Rework basebackup.

* add lsn argument
* do not expose wait_lsn, wait inside list_nonrels()
* fix parameters parsing
* expose get_last_record_rlsn() to atomically read (last,prev) pair

More work is needed to correctly handle basebackup@old_lsn but current
approach already allows to fix test_restart_compute

zenith_utils/src/lsn.rs

@@ -7,6 +7,8 @@ use std::path::Path;
 use std::str::FromStr;
 use std::sync::atomic::{AtomicU64, Ordering};
 
+use crate::seqwait::MonotonicCounter;
+
 /// Transaction log block size in bytes
 pub const XLOG_BLCKSZ: u32 = 8192;
 
@@ -197,6 +199,28 @@ impl AtomicLsn {
     }
 }
 
+/// Pair of LSN's pointing to the end of the last valid record and previous one
+#[derive(Debug, Clone, Copy)]
+pub struct RecordLsn {
+    /// LSN at the end of the current record
+    pub last: Lsn,
+    /// LSN at the end of the previous record
+    pub prev: Lsn,
+}
+
+/// Expose `self.last` as counter to be able to use RecordLsn in SeqWait
+impl MonotonicCounter<Lsn> for RecordLsn {
+    fn cnt_advance(&mut self, lsn: Lsn) {
+        assert!(self.last <= lsn);
+        let new_prev = self.last;
+        self.last = lsn;
+        self.prev = new_prev;
+    }
+    fn cnt_value(&self) -> Lsn {
+        self.last
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;