Merge "Report a config error for unsupported merge mode"

13 files changed, 401 insertions, 15 deletions

diff --git a/doc/source/developer/model-changelog.rst b/doc/source/developer/model-changelog.rst
index c6d8fedbd..a14ff9895 100644
--- a/doc/source/developer/model-changelog.rst
+++ b/doc/source/developer/model-changelog.rst
@@ -100,3 +100,9 @@ Version 10
 :Prior Zuul version: 6.4.0
 :Description: Renames admin_rules to authz_rules in unparsed abide.
               Affects schedulers and web.
+
+Version 11
+----------
+
+:Prior Zuul version: 8.0.1
+:Description: Adds merge_modes to branch cache.  Affects schedulers and web.
diff --git a/tests/fakegithub.py b/tests/fakegithub.py
index 4255d5022..725c083e2 100644
--- a/tests/fakegithub.py
+++ b/tests/fakegithub.py
@@ -238,6 +238,11 @@ class FakeRepository(object):
 
         # List of branch protection rules
         self._branch_protection_rules = defaultdict(FakeBranchProtectionRule)
+        self._repodata = {
+            'allow_merge_commit': True,
+            'allow_squash_merge': True,
+            'allow_rebase_merge': True,
+        }
 
         # fail the next commit requests with 404
         self.fail_not_found = 0
@@ -350,6 +355,8 @@ class FakeRepository(object):
             return self.get_url_collaborators(request)
         if entity == 'commits':
             return self.get_url_commits(request, params=params)
+        if entity == '':
+            return self.get_url_repo()
         else:
             return None
 
@@ -444,6 +451,9 @@ class FakeRepository(object):
         }
         return FakeResponse(data)
 
+    def get_url_repo(self):
+        return FakeResponse(self._repodata)
+
     def pull_requests(self, state=None, sort=None, direction=None):
         # sort and direction are unused currently, but present to match
         # real world call signatures.
@@ -752,7 +762,12 @@ class FakeGithubSession(object):
             return FakeResponse(check_run.as_dict(), 200)
 
     def get_repo(self, request, params=None):
-        org, project, request = request.split('/', 2)
+        parts = request.split('/', 2)
+        if len(parts) == 2:
+            org, project = parts
+            request = ''
+        else:
+            org, project, request = parts
         project_name = '{}/{}'.format(org, project)
 
         repo = self.client.repo_from_project(project_name)
diff --git a/tests/fixtures/layouts/github-merge-mode.yaml b/tests/fixtures/layouts/github-merge-mode.yaml
new file mode 100644
index 000000000..139db886a
--- /dev/null
+++ b/tests/fixtures/layouts/github-merge-mode.yaml
@@ -0,0 +1,21 @@
+- pipeline:
+    name: check
+    manager: independent
+    trigger:
+      github:
+        - event: pull_request_review
+          action: submitted
+          state: approve
+    start:
+      github: {}
+    success:
+      github: {}
+    failure:
+      github: {}
+
+- project:
+    name: org/project
+    merge-mode: rebase
+    gate:
+      jobs:
+        - noop
diff --git a/tests/unit/test_github_driver.py b/tests/unit/test_github_driver.py
index 47cc6c624..e3706440b 100644
--- a/tests/unit/test_github_driver.py
+++ b/tests/unit/test_github_driver.py
@@ -1463,6 +1463,27 @@ class TestGithubDriver(ZuulTestCase):
         self.assertEqual('SUCCESS',
                          self.getJobFromHistory('project-test2').result)
 
+    @simple_layout('layouts/github-merge-mode.yaml', driver='github')
+    def test_merge_method_syntax_check(self):
+        """
+        Tests that the merge mode gets forwarded to the reporter and the
+        PR was rebased.
+        """
+        github = self.fake_github.getGithubClient()
+        repo = github.repo_from_project('org/project')
+        repo._repodata['allow_rebase_merge'] = False
+        self.scheds.execute(lambda app: app.sched.reconfigure(app.config))
+        self.waitUntilSettled()
+
+        tenant = self.scheds.first.sched.abide.tenants.get('tenant-one')
+        loading_errors = tenant.layout.loading_errors
+        self.assertEquals(
+            len(tenant.layout.loading_errors), 1,
+            "An error should have been stored")
+        self.assertIn(
+            "rebase not supported",
+            str(loading_errors[0].error))
+
 
