Provide an is_update_available function

This function does all the needed checks before returning `True`, making it a good external API.

Under the hood, the registry now has an `is_new_remote_image_available`
which is just for checking the presence of a new image, but doesn't do
any verirications on it, and there is also a new `check_signatures_and_logindex` that ensures that these two are valid.

dangerzone/updater/registry.py

@@ -5,6 +5,8 @@ from typing import Dict, Optional, Tuple
 
 import requests
 
+from .. import container_utils as runtime
+from .. import errors as dzerrors
 from . import errors, log
 
 __all__ = [
@@ -114,3 +116,24 @@ def get_manifest_digest(
         tag_manifest_content = get_manifest(image_str).content
 
     return sha256(tag_manifest_content).hexdigest()
+
+
+def is_new_remote_image_available(image_str: str) -> Tuple[bool, str]:
+    """
+    Check if a new remote image is available on the registry.
+    """
+    remote_digest = get_manifest_digest(image_str)
+    image = parse_image_location(image_str)
+    if image.digest:
+        local_digest = image.digest
+    else:
+        try:
+            local_digest = runtime.get_local_image_digest(image_str)
+        except dzerrors.ImageNotPresentException:
+            log.debug("No local image found")
+            return True, remote_digest
+
+    log.debug("Remote digest: %s", remote_digest)
+    log.debug("Local digest: %s", local_digest)
+
+    return (remote_digest != local_digest, remote_digest)

dangerzone/updater/signatures.py

@@ -136,20 +136,40 @@ class Signature:
         return full_digest.replace("sha256:", "")
 
 
-def is_update_available(image: str) -> Tuple[bool, Optional[str]]:
-    remote_digest = registry.get_manifest_digest(image)
-    try:
-        local_digest = runtime.get_local_image_digest(image)
-    except dzerrors.ImageNotPresentException:
-        log.debug("No local image found")
-        return True, remote_digest
-    log.debug("Remote digest: %s", remote_digest)
-    log.debug("Local digest: %s", local_digest)
-    has_update = remote_digest != local_digest
-    if has_update:
-        return True, remote_digest
+def is_update_available(image_str: str, pubkey: str) -> Tuple[bool, Optional[str]]:
+    """
+    Check if a new image is available, doing all the necessary checks ensuring it
+    would be safe to upgrade.
+    """
+    new_image_available, remote_digest = registry.is_new_remote_image_available(
+        image_str
+    )
+    if not new_image_available:
         return False, None
 
+    try:
+        check_signatures_and_logindex(image_str, remote_digest, pubkey)
+        return True, remote_digest
+    except errors.InvalidLogIndex:
+        return False, None
+
+
+def check_signatures_and_logindex(
+    image_str: str, remote_digest: str, pubkey: str
+) -> list[Dict]:
+    signatures = get_remote_signatures(image_str, remote_digest)
+    verify_signatures(signatures, remote_digest, pubkey)
+
+    incoming_log_index = get_log_index_from_signatures(signatures)
+    last_log_index = get_last_log_index()
+
+    if incoming_log_index < last_log_index:
+        raise errors.InvalidLogIndex(
+            f"The incoming log index ({incoming_log_index}) is "
+            f"lower than the last known log index ({last_log_index})"
+        )
+    return signatures
+
 
 def verify_signatures(
     signatures: List[Dict],
@@ -461,22 +481,11 @@ def prepare_airgapped_archive(image_name: str, destination: str) -> None:
 
 def upgrade_container_image(image: str, manifest_digest: str, pubkey: str) -> str:
     """Verify and upgrade the image to the latest, if signed."""
-    update_available, _ = is_update_available(image)
+    update_available, remote_digest = registry.is_new_remote_image_available(image)
     if not update_available:
         raise errors.ImageAlreadyUpToDate("The image is already up to date")
 
-    signatures = get_remote_signatures(image, manifest_digest)
-    verify_signatures(signatures, manifest_digest, pubkey)
-
-    # Only upgrade if the log index is higher than the last known one
-    incoming_log_index = get_log_index_from_signatures(signatures)
-    last_log_index = get_last_log_index()
-
-    if incoming_log_index < last_log_index:
-        raise errors.InvalidLogIndex(
-            "Trying to upgrade to an image with a lower log index"
-        )
-
+    signatures = check_signatures_and_logindex(image, remote_digest, pubkey)
     runtime.container_pull(image, manifest_digest)
 
     # Store the signatures just now to avoid storing them unverified