refactor(bootstrapper): replace _handle_test_mode_failure with phase re-entry

Eliminate `_handle_test_mode_failure` by reusing the existing prebuilt
download path in `_phase_prepare_source`. When a source build fails in
test mode, `_handle_phase_error` now re-resolves with `pre_built=True`
and resets the `WorkItem` to `PREPARE_SOURCE` with `pbi_pre_built=True`,
letting the normal phase dispatch handle the download.

Add `is_test_mode_fallback` flag to `WorkItem` to prevent infinite
fallback loops.

Fix parent_req lookup in fallback resolution: use `self.why[-2]` instead
of `self.why[-1]` since `_track_why` has already pushed the current item
onto the why stack.

Closes: #892

Co-Authored-By: Claude <claude@anthropic.com>
Signed-off-by: Lalatendu Mohanty <lmohanty@redhat.com>

Author: LalatenduMohanty

src/fromager/bootstrapper.py

@@ -139,6 +139,7 @@ class WorkItem:
     cached_wheel_filename: pathlib.Path | None = None
     build_result: SourceBuildResult | None = None
     pbi_pre_built: bool = False
+    is_test_mode_fallback: bool = False
     build_system_deps: set[Requirement] = dataclasses.field(default_factory=set)
     build_backend_deps: set[Requirement] = dataclasses.field(default_factory=set)
     build_sdist_deps: set[Requirement] = dataclasses.field(default_factory=set)
@@ -789,82 +790,6 @@ def _do_build(
             )
             return self._build_wheel(req, resolved_version, sdist_root_dir, build_env)
 
-    def _handle_test_mode_failure(
-        self,
-        req: Requirement,
-        resolved_version: Version,
-        req_type: RequirementType,
-        build_error: Exception,
-    ) -> SourceBuildResult | None:
-        """Handle build failure in test mode by attempting pre-built fallback.
-
-        Args:
-            req: The requirement that failed to build.
-            resolved_version: The version that was attempted.
-            req_type: The type of requirement (for fallback resolution).
-            build_error: The original exception from the build attempt.
-
-        Returns:
-            SourceBuildResult if fallback succeeded, None if fallback also failed.
-        """
-        logger.warning(
-            "test mode: build failed for %s==%s, attempting pre-built fallback: %s",
-            req.name,
-            resolved_version,
-            build_error,
-        )
-
-        try:
-            parent_req = self.why[-1][1] if self.why else None
-            results = self._resolver.resolve(
-                req=req,
-                req_type=req_type,
-                parent_req=parent_req,
-                pre_built=True,  # Force prebuilt for test mode fallback
-            )
-            wheel_url, fallback_version = results[0]
-
-            if fallback_version != resolved_version:
-                logger.warning(
-                    "test mode: version mismatch for %s - requested %s, fallback %s",
-                    req.name,
-                    resolved_version,
-                    fallback_version,
-                )
-
-            wheel_filename, unpack_dir = self._download_prebuilt(
-                req=req,
-                req_type=req_type,
-                resolved_version=fallback_version,
-                wheel_url=wheel_url,
-            )
-
-            logger.info(
-                "test mode: successfully used pre-built wheel for %s==%s",
-                req.name,
-                fallback_version,
-            )
-            # Package succeeded via fallback - no failure to record
-
-            return SourceBuildResult(
-                wheel_filename=wheel_filename,
-                sdist_filename=None,
-                unpack_dir=unpack_dir,
-                sdist_root_dir=None,
-                build_env=None,
-                source_type=SourceType.PREBUILT,
-            )
-
-        except Exception as fallback_error:
-            logger.error(
-                "test mode: pre-built fallback also failed for %s: %s",
-                req.name,
-                fallback_error,
-                exc_info=True,
-            )
-            # Return None to signal failure; bootstrap() will record via re-raised exception
-            return None
-
     def _look_for_existing_wheel(
         self,
         req: Requirement,
@@ -1754,18 +1679,56 @@ def _handle_phase_error(
                     BootstrapPhase.BUILD,
                 )
                 and not item.pbi_pre_built
+                and not item.is_test_mode_fallback
             ):
                 assert item.resolved_version is not None
-                fallback = self._handle_test_mode_failure(
-                    req=item.req,
-                    resolved_version=item.resolved_version,
-                    req_type=item.req_type,
-                    build_error=err,
+                logger.warning(
+                    "test mode: build failed for %s==%s, attempting pre-built fallback: %s",
+                    item.req.name,
+                    item.resolved_version,
+                    err,
                 )
-                if fallback is not None:
-                    item.build_result = fallback
-                    item.phase = BootstrapPhase.PROCESS_INSTALL_DEPS
+                try:
+                    # why[-1] is the current item (pushed by _track_why),
+                    # so the real parent is why[-2].
+                    parent_req = self.why[-2][1] if len(self.why) > 1 else None
+                    results = self._resolver.resolve(
+                        req=item.req,
+                        req_type=item.req_type,
+                        parent_req=parent_req,
+                        pre_built=True,
+                    )
+                    wheel_url, fallback_version = results[0]
+                    if fallback_version != item.resolved_version:
+                        logger.warning(
+                            "test mode: version mismatch for %s"
+                            " - requested %s, fallback %s",
+                            item.req.name,
+                            item.resolved_version,
+                            fallback_version,
+                        )
+                    item.source_url = wheel_url
+                    item.resolved_version = fallback_version
+                    item.pbi_pre_built = True
+                    item.is_test_mode_fallback = True
+                    item.phase = BootstrapPhase.PREPARE_SOURCE
+                    item.build_env = None
+                    item.sdist_root_dir = None
+                    item.unpack_dir = None
+                    item.cached_wheel_filename = None
+                    item.build_result = None
                     return [item]
+                except Exception as fallback_error:
+                    logger.error(
+                        "test mode: pre-built fallback also failed for %s: %s",
+                        item.req.name,
+                        fallback_error,
+                        exc_info=True,
+                    )
+                    self._record_test_mode_failure(
+                        item.req, str(item.resolved_version), err, "bootstrap"
+                    )
+                    return []
             self._record_test_mode_failure(
                 item.req, str(item.resolved_version), err, "bootstrap"
             )