 class TestMultiGithubDriver(ZuulTestCase):
     config_file = 'zuul-multi-github.conf'
diff --git a/tests/unit/test_model_upgrade.py b/tests/unit/test_model_upgrade.py
index 8e4cdc20c..f392e8c3e 100644
--- a/tests/unit/test_model_upgrade.py
+++ b/tests/unit/test_model_upgrade.py
@@ -17,6 +17,7 @@ import json
 from zuul.zk.components import ComponentRegistry
 
 from tests.base import ZuulTestCase, simple_layout, iterate_timeout
+from tests.base import ZuulWebFixture
 
 
 def model_version(version):
@@ -305,6 +306,85 @@ class TestGithubModelUpgrade(ZuulTestCase):
         ], ordered=False)
         self.assertTrue(A.is_merged)
 
+    @model_version(10)
+    @simple_layout('layouts/github-merge-mode.yaml', driver='github')
+    def test_merge_method_syntax_check(self):
+        """
+        Tests that the merge mode gets forwarded to the reporter and the
+        PR was rebased.
+        """
+        webfixture = self.useFixture(
+            ZuulWebFixture(self.changes, self.config,
+                           self.additional_event_queues, self.upstream_root,
+                           self.poller_events,
+                           self.git_url_with_auth, self.addCleanup,
+                           self.test_root))
+        sched = self.scheds.first.sched
+        web = webfixture.web
+
+        github = self.fake_github.getGithubClient()
+        repo = github.repo_from_project('org/project')
+        repo._repodata['allow_rebase_merge'] = False
+        self.scheds.execute(lambda app: app.sched.reconfigure(app.config))
+        self.waitUntilSettled()
+
+        # Verify that there are no errors with model version 9 (we
+        # should be using the defaultdict that indicates all merge
+        # modes are supported).
+        tenant = sched.abide.tenants.get('tenant-one')
+        self.assertEquals(len(tenant.layout.loading_errors), 0)
+
+        # Upgrade our component
+        self.model_test_component_info.model_api = 11
+
+        # Perform a smart reconfiguration which should not clear the
+        # cache; we should continue to see no errors because we should
+        # still be using the defaultdict.
+        self.scheds.first.smartReconfigure()
+        tenant = sched.abide.tenants.get('tenant-one')
+        self.assertEquals(len(tenant.layout.loading_errors), 0)
+
+        # Wait for web to have the same config
+        for _ in iterate_timeout(10, "config is synced"):
+            if (web.tenant_layout_state.get('tenant-one') ==
+                web.local_layout_state.get('tenant-one')):
+                break
+
+        # Repeat the check
+        tenant = web.abide.tenants.get('tenant-one')
+        self.assertEquals(len(tenant.layout.loading_errors), 0)
+
+        # Perform a full reconfiguration which should cause us to
+        # actually query, update the branch cache, and report an
+        # error.
+        self.scheds.first.fullReconfigure()
+        self.waitUntilSettled()
+
+        tenant = sched.abide.tenants.get('tenant-one')
+        loading_errors = tenant.layout.loading_errors
+        self.assertEquals(
+            len(tenant.layout.loading_errors), 1,
+            "An error should have been stored in sched")
+        self.assertIn(
+            "rebase not supported",
+            str(loading_errors[0].error))
+
+        # Wait for web to have the same config
+        for _ in iterate_timeout(10, "config is synced"):
+            if (web.tenant_layout_state.get('tenant-one') ==
+                web.local_layout_state.get('tenant-one')):
+                break
+
+        # Repoat the check for web
+        tenant = web.abide.tenants.get('tenant-one')
+        loading_errors = tenant.layout.loading_errors
+        self.assertEquals(
+            len(tenant.layout.loading_errors), 1,
+            "An error should have been stored in web")
+        self.assertIn(
+            "rebase not supported",
+            str(loading_errors[0].error))
+
 
 class TestDeduplication(ZuulTestCase):
     config_file = "zuul-gerrit-github.conf"
diff --git a/zuul/configloader.py b/zuul/configloader.py
index 67d4494c4..08b517618 100644
--- a/zuul/configloader.py
+++ b/zuul/configloader.py
@@ -1858,6 +1858,9 @@ class TenantParser(object):
         tpc.branches = static_branches
         tpc.dynamic_branches = always_dynamic_branches
 
