fix(resolver): prefer sdists in cache server fallback

`_resolve_from_cache_server` queried for both wheels and sdists,
picking whichever had the higher version. When only a wheel existed,
the wheel URL was returned as `source_url` but written to
build-order.json with `source_url_type: "sdist"`, causing downstream
consumers to fail on the `.whl` basename.

Now only sdists are queried. If the cache server has only wheels for
the package, an empty list is returned — the package is already built
and does not need to appear in the build order.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Andre Lustosa <alustosa@redhat.com>

Author: andre-motta

src/fromager/bootstrap_requirement_resolver.py

@@ -189,40 +189,45 @@ def resolve(
         return [results[0]]
 
     def _resolve_from_cache_server(self, req: Requirement) -> list[tuple[str, Version]]:
-        """Fall back to the remote wheel cache server for a cached version.
+        """Fall back to the remote wheel cache server for a cached sdist.
 
         When age filtering removes all candidates in multi-version mode,
-        queries the remote cache server for the newest available wheel.
+        queries the remote cache server for the newest available sdist.
         Returns at most one version so that transitive dependencies are
         re-processed without rebuilding every old version.
+
+        Only sdists are returned.  If the cache server has only wheels for
+        the package, an empty list is returned because the package is
+        already built and does not need to appear in the build order.
         """
         logger.info(
             "checking cache server %s for existing build",
             self.cache_wheel_server_url,
         )
-        best: tuple[str, Version] | None = None
-        for include_sdists, include_wheels in [(False, True), (True, False)]:
-            try:
-                provider = finders.PyPICacheProvider(
-                    cache_server_url=self.cache_wheel_server_url,
-                    constraints=self.ctx.constraints,
-                    include_sdists=include_sdists,
-                    include_wheels=include_wheels,
-                )
-                results = resolver.find_all_matching_from_provider(provider, req)
-                if results:
-                    url, version = results[0]
-                    if best is None or version > best[1]:
-                        best = (url, version)
-            except Exception as err:
-                logger.warning(
-                    "error checking cache server %s: %s",
-                    self.cache_wheel_server_url,
-                    err,
-                )
-        if best is not None:
-            logger.info("found version %s on cache server", best[1])
-            return [best]
+        try:
+            provider = finders.PyPICacheProvider(
+                cache_server_url=self.cache_wheel_server_url,
+                constraints=self.ctx.constraints,
+                include_sdists=True,
+                include_wheels=False,
+            )
+            results = resolver.find_all_matching_from_provider(provider, req)
+            if results:
+                url, version = results[0]
+                logger.info("found sdist version %s on cache server", version)
+                return [(url, version)]
+        except Exception as err:
+            logger.warning(
+                "error checking cache server %s: %s",
+                self.cache_wheel_server_url,
+                err,
+            )
+
+        logger.debug(
+            "no sdist found on cache server %s for %s",
+            self.cache_wheel_server_url,
+            req.name,
+        )
         return []
 
     def get_cached_resolution(

tests/test_bootstrap_requirement_resolver.py

@@ -699,8 +699,8 @@ def test_resolve_prebuilt_after_source_uses_separate_cache(
 class TestResolveFromCacheServer:
     """Tests for the cache server fallback in _resolve_from_cache_server."""
 
-    def test_returns_newest_version_from_cache(self, tmp_context: WorkContext) -> None:
-        """Falls back to cache server and returns only the newest version."""
+    def test_returns_newest_sdist_from_cache(self, tmp_context: WorkContext) -> None:
+        """Falls back to cache server and returns only the newest sdist."""
         brr = BootstrapRequirementResolver(
             tmp_context,
             multiple_versions=True,
@@ -712,14 +712,35 @@ def test_returns_newest_version_from_cache(self, tmp_context: WorkContext) -> No
             "fromager.bootstrap_requirement_resolver.resolver"
             ".find_all_matching_from_provider",
             return_value=[
-                ("http://cache.test/testpkg-3.0.whl", Version("3.0")),
-                ("http://cache.test/testpkg-2.0.whl", Version("2.0")),
+                ("http://cache.test/testpkg-3.0.tar.gz", Version("3.0")),
+                ("http://cache.test/testpkg-2.0.tar.gz", Version("2.0")),
             ],
         ):
             result = brr._resolve_from_cache_server(req)
 
         assert len(result) == 1
         assert result[0][1] == Version("3.0")
+        assert result[0][0].endswith(".tar.gz")
+
+    def test_returns_empty_when_only_wheels_on_cache(
+        self, tmp_context: WorkContext
+    ) -> None:
+        """Returns empty list when cache server has only wheels (already built)."""
+        brr = BootstrapRequirementResolver(
+            tmp_context,
+            multiple_versions=True,
+            cache_wheel_server_url="http://cache.test/simple",
+        )
+        req = Requirement("testpkg")
+
+        with patch(
+            "fromager.bootstrap_requirement_resolver.resolver"
+            ".find_all_matching_from_provider",
+            return_value=[],
+        ):
+            result = brr._resolve_from_cache_server(req)
+
+        assert result == []
 
     def test_returns_empty_when_cache_has_no_match(
         self, tmp_context: WorkContext
@@ -783,7 +804,7 @@ def test_resolve_uses_cache_fallback_when_age_filter_empties(
             patch.object(
                 brr,
                 "_resolve_from_cache_server",
-                return_value=[("http://cache.test/testpkg-1.0.whl", Version("1.0"))],
+                return_value=[("http://cache.test/testpkg-1.0.tar.gz", Version("1.0"))],
             ) as mock_cache,
         ):
             result = brr.resolve(