 @@ class LocalSigner(object):
             raise Exception("Failed to sign '%s" % input_file)
 
 
+    def get_gpg_version(self):
+        """Return the gpg version"""
+        import subprocess
+        try:
+            return subprocess.check_output((self.gpg_bin, "--version")).split()[2]
+        except subprocess.CalledProcessError as e:
+            raise bb.build.FuncFailed("Could not get gpg version: %s" % e)
+
+
     def verify(self, sig_file):
         """Verify signature"""
         cmd = self.gpg_bin + " --verify "