Support read-only nodes

Change 'zenith.signal' file to a human-readable format, similar to
backup_label. It can contain a "PREV LSN: %X/%X" line, or a special
value to indicate that it's OK to start with invalid LSN ('none'), or
that it's a read-only node and generating WAL is forbidden
('invalid').

The 'zenith pg create' and 'zenith pg start' commands now take a node
name parameter, separate from the branch name. If the node name is not
given, it defaults to the branch name, so this doesn't break existing
scripts.

If you pass "foo@<lsn>" as the branch name, a read-only node anchored
at that LSN is created. The anchoring is performed by setting the
'recovery_target_lsn' option in the postgresql.conf file, and putting
the server into standby mode with 'standby.signal'.

We no longer store the synthetic checkpoint record in the WAL segment.
The postgres startup code has been changed to use the copy of the
checkpoint record in the pg_control file, when starting in zenith
mode.

pageserver/src/basebackup.rs

@@ -13,6 +13,7 @@
 use anyhow::Result;
 use bytes::{BufMut, BytesMut};
 use log::*;
+use std::fmt::Write as FmtWrite;
 use std::io;
 use std::io::Write;
 use std::sync::Arc;
@@ -83,7 +84,7 @@ impl<'a> Basebackup<'a> {
 
         info!(
             "taking basebackup lsn={}, prev_lsn={}",
-            backup_prev, backup_lsn
+            backup_lsn, backup_prev
         );
 
         Ok(Basebackup {
@@ -248,13 +249,7 @@ impl<'a> Basebackup<'a> {
         let mut pg_control = ControlFileData::decode(&pg_control_bytes)?;
         let mut checkpoint = CheckPoint::decode(&checkpoint_bytes)?;
 
-        // Generate new pg_control and WAL needed for bootstrap
-        let checkpoint_segno = self.lsn.segment_number(pg_constants::WAL_SEGMENT_SIZE);
-        let checkpoint_lsn = XLogSegNoOffsetToRecPtr(
-            checkpoint_segno,
-            XLOG_SIZE_OF_XLOG_LONG_PHD as u32,
-            pg_constants::WAL_SEGMENT_SIZE,
-        );
+        // Generate new pg_control needed for bootstrap
         checkpoint.redo = normalize_lsn(self.lsn, pg_constants::WAL_SEGMENT_SIZE).0;
 
         //reset some fields we don't want to preserve
@@ -263,19 +258,24 @@ impl<'a> Basebackup<'a> {
         checkpoint.oldestActiveXid = 0;
 
         //save new values in pg_control
-        pg_control.checkPoint = checkpoint_lsn;
+        pg_control.checkPoint = 0;
         pg_control.checkPointCopy = checkpoint;
         pg_control.state = pg_constants::DB_SHUTDOWNED;
 
         // add zenith.signal file
-        let xl_prev = if self.prev_record_lsn == Lsn(0) {
-            0xBAD0 // magic value to indicate that we don't know prev_lsn
+        let mut zenith_signal = String::new();
+        if self.prev_record_lsn == Lsn(0) {
+            if self.lsn == self.timeline.get_ancestor_lsn() {
+                write!(zenith_signal, "PREV LSN: none")?;
+            } else {
+                write!(zenith_signal, "PREV LSN: invalid")?;
+            }
         } else {
-            self.prev_record_lsn.0
-        };
+            write!(zenith_signal, "PREV LSN: {}", self.prev_record_lsn)?;
+        }
         self.ar.append(
-            &new_tar_header("zenith.signal", 8)?,
-            &xl_prev.to_le_bytes()[..],
+            &new_tar_header("zenith.signal", zenith_signal.len() as u64)?,
+            zenith_signal.as_bytes(),
         )?;
 
         //send pg_control
@@ -284,14 +284,15 @@ impl<'a> Basebackup<'a> {
         self.ar.append(&header, &pg_control_bytes[..])?;
 
         //send wal segment
+        let segno = self.lsn.segment_number(pg_constants::WAL_SEGMENT_SIZE);
         let wal_file_name = XLogFileName(
             1, // FIXME: always use Postgres timeline 1
-            checkpoint_segno,
+            segno,
             pg_constants::WAL_SEGMENT_SIZE,
         );
         let wal_file_path = format!("pg_wal/{}", wal_file_name);
         let header = new_tar_header(&wal_file_path, pg_constants::WAL_SEGMENT_SIZE as u64)?;
-        let wal_seg = generate_wal_segment(&pg_control);
+        let wal_seg = generate_wal_segment(segno, pg_control.system_identifier);
         assert!(wal_seg.len() == pg_constants::WAL_SEGMENT_SIZE);
         self.ar.append(&header, &wal_seg[..])?;
         Ok(())

pageserver/src/layered_repository.rs

@@ -678,6 +678,10 @@ pub struct LayeredTimeline {
 
 /// Public interface functions
 impl Timeline for LayeredTimeline {
+    fn get_ancestor_lsn(&self) -> Lsn {
+        self.ancestor_lsn
+    }
+
     /// Wait until WAL has been received up to the given LSN.
     fn wait_lsn(&self, lsn: Lsn) -> Result<()> {
         // This should never be called from the WAL receiver thread, because that could lead

pageserver/src/repository.rs

@@ -119,6 +119,9 @@ pub trait Timeline: Send + Sync {
     /// Get a list of all existing non-relational objects
     fn list_nonrels(&self, lsn: Lsn) -> Result<HashSet<RelishTag>>;
 
+    /// Get the LSN where this branch was created
+    fn get_ancestor_lsn(&self) -> Lsn;
+
     //------------------------------------------------------------------------------
     // Public PUT functions, to update the repository with new page versions.
     //