+        tpc.merge_modes = tpc.project.source.getProjectMergeModes(
+            tpc.project, tenant, min_ltime)
+
     def _loadProjectKeys(self, connection_name, project):
         project.private_secrets_key, project.public_secrets_key = (
             self.keystorage.getProjectSecretsKeys(
@@ -2577,11 +2580,22 @@ class TenantParser(object):
             # Set a merge mode if we don't have one for this project.
             # This can happen if there are only regex project stanzas
             # but no specific project stanzas.
+            (trusted, project) = tenant.getProject(project_name)
             project_metadata = layout.getProjectMetadata(project_name)
             if project_metadata.merge_mode is None:
-                (trusted, project) = tenant.getProject(project_name)
                 mode = project.source.getProjectDefaultMergeMode(project)
                 project_metadata.merge_mode = model.MERGER_MAP[mode]
+            tpc = tenant.project_configs[project.canonical_name]
+            if tpc.merge_modes is not None:
+                source_context = model.ProjectContext(
+                    project.canonical_name, project.name)
+                with project_configuration_exceptions(source_context,
+                                                      layout.loading_errors):
+                    if project_metadata.merge_mode not in tpc.merge_modes:
+                        mode = model.get_merge_mode_name(
+                            project_metadata.merge_mode)
+                        raise Exception(f'Merge mode {mode} not supported '
+                                        f'by project {project_name}')
 
     def _parseLayout(self, tenant, data, loading_errors, layout_uuid=None):
         # Don't call this method from dynamic reconfiguration because
diff --git a/zuul/connection/__init__.py b/zuul/connection/__init__.py
index 53be27d38..3d4ceec55 100644
--- a/zuul/connection/__init__.py
+++ b/zuul/connection/__init__.py
@@ -15,10 +15,8 @@
 import abc
 import logging
 
-from typing import List, Optional
-
 from zuul.lib.logutil import get_annotated_logger
-from zuul.model import Project
+from zuul import model
 
 
 class ReadOnlyBranchCacheError(RuntimeError):
@@ -143,8 +141,8 @@ class ZKBranchCacheMixin:
     read_only = False
 
     @abc.abstractmethod
-    def isBranchProtected(self, project_name: str, branch_name: str,
-                          zuul_event_id) -> Optional[bool]:
+    def isBranchProtected(self, project_name, branch_name,
+                          zuul_event_id):
         """Return if the branch is protected or None if the branch is unknown.
 
         :param str project_name:
@@ -157,9 +155,35 @@ class ZKBranchCacheMixin:
         pass
 
     @abc.abstractmethod
-    def _fetchProjectBranches(self, project: Project,
-                              exclude_unprotected: bool) -> List[str]:
-        pass
+    def _fetchProjectBranches(self, project, exclude_unprotected):
+        """Perform a remote query to determine the project's branches.
+
+        Connection subclasses should implement this method.
+
+        :param model.Project project:
+            The project.
+        :param bool exclude_unprotected:
+            Whether the query should exclude unprotected branches from
+            the response.
+
+        :returns: A list of branch names.
+        """
+
+    def _fetchProjectMergeModes(self, project):
+        """Perform a remote query to determine the project's supported merge
+           modes.
+
+        Connection subclasses should implement this method if they are
+        able to determine which merge modes apply for a project.  The
+        default implemantion returns that all merge modes are valid.
+
+        :param model.Project project:
+            The project.
+
+        :returns: A list of merge modes as model IDs.
+
+        """
+        return model.ALL_MERGE_MODES
 
     def clearConnectionCacheOnBranchEvent(self, event):
         """Update event and clear connection cache if needed.
@@ -214,8 +238,7 @@ class ZKBranchCacheMixin:
 
         # Update them if we have them
         if protected_branches is not None:
-            protected_branches = self._fetchProjectBranches(
-                project, True)
+            protected_branches = self._fetchProjectBranches(project, True)
             self._branch_cache.setProjectBranches(
                 project.name, True, protected_branches)
 
@@ -223,6 +246,10 @@ class ZKBranchCacheMixin:
             all_branches = self._fetchProjectBranches(project, False)
             self._branch_cache.setProjectBranches(
                 project.name, False, all_branches)
+
+        merge_modes = self._fetchProjectMergeModes(project)
+        self._branch_cache.setProjectMergeModes(
+            project.name, merge_modes)
         self.log.info("Got branches for %s" % project.name)
 
     def getProjectBranches(self, project, tenant, min_ltime=-1):
@@ -282,6 +309,62 @@ class ZKBranchCacheMixin:
 
         return sorted(branches)
 
