fix(keyrinctl): yield file not found errors instead of blocking on stdin

Handle missing or wrong certificate paths in a way that does not lead to
a blocking command by reading from stdin. Instead throw either file not
found errors or expect optional outputs.

libkeyringctl/cli.py

@@ -133,13 +133,15 @@ def main() -> None:
                     )
                 )
             elif "export" == args.subcommand:
-                print(
-                    export(
+                result = export(
                     working_dir=working_dir,
                     keyring_root=keyring_root,
                     sources=args.source,
                     output=args.output,
-                    ),
+                )
+                if result:
+                    print(
+                        result,
                         end="",
                     )
             elif "build" == args.subcommand:

libkeyringctl/keyring.py

@@ -62,8 +62,6 @@ def get_cert_paths(paths: Iterable[Path]) -> Set[Path]:
     visit: List[Path] = list(paths)
     while visit:
         path = visit.pop()
-        if not path.exists():
-            continue
         # this level contains a certificate, abort depth search
         if list(path.glob("*.asc")):
             cert_paths.add(path)
@@ -580,7 +578,7 @@ def convert(
         get_fingerprints(
             working_dir=working_dir,
             sources=source,
-            paths=[keyring_root],
+            paths=[keyring_root] if keyring_root.exists() else [],
         ).keys()
     )
 
@@ -832,7 +830,7 @@ def export(
     keyring_root: Path,
     sources: Optional[List[Path]] = None,
     output: Optional[Path] = None,
-) -> str:
+) -> Optional[str]:
     """Export all provided PGP packet files to a single output file
 
     If sources contains directories, any .asc files below them are considered.
@@ -876,6 +874,9 @@ def export(
         )
         certificates.append(output_path)
 
+    if not certificates:
+        return None
+
     return keyring_merge(certificates, output, force=True)