fix(resolver): indicate resolver type in error messages

Add provider description to error messages when resolution fails.
Previously, errors from custom resolvers (GitHub, GitLab, etc.) were
indistinguishable from PyPI resolver errors, making debugging difficult.

Closes: #858

Signed-off-by: Lalatendu Mohanty <lmohanty@redhat.com>

Author: LalatenduMohanty

src/fromager/resolver.py

@@ -163,8 +163,9 @@ def resolve_from_provider(
         result = rslvr.resolve([req])
     except resolvelib.resolvers.ResolverException as err:
         constraint = provider.constraints.get_constraint(req.name)
+        provider_desc = provider.get_provider_description()
         raise resolvelib.resolvers.ResolverException(
-            f"Unable to resolve requirement specifier {req} with constraint {constraint}"
+            f"Unable to resolve requirement specifier {req} with constraint {constraint} using {provider_desc}"
         ) from err
     # resolvelib actually just returns one candidate per requirement.
     # result.mapping is map from an identifier to its resolved candidate
@@ -395,6 +396,13 @@ def cache_key(self) -> str:
         """
         raise NotImplementedError()
 
+    def get_provider_description(self) -> str:
+        """Return a human-readable description of the provider type
+
+        This is used in error messages to indicate what resolver was being used.
+        """
+        return type(self).__name__
+
     def find_candidates(self, identifier: str) -> Candidates:
         """Find unfiltered candidates"""
         raise NotImplementedError()
@@ -552,6 +560,9 @@ def find_matches(
 class PyPIProvider(BaseProvider):
     """Lookup package and versions from a simple Python index (PyPI)"""
 
+    def get_provider_description(self) -> str:
+        return f"PyPI resolver (searching at {self.sdist_server_url})"
+
     def __init__(
         self,
         include_sdists: bool = True,
@@ -707,6 +718,9 @@ def _re_match_function(
             logger.debug(f"{identifier}: could not parse version from {value}: {err}")
             return None
 
+    def get_provider_description(self) -> str:
+        return "custom resolver (GenericProvider)"
+
     @property
     def cache_key(self) -> str:
         raise NotImplementedError("GenericProvider does not implement caching")
@@ -757,6 +771,9 @@ def __init__(
         self.organization = organization
         self.repo = repo
 
+    def get_provider_description(self) -> str:
+        return f"GitHub tag resolver (repository: {self.organization}/{self.repo})"
+
     @property
     def cache_key(self) -> str:
         return f"{self.organization}/{self.repo}"
@@ -828,6 +845,9 @@ def __init__(
             f"{self.server_url}/api/v4/projects/{encoded_path}/repository/tags"
         )
 
+    def get_provider_description(self) -> str:
+        return f"GitLab tag resolver (project: {self.server_url}/{self.project_path})"
+
     @property
     def cache_key(self) -> str:
         return f"{self.server_url}/{self.project_path}"

tests/test_resolver.py

@@ -1033,3 +1033,68 @@ def test_pep592_support_constraint_mismatch() -> None:
 def test_extract_filename_from_url(url, filename) -> None:
     result = resolver.extract_filename_from_url(url)
     assert result == filename
+
+
+def test_custom_resolver_error_message_missing_tag() -> None:
+    """Test that error message indicates custom resolver when tag doesn't exist.
+
+    This reproduces issue #858 where the error message mentions PyPI and sdists
+    even when using a custom resolver like GitHubTagProvider.
+    """
+    with requests_mock.Mocker() as r:
+        # Mock GitHub API to return empty tags (simulating missing tag)
+        r.get(
+            "https://api.github.com:443/repos/test-org/test-repo/tags",
+            json=[],  # Empty tags list - tag doesn't exist
+        )
+
+        provider = resolver.GitHubTagProvider(organization="test-org", repo="test-repo")
+
+        with pytest.raises(resolvelib.resolvers.ResolverException) as exc_info:
+            resolver.resolve_from_provider(provider, Requirement("test-package==1.0.0"))
+
+        error_message = str(exc_info.value)
+        assert (
+            "GitHub" in error_message
+            or "test-org/test-repo" in error_message
+            or "custom resolver" in error_message.lower()
+        ), (
+            f"Error message should indicate custom resolver was used (GitHub tag resolver), "
+            f"but got: {error_message}"
+        )
+        # Should NOT mention PyPI when using GitHub resolver
+        assert "pypi.org" not in error_message.lower(), (
+            f"Error message incorrectly mentions PyPI when using GitHub resolver: {error_message}"
+        )
+
+
+def test_custom_resolver_error_message_via_resolve() -> None:
+    """Test error message when using resolve() function with custom resolver override."""
+
+    def custom_resolver_provider(*args, **kwargs):
+        """Custom resolver that returns GitHubTagProvider."""
+        return resolver.GitHubTagProvider(organization="test-org", repo="test-repo")
+
+    with requests_mock.Mocker() as r:
+        # Mock GitHub API to return empty tags
+        r.get(
+            "https://api.github.com:443/repos/test-org/test-repo/tags",
+            json=[],
+        )
+
+        provider = custom_resolver_provider()
+
+        with pytest.raises(resolvelib.resolvers.ResolverException) as exc_info:
+            resolver.resolve_from_provider(provider, Requirement("test-package==1.0.0"))
+
+        error_message = str(exc_info.value)
+        # After fix for issue #858, the error message should indicate that a GitHub resolver was used
+        assert (
+            "GitHub" in error_message
+            or "test-org/test-repo" in error_message
+            or "custom resolver" in error_message.lower()
+        ), f"Error message should indicate GitHub resolver was used: {error_message}"
+        # Should NOT mention PyPI when using GitHub resolver
+        assert "pypi.org" not in error_message.lower(), (
+            f"Error message incorrectly mentions PyPI when using GitHub resolver: {error_message}"
+        )