+    def getProjectMergeModes(self, project, tenant, min_ltime=-1):
+        """Get the merge modes for the given project.
+
+        :param zuul.model.Project project:
+            The project for which the merge modes are returned.
+        :param zuul.model.Tenant tenant:
+            The related tenant.
+        :param int min_ltime:
+            The minimum ltime to determine if we need to refresh the cache.
+
+        :returns: The list of merge modes by model id.
+        """
+        merge_modes = None
+
+        if self._branch_cache:
+            try:
+                merge_modes = self._branch_cache.getProjectMergeModes(
+                    project.name, min_ltime)
+            except LookupError:
+                if self.read_only:
+                    # A scheduler hasn't attempted to fetch them yet
+                    raise ReadOnlyBranchCacheError(
+                        "Will not fetch merge modes as read-only is set")
+                else:
+                    merge_modes = None
+
+        if merge_modes is not None:
+            return merge_modes
+        elif self.read_only:
+            # A scheduler has previously attempted a fetch, but got
+            # the None due to an error; we can't retry since we're
+            # read-only.
+            raise RuntimeError(
+                "Will not fetch merge_modes as read-only is set")
+
+        # We need to perform a query
+        try:
+            merge_modes = self._fetchProjectMergeModes(project)
+        except Exception:
+            # We weren't able to get the merge modes.  We need to tell
+            # future schedulers to try again but tell zuul-web that we
+            # tried and failed.  Set the merge modes to None to indicate
+            # that we have performed a fetch and retrieved no data.  Any
+            # time we encounter None in the cache, we will try again.
+            if self._branch_cache:
+                self._branch_cache.setProjectMergeModes(
+                    project.name, None)
+            raise
+        self.log.info("Got merge modes for %s" % project.name)
+
+        if self._branch_cache:
+            self._branch_cache.setProjectMergeModes(
+                project.name, merge_modes)
+
+        return merge_modes
+
     def checkBranchCache(self, project_name: str, event,
                          protected: bool = None) -> None:
         """Clear the cache for a project when a branch event is processed
diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index a2a55b050..631d98998 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -48,6 +48,7 @@ from zuul.driver.github.graphql import GraphQLClient
 from zuul.lib import tracing
 from zuul.web.handler import BaseWebController
 from zuul.lib.logutil import get_annotated_logger
+from zuul import model
 from zuul.model import Ref, Branch, Tag, Project
 from zuul.exceptions import MergeFailure
 from zuul.driver.github.githubmodel import PullRequest, GithubTriggerEvent
@@ -64,6 +65,12 @@ GITHUB_BASE_URL = 'https://api.github.com'
 PREVIEW_JSON_ACCEPT = 'application/vnd.github.machine-man-preview+json'
 PREVIEW_DRAFT_ACCEPT = 'application/vnd.github.shadow-cat-preview+json'
 PREVIEW_CHECKS_ACCEPT = 'application/vnd.github.antiope-preview+json'
+ALL_MERGE_MODES = [
+    model.MERGER_MERGE,
+    model.MERGER_MERGE_RESOLVE,
+    model.MERGER_SQUASH_MERGE,
+    model.MERGER_REBASE,
+]
 
 # NOTE (felix): Using log levels for file comments / annotations is IMHO more
 # convenient than the values Github expects. Having in mind that those comments
@@ -956,8 +963,9 @@ class GithubClientManager:
                 if 'cache-control' in response.headers:
                     del response.headers['cache-control']
 
+        self._cache = DictCache()
         self.cache_adapter = cachecontrol.CacheControlAdapter(
-            DictCache(),
+            self._cache,
             cache_etags=True,
             heuristic=NoAgeHeuristic())
 
