remove snapshot id; replace with timeline id

The snapshot id doesn't make sense when two snapshots are squashed.
Better to use the timeline id anyway.

It's still a little strange to squash two timelines together, but it
must be possible to flatten history even if there have been many branch
events, so this is should be a control plane / snapshot management
problem.

Cargo.lock

@@ -1972,6 +1972,7 @@ dependencies = [
  "anyhow",
  "aversion",
  "bookfile",
+ "hex",
  "rand",
  "serde",
  "structopt",

snapfile/Cargo.toml

@@ -15,6 +15,7 @@ serde = { version = "1.0", features = ["derive"] }
 rand = "0.8.3"
 structopt = "0.3"
 zenith_utils = { path = "../zenith_utils" }
+hex = "0.4.3"
 
 [dev-dependencies]
 tempfile = "3.2"

snapfile/src/lib.rs

@@ -27,23 +27,27 @@ pub use versioned::{PageIndex, PageLocation, Predecessor, SnapFileMeta};
 use zenith_utils::lsn::Lsn;
 
 impl SnapFileMeta {
-    pub fn new(previous: Option<SnapFileMeta>, lsn: Lsn) -> Self {
+    pub fn new(previous: Option<SnapFileMeta>, timeline: [u8; 16], lsn: Lsn) -> Self {
         // Store the metadata of the predecessor snapshot, if there is one.
         let predecessor = previous.map(|prev| Predecessor {
-            id: prev.snap_id,
+            timeline: prev.timeline,
             lsn: prev.lsn,
         });
 
-        let snap_id: u64 = rand::random();
         SnapFileMeta {
-            snap_id,
+            timeline,
             predecessor,
             lsn: lsn.into(),
         }
     }
 
     fn to_filename(&self) -> OsString {
-        format!("{:x}.zdb", self.snap_id).into()
+        let timeline_string = hex::encode(self.timeline);
+        let pred_lsn = match &self.predecessor {
+            None => 0,
+            Some(pred) => pred.lsn,
+        };
+        format!("{}_{:x}_{:x}.zdb", timeline_string, pred_lsn, self.lsn).into()
     }
 }
 
@@ -266,13 +270,15 @@ mod tests {
     use std::path::PathBuf;
     use tempfile::TempDir;
 
+    const TEST_TIMELINE: [u8; 16] = [99u8; 16];
+
     #[test]
     fn snap_two_pages() {
         // When `dir` goes out of scope the directory will be unlinked.
         let dir = TempDir::new().unwrap();
         let snap_meta = {
             // Write out a new snapshot file with two pages.
-            let meta = SnapFileMeta::new(None, Lsn(1234));
+            let meta = SnapFileMeta::new(None, TEST_TIMELINE, Lsn(1234));
             let mut snap = SnapWriter::new(dir.path(), meta).unwrap();
             // Write the pages out of order, because why not?
             let page99 = [99u8; 8192];
@@ -313,7 +319,7 @@ mod tests {
         let dir = TempDir::new().unwrap();
         let snap_meta = {
             // Write out a new snapshot file with no pages.
-            let meta = SnapFileMeta::new(None, Lsn(1234));
+            let meta = SnapFileMeta::new(None, TEST_TIMELINE, Lsn(1234));
             let snap = SnapWriter::new(dir.path(), meta).unwrap();
             snap.finish().unwrap()
         };

snapfile/src/squash.rs

@@ -42,7 +42,7 @@ pub fn squash(older: &Path, newer: &Path, out_dir: &Path) -> Result<()> {
 
     // Check that snap1 is the predecessor of snap2.
     match meta2.predecessor {
-        Some(pred) if pred.id == meta1.snap_id => {}
+        Some(pred) if pred.timeline == meta1.timeline => {}
         _ => {
             bail!(
                 "snap file {:?} is not the predecessor of {:?}",
@@ -55,11 +55,12 @@ pub fn squash(older: &Path, newer: &Path, out_dir: &Path) -> Result<()> {
     // The new combined snapshot will have most fields from meta2 (the later
     // snapshot), but will have the predecessor from meta1.
     let new_meta = SnapFileMeta {
-        // FIXME: Wow, this seems wrong. Need to sort out what to do here.
-        // should snap_id even exist? The fact that we plan on squashing
-        // snapshots often implies that maybe they shouldn't.
-        // How do we identify predecessor? timeline_id + lsn?
-        snap_id: meta2.snap_id,
+        // There is some danger in squashing snapshots across two timelines,
+        // in that it's possible to get confused about what the history
+        // looks like. Ultimately, it should be possible to squash our way
+        // to a "complete" snapshot (that contains all pages), so this must
+        // be possible.
+        timeline: meta2.timeline,
         predecessor: meta1.predecessor,
         lsn: meta2.lsn,
     };

snapfile/src/versioned.rs

@@ -30,21 +30,22 @@ pub(crate) const CHAPTER_PAGE_INDEX: u64 = 3;
 /// of that snapshot.
 #[derive(Debug, PartialEq, Serialize, Deserialize)]
 pub struct Predecessor {
-    /// This is the id number of the previous snapshot.
+    /// This is the ID number of the predecessor timeline.
     ///
-    /// This must match the snap_id of the previous snapshot.
-    pub id: u64,
+    /// This may match the current snapshot's timeline id, but
+    /// it may not (if the precessor was the branch point).
+    pub timeline: [u8; 16],
 
-    /// This is the LSN of the previous snapshot.
+    /// This is the LSN of the predecessor snapshot.
     pub lsn: u64,
 }
 
 #[derive(Debug, PartialEq, Serialize, Deserialize, Versioned, UpgradeLatest)]
 pub struct SnapFileMetaV1 {
-    /// This is a unique ID number for this snapshot.
+    /// This is a unique ID number for this timeline.
     ///
     /// This number guarantees that snapshot history is unique.
-    pub snap_id: u64,
+    pub timeline: [u8; 16],
 
     /// Information about the predecessor snapshot.
     ///