some code reading comments on LSN

compute_tools/src/compute.rs

@@ -1280,7 +1280,7 @@ impl ComputeNode {
         let start_time = Utc::now();
 
         let mut sync_handle = maybe_cgexec(&self.params.pgbin)
-            .args(["--sync-safekeepers"])
+            .args(["--sync-safekeepers"]) // CF walproposer.c:289
             .env("PGDATA", &self.params.pgdata) // we cannot use -D in this mode
             .envs(if let Some(storage_auth_token) = &storage_auth_token {
                 vec![("NEON_AUTH_TOKEN", storage_auth_token)]

libs/postgres_ffi/src/waldecoder_handler.rs

@@ -162,9 +162,9 @@ impl WalStreamDecoderHandler for WalStreamDecoder {
                     // Fast path for the common case that the whole record fits on the page.
                     let pageleft = self.lsn.remaining_in_block() as u32;
                     if self.inputbuf.remaining() >= xl_tot_len as usize && xl_tot_len <= pageleft {
-                        self.lsn += xl_tot_len as u64;
+                        self.lsn += xl_tot_len as u64; /* we set self.lsn to the exclusive end of the record */
                         let recordbuf = self.inputbuf.copy_to_bytes(xl_tot_len as usize);
-                        return Ok(Some(self.complete_record(recordbuf)?));
+                        return Ok(Some(self.complete_record(recordbuf)?)); // this returns (start LSN of NEXT!? record, this record's Bytes), wtf
                     } else {
                         // Need to assemble the record from pieces. Remember the size of the
                         // record, and loop back. On next iterations, we will reach the branch

libs/wal_decoder/src/models.rs

@@ -80,8 +80,8 @@ pub struct InterpretedWalRecord {
     /// by the pageserver
     pub batch: SerializedValueBatch,
     /// Byte offset within WAL for the start of the next PG WAL record.
-    /// Usually this is the end LSN of the current record, but in case of
-    /// XLOG SWITCH records it will be within the next segment.
+    /// Usually this is the byte following the last byte of this record here,
+    /// but in case of XLOG SWITCH records it will be within the next segment.
     pub next_record_lsn: Lsn,
     /// Whether to flush all uncommitted modifications to the storage engine
     /// before ingesting this record. This is currently only used for legacy PG
@@ -213,6 +213,7 @@ pub struct XactCommon {
     pub origin_id: u16,
     // Fields below are only used for logging
     pub xl_xid: TransactionId,
+    // The `next_record_lsn` returned by wal decoder when we decoded this record.
     pub lsn: Lsn,
 }
 
@@ -255,6 +256,7 @@ pub enum XlogRecord {
 #[derive(Clone, Serialize, Deserialize)]
 pub struct RawXlogRecord {
     pub info: u8,
+    // The `next_record_lsn` returned by wal decoder when we decoded this record.
     pub lsn: Lsn,
     pub buf: Bytes,
 }

libs/wal_decoder/src/serialized_batch.rs

@@ -80,6 +80,8 @@ impl Eq for OrderedValueMeta {}
 #[derive(Serialize, Deserialize, Clone)]
 pub struct SerializedValueMeta {
     pub key: CompactKey,
+    // The `next_record_lsn` emitted by the wal_decoder for the WAL record
+    // that corresponds to this value.
     pub lsn: Lsn,
     /// Starting offset of the value for the (key, LSN) tuple
     /// in [`SerializedValueBatch::raw`]
@@ -92,6 +94,8 @@ pub struct SerializedValueMeta {
 #[derive(Serialize, Deserialize, Clone)]
 pub struct ObservedValueMeta {
     pub key: CompactKey,
+    // The `next_record_lsn` emitted by the wal_decoder for the WAL record
+    // that corresponds to this value.
     pub lsn: Lsn,
 }
 
@@ -109,7 +113,12 @@ pub struct SerializedValueBatch {
     /// by LSN. Note that entries for a key do not have to be contiguous.
     pub metadata: Vec<ValueMeta>,
 
-    /// The highest LSN of any value in the batch
+    /// The highest LSN of any value in the batch.
+    ///
+    /// The "LSN of a Value" is the `next_record_lsn` that the wal_decoder
+    /// emitted for that value, i.e., the LSN of a Value is
+    /// an LSN that is >= the next byte after the last byte of this value's
+    /// WAL record.
     pub max_lsn: Lsn,
 
     /// Number of values encoded by [`Self::raw`]

pageserver/src/pgdatadir_mapping.rs

@@ -1494,7 +1494,7 @@ pub struct DatadirModification<'a> {
     /// in the state in 'tline' yet.
     pub tline: &'a Timeline,
 
-    /// Current LSN of the modification
+    /// Current LSN of the modification.
     lsn: Lsn,
 
     // The modifications are not applied directly to the underlying key-value store.