@@ -1776,6 +1784,42 @@ class GithubConnection(ZKChangeCacheMixin, ZKBranchCacheMixin, BaseConnection):
 
         return branches
 
+    def _fetchProjectMergeModes(self, project):
+        github = self.getGithubClient(project.name)
+        url = github.session.build_url('repos', project.name)
+        headers = {'Accept': 'application/vnd.github.loki-preview+json'}
+        merge_modes = []
+
+        # GitHub API bug: if the allow_* attributes below are changed,
+        # the ETag is not updated, meaning that once we cache the repo
+        # URL, we'll never update it.  To avoid this, clear this URL
+        # from the cache before performing the request.
+        self._github_client_manager._cache.data.pop(url, None)
+
+        resp = github.session.get(url, headers=headers)
+
+        if resp.status_code == 403:
+            self.log.error(str(resp))
+            rate_limit = github.rate_limit()
+            if rate_limit['resources']['core']['remaining'] == 0:
+                self.log.warning(
+                    "Rate limit exceeded, using full merge method list")
+            return ALL_MERGE_MODES
+        elif resp.status_code == 404:
+            raise Exception("Got status code 404 when fetching "
+                            "project %s" % project.name)
+
+        resp = resp.json()
+        if resp.get('allow_merge_commit'):
+            merge_modes.append(model.MERGER_MERGE)
+            merge_modes.append(model.MERGER_MERGE_RESOLVE)
+        if resp.get('allow_squash_merge'):
+            merge_modes.append(model.MERGER_SQUASH_MERGE)
+        if resp.get('allow_rebase_merge'):
+            merge_modes.append(model.MERGER_REBASE)
+
+        return merge_modes
+
     def isBranchProtected(self, project_name: str, branch_name: str,
                           zuul_event_id=None) -> Optional[bool]:
         github = self.getGithubClient(
diff --git a/zuul/driver/github/githubsource.py b/zuul/driver/github/githubsource.py
index bdc373f79..0a94a1730 100644
--- a/zuul/driver/github/githubsource.py
+++ b/zuul/driver/github/githubsource.py
@@ -135,6 +135,9 @@ class GithubSource(BaseSource):
     def getProjectBranches(self, project, tenant, min_ltime=-1):
         return self.connection.getProjectBranches(project, tenant, min_ltime)
 
+    def getProjectMergeModes(self, project, tenant, min_ltime=-1):
+        return self.connection.getProjectMergeModes(project, tenant, min_ltime)
+
     def getProjectBranchCacheLtime(self):
         return self.connection._branch_cache.ltime
 
diff --git a/zuul/model.py b/zuul/model.py
index e339b1ffa..2ca7b425b 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -64,6 +64,7 @@ MERGER_MAP = {
     'squash-merge': MERGER_SQUASH_MERGE,
     'rebase': MERGER_REBASE,
 }
+ALL_MERGE_MODES = list(MERGER_MAP.values())
 
 PRECEDENCE_NORMAL = 0
 PRECEDENCE_LOW = 1
@@ -6968,6 +6969,7 @@ class TenantProjectConfig(object):
         self.extra_config_dirs = ()
         # Load config from a different branch if this is a config project
         self.load_branch = None
+        self.merge_modes = None
 
     def isAlwaysDynamicBranch(self, branch):
         if self.always_dynamic_branches is None:
diff --git a/zuul/model_api.py b/zuul/model_api.py
index 9542cf415..6c93a5177 100644
--- a/zuul/model_api.py
+++ b/zuul/model_api.py
@@ -14,4 +14,4 @@
 
 # When making ZK schema changes, increment this and add a record to
 # docs/developer/model-changelog.rst
-MODEL_API = 10
+MODEL_API = 11
diff --git a/zuul/source/__init__.py b/zuul/source/__init__.py
index faf97b48b..dd11aa9b6 100644
--- a/zuul/source/__init__.py
+++ b/zuul/source/__init__.py
@@ -15,6 +15,8 @@
 import abc
 import time
 
+from zuul import model
+
 
 class BaseSource(object, metaclass=abc.ABCMeta):
     """Base class for sources.
@@ -183,6 +185,20 @@ class BaseSource(object, metaclass=abc.ABCMeta):
 
         """
 