tests/test_bootstrapper_iterative.py

@@ -543,30 +543,43 @@ def test_resolve_error_in_multiple_versions_mode_raises(
     def test_build_phase_test_mode_fallback_success(
         self, tmp_context: WorkContext
     ) -> None:
+        """Fallback re-enters PREPARE_SOURCE as prebuilt."""
         bt = bootstrapper.Bootstrapper(tmp_context, test_mode=True)
         item = _make_build_item(phase=BootstrapPhase.PREPARE_SOURCE)
         item.pbi_pre_built = False
         err = RuntimeError("build failed")
 
-        mock_fallback = Mock(spec=SourceBuildResult)
-        with patch.object(bt, "_handle_test_mode_failure", return_value=mock_fallback):
+        fallback_url = "https://pypi.org/testpkg-1.0-py3-none-any.whl"
+        with patch.object(
+            bt._resolver,
+            "resolve",
+            return_value=[(fallback_url, Version("1.0"))],
+        ):
             result = bt._handle_phase_error(item, err)
 
         assert len(result) == 1
         assert result[0] is item
-        assert item.build_result is mock_fallback
-        assert item.phase == BootstrapPhase.PROCESS_INSTALL_DEPS
+        assert item.phase == BootstrapPhase.PREPARE_SOURCE
+        assert item.pbi_pre_built is True
+        assert item.is_test_mode_fallback is True
+        assert item.source_url == fallback_url
+        assert item.build_result is None
         assert len(bt.failed_packages) == 0
 
     def test_build_phase_test_mode_fallback_failure(
         self, tmp_context: WorkContext
     ) -> None:
+        """When prebuilt resolution fails, build failure is recorded and item is skipped."""
         bt = bootstrapper.Bootstrapper(tmp_context, test_mode=True)
         item = _make_build_item(phase=BootstrapPhase.BUILD)
         item.pbi_pre_built = False
         err = RuntimeError("build failed")
 
-        with patch.object(bt, "_handle_test_mode_failure", return_value=None):
+        with patch.object(
+            bt._resolver,
+            "resolve",
+            side_effect=RuntimeError("no prebuilt available"),
+        ):
             result = bt._handle_phase_error(item, err)
 
         assert result == []
@@ -587,6 +600,22 @@ def test_build_phase_test_mode_prebuilt_skips_fallback(
         assert len(bt.failed_packages) == 1
         assert bt.failed_packages[0]["failure_type"] == "bootstrap"
 
+    def test_build_phase_test_mode_fallback_item_skips_second_fallback(
+        self, tmp_context: WorkContext
+    ) -> None:
+        """A fallback item that fails does not attempt another fallback."""
+        bt = bootstrapper.Bootstrapper(tmp_context, test_mode=True)
+        item = _make_build_item(phase=BootstrapPhase.PREPARE_SOURCE)
+        item.pbi_pre_built = False
+        item.is_test_mode_fallback = True
+        err = RuntimeError("fallback download failed")
+
+        result = bt._handle_phase_error(item, err)
+
+        assert result == []
+        assert len(bt.failed_packages) == 1
+        assert bt.failed_packages[0]["failure_type"] == "bootstrap"
+
     def test_non_build_phase_test_mode_records_failure(
         self, tmp_context: WorkContext
     ) -> None: