keyringctl: Deal with multi-certificate per user files

keyringctl:
Add `sanitize_certificate_file()` to potentially split per-user input
files that contain more than one certificate.
Change `packet_split()` to add documentation and rename the key
parameter to certificate, as it is more generic.
Change `convert_certificate()` to use named parameters when calling
`packet_split()`.
Change `convert()` to call `convert_certificate()` on a list of
sanitized certificates (generated using `sanitized_certificate_file()`)
to be able to deal with multi-certificate files per user.

keyringctl

@@ -125,7 +125,7 @@ def convert_certificate(working_dir: Path, certificate: Path, name_override: Opt
 
     debug(f'Processing certificate {certificate}')
 
-    for packet in packet_split(working_dir, certificate):
+    for packet in packet_split(working_dir=working_dir, certificate=certificate):
         debug(f'Processing packet {packet.name}')
         if packet.name.endswith('--PublicKey'):
             pubkey = packet
@@ -432,10 +432,67 @@ def packet_dump_field(packet: Path, field: str) -> str:
     return lines[0].split(maxsplit=1)[1]
 
 
-def packet_split(working_dir: Path, key: Path) -> Iterable[Path]:
+def sanitize_certificate_file(working_dir: Path, certificate: Path) -> List[Path]:
+    """Sanitize a certificate file, potentially containing several certificates
+
+    If the input file holds several certificates, they are split into respective files below working_dir and their
+    paths are returned. Else the path to the input certificate file is returned.
+
+    Parameters
+    ----------
+    working_dir: Path
+        The path of the working directory below which to create split certificates
+    certificate: Path
+        The absolute path of a file containing one or several PGP certificates
+
+    Returns
+    -------
+    List[Path]
+        A list of paths that either contain the input certificate or several split certificates
+    """
+
+    begin_cert = "-----BEGIN PGP PUBLIC KEY BLOCK-----"
+    end_cert = "-----END PGP PUBLIC KEY BLOCK-----"
+    certificate_list: List[Path] = []
+
+    with open(file=certificate, mode="r") as certificate_file:
+        file = certificate_file.read()
+        if file.count(begin_cert) > 1 and file.count(end_cert) > 1:
+            debug(f"Several public keys detected in file: {certificate}")
+            for split_cert in [f"{cert}{end_cert}\n" for cert in list(filter(None, file.split(f"{end_cert}\n")))]:
+                split_cert_dir = Path(mkdtemp(dir=working_dir)).absolute()
+                split_cert_path = split_cert_dir / certificate.name
+                with open(file=split_cert_path, mode="w") as split_cert_file:
+                    split_cert_file.write(split_cert)
+                    debug(f"Writing split cert to file: {split_cert_path}")
+                    certificate_list.append(split_cert_path)
+            return certificate_list
+        else:
+            return [certificate]
+
+
+def packet_split(working_dir: Path, certificate: Path) -> Iterable[Path]:
+    """Split a file containing a PGP certificate into separate packet files
+
+    The files are split using sq
+
+    Parameters
+    ----------
+    working_dir: Path
+        The path of the working directory below which to create the output files
+    certificate: Path
+        The absolute path of a file containing one PGP certificate
+
+    Returns
+    -------
+    Iterable[Path]
+        An iterable over the naturally sorted list of packet files derived from certificate
+    """
+
     packet_dir = Path(mkdtemp(dir=working_dir)).absolute()
+
     with cwd(packet_dir):
-        system(['sq', 'packet', 'split', '--prefix', '', str(key)])
+        system(['sq', 'packet', 'split', '--prefix', '', str(certificate)])
     return natural_sort_path(packet_dir.iterdir())
 
 
@@ -463,13 +520,15 @@ def convert(
     directories: List[Path] = []
     if source.is_dir():
         for key in source.iterdir():
-            directories.append(
-                convert_certificate(working_dir=working_dir, certificate=key, name_override=name_override)
-            )
+            for sane_cert in sanitize_certificate_file(working_dir=working_dir, certificate=key):
+                directories.append(
+                    convert_certificate(working_dir=working_dir, certificate=sane_cert, name_override=name_override)
+                )
     else:
-        directories.append(
-            convert_certificate(working_dir=working_dir, certificate=source, name_override=name_override)
-        )
+        for sane_cert in sanitize_certificate_file(working_dir=working_dir, certificate=source):
+            directories.append(
+                convert_certificate(working_dir=working_dir, certificate=sane_cert, name_override=name_override)
+            )
 
     for path in directories:
         (target_dir / path.name).mkdir(exist_ok=True)