Packages requiring manual downloads: improve error message (#42017)

Spack packages may not have a public download option, and can implement
`download_instr` to inform users how to obtain the artifacts needed to
build. `spack checksum` however did not account for this and would print
out a confusing error message when invoked on such packages ("Could not
find any remote versions").

This PR updates the error message to output the manual download instructions
if `spack checksum` is invoked on a package with `manual_download = True`.

lib/spack/spack/cmd/checksum.py

@@ -17,7 +17,12 @@
 import spack.util.crypto
 import spack.util.web as web_util
 from spack.cmd.common import arguments
-from spack.package_base import PackageBase, deprecated_version, preferred_version
+from spack.package_base import (
+    ManualDownloadRequiredError,
+    PackageBase,
+    deprecated_version,
+    preferred_version,
+)
 from spack.util.editor import editor
 from spack.util.format import get_version_lines
 from spack.version import Version
@@ -86,6 +91,10 @@ def checksum(parser, args):
     # Get the package we're going to generate checksums for
     pkg = spack.repo.PATH.get_pkg_class(spec.name)(spec)
 
+    # Skip manually downloaded packages
+    if pkg.manual_download:
+        raise ManualDownloadRequiredError(pkg.download_instr)
+
     versions = [Version(v) for v in args.versions]
 
     # Define placeholder for remote versions.

lib/spack/spack/package_base.py

@@ -1413,13 +1413,9 @@ def download_instr(self):
             (str):  default manual download instructions
         """
         required = (
-            "Manual download is required for {0}. ".format(self.spec.name)
-            if self.manual_download
-            else ""
-        )
-        return "{0}Refer to {1} for download instructions.".format(
-            required, self.spec.package.homepage
+            f"Manual download is required for {self.spec.name}. " if self.manual_download else ""
         )
+        return f"{required}Refer to {self.homepage} for download instructions."
 
     def do_fetch(self, mirror_only=False):
         """
@@ -2578,3 +2574,7 @@ class DependencyConflictError(spack.error.SpackError):
 
     def __init__(self, conflict):
         super().__init__("%s conflicts with another file in the flattened directory." % (conflict))
+
+
+class ManualDownloadRequiredError(InvalidPackageOpError):
+    """Raised when attempting an invalid operation on a package that requires a manual download."""

lib/spack/spack/test/cmd/checksum.py

@@ -12,6 +12,7 @@
 import spack.repo
 import spack.spec
 from spack.main import SpackCommand
+from spack.package_base import ManualDownloadRequiredError
 from spack.stage import interactive_version_filter
 from spack.version import Version
 
@@ -272,3 +273,22 @@ def test_checksum_verification_fails(install_mockery, capsys):
     assert out.count("Correct") == 0
     assert "No previous checksum" in out
     assert "Invalid checksum" in out
+
+
+def test_checksum_manual_download_fails(mock_packages, monkeypatch):
+    """Confirm that checksumming a manually downloadable package fails."""
+    name = "zlib"
+    pkg_cls = spack.repo.PATH.get_pkg_class(name)
+    versions = [str(v) for v in pkg_cls.versions]
+    monkeypatch.setattr(spack.package_base.PackageBase, "manual_download", True)
+
+    # First check that the exception is raised with the default download
+    # instructions.
+    with pytest.raises(ManualDownloadRequiredError, match=f"required for {name}"):
+        spack_checksum(name, *versions)
+
+    # Now check that the exception is raised with custom download instructions.
+    error = "Cannot calculate the checksum for a manually downloaded package."
+    monkeypatch.setattr(spack.package_base.PackageBase, "download_instr", error)
+    with pytest.raises(ManualDownloadRequiredError, match=error):
+        spack_checksum(name, *versions)