storcon: do not retry sk migration ops if the quorum is reached

test_runner/fixtures/neon_fixtures.py

@@ -1540,6 +1540,17 @@ class NeonEnv:
 
         raise RuntimeError(f"Pageserver with ID {id} not found")
 
+    def get_safekeeper(self, id: int) -> Safekeeper:
+        """
+        Look up a safekeeper by its ID.
+        """
+
+        for sk in self.safekeepers:
+            if sk.id == id:
+                return sk
+
+        raise RuntimeError(f"Safekeeper with ID {id} not found")
+
     def get_tenant_pageserver(self, tenant_id: TenantId | TenantShardId):
         """
         Get the NeonPageserver where this tenant shard is currently attached, according

test_runner/regress/test_safekeeper_migration.py

@@ -196,3 +196,33 @@ def test_safekeeper_migration_common_set_failpoints(neon_env_builder: NeonEnvBui
     assert (
         f"timeline {env.initial_tenant}/{env.initial_timeline} deleted" in exc.value.response.text
     )
+
+
+def test_migrate_from_unavailable_sk(neon_env_builder: NeonEnvBuilder):
+    """
+    Test that we can migrate from an unavailable safekeeper
+    if the quorum is still alive.
+    """
+    neon_env_builder.num_safekeepers = 4
+    neon_env_builder.storage_controller_config = {
+        "timelines_onto_safekeepers": True,
+        "timeline_safekeeper_count": 3,
+    }
+    env = neon_env_builder.init_start()
+    env.pageserver.allowed_errors.extend(ALLOWED_PAGESERVER_ERRORS)
+
+    mconf = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+    assert len(mconf["sk_set"]) == 3
+
+    another_sk = [sk.id for sk in env.safekeepers if sk.id not in mconf["sk_set"]][0]
+
+    unavailable_sk = mconf["sk_set"][0]
+    env.get_safekeeper(unavailable_sk).stop()
+
+    new_sk_set = mconf["sk_set"][1:] + [another_sk]
+
+    env.storage_controller.migrate_safekeepers(env.initial_tenant, env.initial_timeline, new_sk_set)
+
+    mconf = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+    assert mconf["sk_set"] == new_sk_set
+    assert mconf["generation"] == 3