Safekeeper::Storage::write_wal: clarify behavior (#679)

It previously took &SafeKeeperState similar to persist(), but only for its
`server` member.
Now it takes &ServerInfo only, so there it's clear the state is not persisted.
Also added a comment about sync.

Author: yeputons

walkeeper/src/safekeeper.rs

@@ -275,8 +275,8 @@ impl AcceptorProposerMessage {
 pub trait Storage {
     /// Persist safekeeper state on disk, optionally syncing it.
     fn persist(&mut self, s: &SafeKeeperState, sync: bool) -> Result<()>;
-    /// Write piece of wal in buf to disk.
-    fn write_wal(&mut self, s: &SafeKeeperState, startpos: Lsn, buf: &[u8]) -> Result<()>;
+    /// Write piece of wal in buf to disk and sync it.
+    fn write_wal(&mut self, server: &ServerInfo, startpos: Lsn, buf: &[u8]) -> Result<()>;
 }
 
 /// SafeKeeper which consumes events (messages from compute) and provides
@@ -423,7 +423,7 @@ where
         let mut last_rec_lsn = Lsn(0);
         if !msg.wal_data.is_empty() {
             self.storage
-                .write_wal(&self.s, msg.h.begin_lsn, &msg.wal_data)?;
+                .write_wal(&self.s.server, msg.h.begin_lsn, &msg.wal_data)?;
 
             // figure out last record's end lsn for reporting (if we got the
             // whole record)
@@ -546,7 +546,7 @@ mod tests {
             Ok(())
         }
 
-        fn write_wal(&mut self, _s: &SafeKeeperState, _startpos: Lsn, _buf: &[u8]) -> Result<()> {
+        fn write_wal(&mut self, _server: &ServerInfo, _startpos: Lsn, _buf: &[u8]) -> Result<()> {
             Ok(())
         }
     }

walkeeper/src/timeline.rs

@@ -18,8 +18,8 @@ use zenith_utils::zid::{ZTenantId, ZTimelineId};
 
 use crate::replication::{HotStandbyFeedback, END_REPLICATION_MARKER};
 use crate::safekeeper::{
-    AcceptorProposerMessage, ProposerAcceptorMessage, SafeKeeper, SafeKeeperState, Storage,
-    SK_FORMAT_VERSION, SK_MAGIC,
+    AcceptorProposerMessage, ProposerAcceptorMessage, SafeKeeper, SafeKeeperState, ServerInfo,
+    Storage, SK_FORMAT_VERSION, SK_MAGIC,
 };
 use crate::WalAcceptorConf;
 use postgres_ffi::xlog_utils::{XLogFileName, XLOG_BLCKSZ};
@@ -337,14 +337,14 @@ impl Storage for FileStorage {
         Ok(())
     }
 
-    fn write_wal(&mut self, s: &SafeKeeperState, startpos: Lsn, buf: &[u8]) -> Result<()> {
+    fn write_wal(&mut self, server: &ServerInfo, startpos: Lsn, buf: &[u8]) -> Result<()> {
         let mut bytes_left: usize = buf.len();
         let mut bytes_written: usize = 0;
         let mut partial;
         let mut start_pos = startpos;
         const ZERO_BLOCK: &[u8] = &[0u8; XLOG_BLCKSZ];
-        let wal_seg_size = s.server.wal_seg_size as usize;
-        let ztli = s.server.ztli;
+        let wal_seg_size = server.wal_seg_size as usize;
+        let ztli = server.ztli;
 
         /* Extract WAL location for this block */
         let mut xlogoff = start_pos.segment_offset(wal_seg_size) as usize;
@@ -365,7 +365,7 @@ impl Storage for FileStorage {
             /* Open file */
             let segno = start_pos.segment_number(wal_seg_size);
             // note: we basically don't support changing pg timeline
-            let wal_file_name = XLogFileName(s.server.tli, segno, wal_seg_size);
+            let wal_file_name = XLogFileName(server.tli, segno, wal_seg_size);
             let wal_file_path = self
                 .conf
                 .data_dir