bases: improve errors for ESM bases

Bases that were once supported and no longer supported in the current
version provide a reference to the version/channel of Snapcraft where
they once were.

Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>

snapcraft/errors.py

@@ -16,6 +16,8 @@
 
 """Snapcraft error definitions."""
 
+from typing import Optional
+
 from craft_cli import CraftError
 
 
@@ -117,6 +119,27 @@ class LegacyFallback(Exception):
     """Fall back to legacy snapcraft implementation."""
 
 
+class MaintenanceBase(SnapcraftError):
+    """Error for bases under ESM and no longer supported in this release."""
+
+    def __init__(self, base) -> None:
+        channel: Optional[str] = None
+        if base == "core":
+            channel = "4.x"
+        elif base == "core18":
+            channel = "7.x"
+
+        resolution: Optional[str] = None
+        if channel:
+            resolution = f"Install from or refresh to the {channel!r} channel."
+
+        super().__init__(
+            f"{base!r} is not supported on this version of Snapcraft.",
+            resolution=resolution,
+            docs_url="https://snapcraft.io/docs/base-snaps",
+        )
+
+
 class StoreCredentialsUnauthorizedError(SnapcraftError):
     """Error raised for 401 responses from the Snap Store."""
 

tests/unit/test_errors.py

@@ -0,0 +1,44 @@
+# -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
+#
+# Copyright 2022 Canonical Ltd.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Snapcraft error tests."""
+
+import pytest
+
+from snapcraft import errors
+
+
+@pytest.mark.parametrize("base,channel", [("core", "4.x"), ("core18", "7.x")])
+def test_maintenance_base(base, channel):
+    message = f"{base!r} is not supported on this version of Snapcraft."
+    resolution = f"Install from or refresh to the {channel!r} channel."
+
+    error = errors.MaintenanceBase(base)
+
+    assert str(error) == message
+    assert error.resolution == resolution
+    assert error.docs_url == "https://snapcraft.io/docs/base-snaps"
+
+
+def test_maintenance_base_fallback():
+    """For when the base is not explicitly handled in the exception."""
+    error = errors.MaintenanceBase("unknown-base")
+
+    assert (
+        str(error) == f"'unknown-base' is not supported on this version of Snapcraft."
+    )
+    assert error.resolution == None
+    assert error.docs_url == "https://snapcraft.io/docs/base-snaps"