💅 Bundle attestation existence check together

This patch moves said check out of the signing loop and performs the
check early in the process. It is then able to report multiple
problems in a single error.

attestations.py

@@ -69,13 +69,47 @@ def collect_dists(packages_dir: Path) -> list[Path]:
     return dist_paths
 
 
-def attest_dist(dist_path: Path, signer: Signer) -> None:
+def assert_attestations_do_not_pre_exist(
+        dist_to_attestation_map: dict[Path, Path],
+) -> None:
+    existing_attestations = {
+        f'* {dist !s} -> {dist_attestation !s}'
+        for dist, dist_attestation in dist_to_attestation_map.items()
+        if dist_attestation.exists()
+    }
+    if not existing_attestations:
+        return
+
+    existing_attestations_list = '\n'.join(map(str, existing_attestations))
+    error_message = (
+        'The following distributions already have publish attestations:'
+        f'{existing_attestations_list}',
+    )
+    die(error_message)
+
+
+def compose_attestation_mapping(dist_paths: list[Path]) -> dict[Path, Path]:
+    dist_to_attestation_map = {
+        dist_path: dist_path.with_suffix(
+            f'{dist_path.suffix}.publish.attestation',
+        )
+        for dist_path in dist_paths
+    }
+
     # We are the publishing step, so there should be no pre-existing publish
     # attestation. The presence of one indicates user confusion.
-    attestation_path = Path(f'{dist_path}.publish.attestation')
-    if attestation_path.exists():
-        die(f'{dist_path} already has a publish attestation: {attestation_path}')
+    # Make sure there's no publish attestations on disk.
+    # We do this up-front to prevent partial signing.
+    assert_attestations_do_not_pre_exist(dist_to_attestation_map)
 
+    return dist_to_attestation_map
+
+
+def attest_dist(
+        dist_path: Path,
+        attestation_path: Path,
+        signer: Signer,
+) -> None:
     dist = Distribution.from_file(dist_path)
     attestation = Attestation.sign(signer, dist)
 
@@ -92,7 +126,9 @@ def get_identity_token() -> IdentityToken:
 
 
 def main() -> None:
-    packages_dir = Path(sys.argv[1])
+    dist_to_attestation_map = compose_attestation_mapping(
+        collect_dists(Path(sys.argv[1])),
+    )
 
     try:
         identity = get_identity_token()
@@ -103,12 +139,10 @@ def main() -> None:
         # since permissions can't be to blame at this stage.
         die(_TOKEN_RETRIEVAL_FAILED_MESSAGE.format(identity_error=identity_error))
 
-    dist_paths = collect_dists(packages_dir)
-
-    with SigningContext.production().signer(identity, cache=True) as s:
-        debug(f'attesting to dists: {dist_paths}')
-        for dist_path in dist_paths:
-            attest_dist(dist_path, s)
+    with SigningContext.production().signer(identity, cache=True) as signer:
+        debug(f'attesting to dists: {dist_to_attestation_map.keys()}')
+        for dist_path, attestation_path in dist_to_attestation_map.items():
+            attest_dist(dist_path, attestation_path, signer)
 
 
 if __name__ == '__main__':