Put back check that a timeline must have ancestor or some layer files.

pageserver/src/tenant.rs

@@ -471,7 +471,7 @@ impl Tenant {
                         .create_timeline_data(
                             timeline_id,
                             up_to_date_metadata.clone(),
-                            ancestor,
+                            ancestor.clone(),
                             None,
                         )
                         .with_context(|| {
@@ -499,6 +499,13 @@ impl Tenant {
                 .context("failed to reconcile with remote")?
         }
 
+        // Sanity check: a timeline should have some content.
+        anyhow::ensure!(
+            ancestor.is_some() ||
+                timeline.layers.read().unwrap().iter_historic_layers().next().is_some(),
+            "Timeline has no ancestor and no layer files"
+        );
+
         // Save the metadata file to local disk.
         if !picked_local {
             save_metadata(

test_runner/regress/test_broken_timeline.py

@@ -20,6 +20,7 @@ def test_broken_timeline(neon_env_builder: NeonEnvBuilder):
             ".*is not active. Current state: Broken.*",
             ".*will not become active. Current state: Broken.*",
             ".*failed to load metadata.*",
+            ".*could not load tenant.*load local timeline.*",
         ]
     )
 
@@ -86,12 +87,14 @@ def test_broken_timeline(neon_env_builder: NeonEnvBuilder):
         f"As expected, compute startup failed eagerly for timeline with corrupt metadata: {err}"
     )
 
-    # Second timeline has no ancestors, only the metadata file and no layer files
-    # This will fail with an error like "extracting base backup failed" and cause
-    # "could not find data for key"
-    with pytest.raises(Exception, match=".*could not find data for key.*") as err:
+    # Second timeline has no ancestors, only the metadata file and no layer files.
+    # That is checked explicitly in the pageserver, and causes the tenant to be marked
+    # as broken.
+    with pytest.raises(
+        Exception, match=f"Tenant {tenant2} will not become active. Current state: Broken"
+    ) as err:
         pg2.start()
-    log.info(f"As expected, compute startup failed eagerly for timeline with missing layers: {err}")
+    log.info(f"As expected, compute startup failed for timeline with missing layers: {err}")
 
     # Third timeline will also fail during basebackup, because the layer file is corrupt.
     # (We don't check layer file contents on startup, when loading the timeline)