depgraph: Do not allow slotted deps to be satisfied by wrong slots

lib/_emerge/depgraph.py

@@ -4041,6 +4041,16 @@ def _wrapped_add_pkg_dep_string(
                         inst_pkg, modified_use=self._pkg_use_enabled(inst_pkg)
                     )
                 ]
+                # Do not allow slotted deps to be satisfied by wrong slots.
+                # Otherwise, slot-operator-dependent packages may rebuild
+                # before the slotted package they are dependent on.
+                if child and atom.slot_operator == "=":
+                    inst_pkgs = [
+                        inst_pkg
+                        for inst_pkg in inst_pkgs
+                        if inst_pkg.slot == child.slot
+                        and inst_pkg.sub_slot == child.sub_slot
+                    ]
                 if inst_pkgs:
                     for inst_pkg in inst_pkgs:
                         if self._pkg_visibility_check(inst_pkg):
@@ -4160,6 +4170,14 @@ def _wrapped_add_pkg_dep_string(
                             inst_pkg, modified_use=self._pkg_use_enabled(inst_pkg)
                         )
                     ]
+                    # Do not allow slotted deps to be satisfied by wrong slots.
+                    if child and atom.slot_operator == "=":
+                        inst_pkgs = [
+                            inst_pkg
+                            for inst_pkg in inst_pkgs
+                            if inst_pkg.slot == child.slot
+                            and inst_pkg.sub_slot == child.sub_slot
+                        ]
                     if inst_pkgs:
                         for inst_pkg in inst_pkgs:
                             if self._pkg_visibility_check(inst_pkg):

lib/portage/tests/resolver/test_perl_rebuild_bug.py

@@ -0,0 +1,121 @@
+# Copyright 2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+from portage.tests import TestCase
+from portage.tests.resolver.ResolverPlayground import (
+    ResolverPlayground,
+    ResolverPlaygroundTestCase,
+)
+
+
+class PerlRebuildBugTestCase(TestCase):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def testPerlRebuildBug(self):
+        """
+        The infamous Perl rebuild bug.
+
+        A non-slotted build-time dependency cycle is created by:
+        dev-lang/perl -> sys-libs/zlib -> sys-devel/automake -> dev-lang/perl
+        Everything else depends on this cycle.
+
+        Bug in solving for smallest cycle causes slot in RDEPEND of
+        dev-perl/Locale-gettext to be ignored, so all dependencies other than
+        perl's >=sys-libs/zlib-1.2.12 are satisfied by already-installed
+        packages. dev-perl/Locale-gettext and sys-devel/automake become leaves
+        of the depgraph after satisfied packages are ignored. They become
+        emerged first. This causes an issue because dev-perl/Locale-gettext is
+        now built before the slot upgrade of dev-lang/perl.
+        """
+        ebuilds = {
+            "dev-lang/perl-5.36.0-r2": {
+                "EAPI": "5",
+                "DEPEND": ">=sys-libs/zlib-1.2.12",
+                "RDEPEND": ">=sys-libs/zlib-1.2.12",
+                "SLOT": "0/5.36",
+            },
+            "dev-perl/Locale-gettext-1.70.0-r1": {
+                "EAPI": "5",
+                "DEPEND": "dev-lang/perl",
+                "RDEPEND": "dev-lang/perl:=",
+            },
+            "sys-apps/help2man-1.49.3": {
+                "EAPI": "5",
+                "DEPEND": "dev-lang/perl dev-perl/Locale-gettext",
+                "RDEPEND": "dev-lang/perl dev-perl/Locale-gettext",
+            },
+            "sys-devel/automake-1.16.5": {
+                "EAPI": "5",
+                "DEPEND": "dev-lang/perl",
+                "RDEPEND": "dev-lang/perl",
+            },
+            "sys-libs/zlib-1.2.13-r1": {
+                "EAPI": "5",
+                "DEPEND": "sys-devel/automake",
+            },
+        }
+
+        installed = {
+            "dev-lang/perl-5.34.0-r3": {
+                "EAPI": "5",
+                "DEPEND": "sys-libs/zlib",
+                "RDEPEND": "sys-libs/zlib",
+                "SLOT": "0/5.34",
+            },
+            "dev-perl/Locale-gettext-1.70.0-r1": {
+                "EAPI": "5",
+                "DEPEND": "dev-lang/perl",
+                "RDEPEND": "dev-lang/perl:0/5.34=",
+            },
+            "sys-apps/help2man-1.48.5": {
+                "EAPI": "5",
+                "DEPEND": "dev-lang/perl dev-perl/Locale-gettext",
+                "RDEPEND": "dev-lang/perl dev-perl/Locale-gettext",
+            },
+            "sys-devel/automake-1.16.4": {
+                "EAPI": "5",
+                "DEPEND": "dev-lang/perl",
+                "RDEPEND": "dev-lang/perl",
+            },
+            "sys-libs/zlib-1.2.11-r4": {
+                "EAPI": "5",
+                "DEPEND": "sys-devel/automake",
+            },
+        }
+
+        world = ["sys-apps/help2man"]
+
+        test_cases = (
+            ResolverPlaygroundTestCase(
+                ["@world"],
+                options={"--deep": True, "--update": True, "--verbose": True},
+                success=True,
+                ambiguous_merge_order=True,
+                merge_order_assertions=(
+                    (
+                        "dev-lang/perl-5.36.0-r2",
+                        "dev-perl/Locale-gettext-1.70.0-r1",
+                    ),
+                ),
+                mergelist=[
+                    "sys-devel/automake-1.16.5",
+                    "sys-libs/zlib-1.2.13-r1",
+                    "dev-lang/perl-5.36.0-r2",
+                    "dev-perl/Locale-gettext-1.70.0-r1",
+                    "sys-apps/help2man-1.49.3",
+                ],
+            ),
+        )
+
+        playground = ResolverPlayground(
+            ebuilds=ebuilds,
+            installed=installed,
+            world=world,
+        )
+        try:
+            for test_case in test_cases:
+                playground.run_TestCase(test_case)
+                self.assertEqual(test_case.test_success, True, test_case.fail_msg)
+        finally:
+            playground.cleanup()