pageserver: return 412 PreconditionFailed in get_timestamp_of_lsn if timestamp is not found (#11491)

## Problem
Now `get_timestamp_of_lsn` returns `404 NotFound` if there is no clog
pages for given LSN, and it's difficult to distinguish from other 404
errors. A separate status code for this error will allow the control
plane to handle this case.
- Closes: https://github.com/neondatabase/neon/issues/11439
- Corresponding PR in control plane:
https://github.com/neondatabase/cloud/pull/27125

## Summary of changes
- Return `412 PreconditionFailed` instead of `404 NotFound` if no
timestamp is fond for given LSN.

I looked briefly through the current error handling code in cloud.git
and the status code change should not affect anything for the existing
code. Change from the corresponding PR also looks fine and should work
with the current PS status code. Additionally, here is OK to merge it
from control plane team:
https://github.com/neondatabase/neon/issues/11439#issuecomment-2789327552

---------

Co-authored-by: John Spray <john@neon.tech>

pageserver/src/http/openapi_spec.yml

@@ -212,6 +212,12 @@ paths:
               schema:
                 type: string
                 format: date-time
+        "412":
+          description: No timestamp is found for given LSN, e.g. if there had been no commits till LSN
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/PreconditionFailedError"
 
   /v1/tenant/{tenant_id}/timeline/{timeline_id}/get_lsn_by_timestamp:
     parameters:

pageserver/src/http/routes.rs

@@ -989,7 +989,7 @@ async fn get_lsn_by_timestamp_handler(
     if !tenant_shard_id.is_shard_zero() {
         // Requires SLRU contents, which are only stored on shard zero
         return Err(ApiError::BadRequest(anyhow!(
-            "Size calculations are only available on shard zero"
+            "Lsn calculations by timestamp are only available on shard zero"
         )));
     }
 
@@ -1064,7 +1064,7 @@ async fn get_timestamp_of_lsn_handler(
     if !tenant_shard_id.is_shard_zero() {
         // Requires SLRU contents, which are only stored on shard zero
         return Err(ApiError::BadRequest(anyhow!(
-            "Size calculations are only available on shard zero"
+            "Timestamp calculations by lsn are only available on shard zero"
         )));
     }
 
@@ -1090,8 +1090,8 @@ async fn get_timestamp_of_lsn_handler(
             .to_string();
             json_response(StatusCode::OK, time)
         }
-        None => Err(ApiError::NotFound(
-            anyhow::anyhow!("Timestamp for lsn {} not found", lsn).into(),
+        None => Err(ApiError::PreconditionFailed(
+            format!("Timestamp for lsn {} not found", lsn).into(),
         )),
     }
 }

pageserver/src/pgdatadir_mapping.rs

@@ -691,7 +691,7 @@ impl Timeline {
         Ok(buf.get_u32_le())
     }
 
-    /// Get size of an SLRU segment
+    /// Does the slru segment exist?
     pub(crate) async fn get_slru_segment_exists(
         &self,
         kind: SlruKind,
@@ -844,9 +844,9 @@ impl Timeline {
         .await
     }
 
-    /// Obtain the possible timestamp range for the given lsn.
+    /// Obtain the timestamp for the given lsn.
     ///
-    /// If the lsn has no timestamps, returns None. returns `(min, max, median)` if it has timestamps.
+    /// If the lsn has no timestamps (e.g. no commits), returns None.
     pub(crate) async fn get_timestamp_for_lsn(
         &self,
         probe_lsn: Lsn,