ssh: call ssh-keygen once for each key to import

ssh-keygen -l supports an input file that has multiple keys. As a
result, it will output multiple key fingerprints.

That being said, ssh-keygen will ignore empty newlines from the input
(and maybe other things?).
It makes it slightly challenging to associate each key with its
fingerprint because the number of lines in the input and output can
differ, e.g.:

             input               |               output
  -----------------------------------------------------------------------
  ssh-rsa AA[...] user@host     ◀-▶ 256 SHA256:[...] user@host (RSA)
  <empty line>                   ┌▶ 3072 SHA256:[...] user@host (ED25519)
  ssh-ed25519 AA[...] user@host ◀┘

To simplify this process, we will do one call to ssh-keygen -l for each
key from the input.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/server/controllers/ssh.py

@@ -91,25 +91,23 @@ class SSHController(SubiquityController):
                                       identities=None, error=e.stderr)
         keys_material: str = cp.stdout.replace('\r', '').strip()
 
-        # ssh-keygen supports multiple keys at once.
+        # ssh-keygen supports multiple keys at once, but it is simpler to
+        # associate each key with its resulting fingerprint if we call
+        # ssh-keygen multiple times.
         fingerprint_command = ('ssh-keygen', '-l', '-f', '-')
-        try:
-            cp = await arun_command(fingerprint_command, check=True,
-                                    input=keys_material)
-        except subprocess.CalledProcessError as e:
-            log.exception("ssh-import-id failed. stderr: %s", e.stderr)
-            return SSHFetchIdResponse(
-                    tatus=SSHFetchIdStatus.FINGERPRINT_ERROR,
-                    identities=None, error=e.stderr)
+        for key_material in (mat for mat in keys_material.splitlines() if mat):
+            try:
+                cp = await arun_command(fingerprint_command, check=True,
+                                        input=key_material)
+            except subprocess.CalledProcessError as e:
+                log.exception("ssh-import-id failed. stderr: %s", e.stderr)
+                return SSHFetchIdResponse(
+                        tatus=SSHFetchIdStatus.FINGERPRINT_ERROR,
+                        identities=None, error=e.stderr)
 
-        fingerprints: str = cp.stdout.replace(
+            fingerprint: str = cp.stdout.replace(
                 f'# ssh-import-id {user_id}', '').strip()
 
-        zipped = zip(
-                [mat for mat in keys_material.splitlines() if mat],
-                fingerprints.splitlines())
-
-        for key_material, fingerprint in zipped:
             key_type, key, key_comment = key_material.split(' ', maxsplit=2)
             identities.append(SSHIdentity(
                 key_type=key_type, key=key, key_comment=key_comment,