+    def getProjectMergeModes(self, project, tenant, min_ltime=-1):
+        """Get supported merge modes for a project
+
+        This method is called very frequently, and should generally
+        return quickly.  The connection is expected to cache merge
+        modes for all projects queried.
+
+        The default implementation indicates that all merge modes are
+        supported.
+
+        """
+
+        return model.ALL_MERGE_MODES
+
     @abc.abstractmethod
     def getProjectBranchCacheLtime(self):
         """Return the current ltime of the project branch cache."""
diff --git a/zuul/zk/branch_cache.py b/zuul/zk/branch_cache.py
index 6fa531fad..0a8872158 100644
--- a/zuul/zk/branch_cache.py
+++ b/zuul/zk/branch_cache.py
@@ -13,11 +13,15 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
+
+import collections
 import logging
 import json
 
 from zuul.zk.zkobject import ZKContext, ShardedZKObject
 from zuul.zk.locks import SessionAwareReadLock, SessionAwareWriteLock, locked
+from zuul.zk.components import COMPONENT_REGISTRY
+from zuul import model
 
 from kazoo.exceptions import NoNodeError
 
@@ -63,15 +67,28 @@ class BranchCacheZKObject(ShardedZKObject):
     def __init__(self):
         super().__init__()
         self._set(protected={},
-                  remainder={})
+                  remainder={},
+                  merge_modes={})
 
     def serialize(self, context):
         data = {
             "protected": self.protected,
             "remainder": self.remainder,
         }
+        # This is mostly here to enable unit tests of upgrades, it's
+        # safe to move into the dict above at any time.
+        if (COMPONENT_REGISTRY.model_api >= 11):
+            data["merge_modes"] = self.merge_modes
         return json.dumps(data, sort_keys=True).encode("utf8")
 
+    def deserialize(self, raw, context):
+        data = super().deserialize(raw, context)
+        # MODEL_API < 11
+        if "merge_modes" not in data:
+            data["merge_modes"] = collections.defaultdict(
+                lambda: model.ALL_MERGE_MODES)
+        return data
+
     def _save(self, context, data, create=False):
         super()._save(context, data, create)
         zstat = context.client.exists(self.getPath())
@@ -119,10 +136,12 @@ class BranchCache:
             if projects is None:
                 self.cache.protected.clear()
                 self.cache.remainder.clear()
+                self.cache.merge_modes.clear()
             else:
                 for p in projects:
                     self.cache.protected.pop(p, None)
                     self.cache.remainder.pop(p, None)
+                    self.cache.merge_modes.pop(p, None)
 
     def getProjectBranches(self, project_name, exclude_unprotected,
                            min_ltime=-1, default=RAISE_EXCEPTION):
@@ -255,6 +274,68 @@ class BranchCache:
                     if branch not in remainder_branches:
                         remainder_branches.append(branch)
 
+    def getProjectMergeModes(self, project_name,
+                             min_ltime=-1, default=RAISE_EXCEPTION):
+        """Get the merge modes for the given project.
+
+        Checking the branch cache we need to distinguish three different
+        cases:
+
+            1. cache miss (not queried yet)
+            2. cache hit (including empty list of merge modes)
+            3. error when fetching merge modes
+
+        If the cache doesn't contain any merge modes for the project and no
+        default value is provided a LookupError is raised.
+
+        If there was an error fetching the merge modes, the return value
+        will be None.
+
+        Otherwise the list of merge modes will be returned.
+
+        :param str project_name:
+            The project for which the merge modes are returned.
+        :param int min_ltime:
+            The minimum cache ltime to consider the cache valid.
+        :param any default:
+            Optional default value to return if no cache entry exits.
+
+        :returns: The list of merge modes by model id, or None if there was
+            an error when fetching the merge modes.
+        """
+        if self.ltime < min_ltime:
+            with locked(self.rlock):
+                self.cache.refresh(self.zk_context)
+
+        merge_modes = None
+        try:
+            merge_modes = self.cache.merge_modes[project_name]
+        except KeyError:
+            if default is RAISE_EXCEPTION:
+                raise LookupError(
+                    f"No merge modes for project {project_name}")
+            else:
+                return default
+
+        return merge_modes
+
+    def setProjectMergeModes(self, project_name, merge_modes):
+        """Set the supported merge modes for the given project.
+
+        Use None as a sentinel value for the merge modes to indicate
+        that there was a fetch error.
+
+        :param str project_name:
+            The project for the merge modes.
+        :param list[int] merge_modes:
+            The list of merge modes (by model ID) or None.
+
+        """
+
+        with locked(self.wlock):
+            with self.cache.activeContext(self.zk_context):
+                self.cache.merge_modes[project_name] = merge_modes
+
     @property
     def ltime(self):
         return self.cache._zstat.last_modified_transaction_id