feat(pageserver): integrate lsn lease into synthetic size (#8220)

Part of #7497, closes #8071. (accidentally closed #8208, reopened here)

## Problem

After the changes in #8084, we need synthetic size to also account for
leased LSNs so that users do not get free retention by running a small
ephemeral endpoint for a long time.

## Summary of changes

This PR integrates LSN leases into the synthetic size calculation. We
model leases as read-only branches started at the leased LSN (except it
does not have a timeline id).

Other changes:
- Add new unit tests testing whether a lease behaves like a read-only
branch.
- Change `/size_debug` response to include lease point in the SVG
visualization.
- Fix `/lsn_lease` HTTP API to do proper parsing for POST.



Signed-off-by: Yuchen Liang <yuchen@neon.tech>
Co-authored-by: Joonas Koivunen <joonas@neon.tech>
Co-authored-by: Christian Schwarz <christian@neon.tech>

libs/pageserver_api/src/models.rs

@@ -228,6 +228,11 @@ pub struct TimelineCreateRequest {
     pub pg_version: Option<u32>,
 }
 
+#[derive(Serialize, Deserialize, Clone)]
+pub struct LsnLeaseRequest {
+    pub lsn: Lsn,
+}
+
 #[derive(Serialize, Deserialize)]
 pub struct TenantShardSplitRequest {
     pub new_shard_count: u8,

libs/tenant_size_model/src/calculation.rs

@@ -34,10 +34,10 @@ struct SegmentSize {
 }
 
 struct SizeAlternatives {
-    // cheapest alternative if parent is available.
+    /// cheapest alternative if parent is available.
     incremental: SegmentSize,
 
-    // cheapest alternative if parent node is not available
+    /// cheapest alternative if parent node is not available
     non_incremental: Option<SegmentSize>,
 }
 

libs/tenant_size_model/src/svg.rs

@@ -3,10 +3,17 @@ use std::fmt::Write;
 
 const SVG_WIDTH: f32 = 500.0;
 
+/// Different branch kind for SVG drawing.
+#[derive(PartialEq)]
+pub enum SvgBranchKind {
+    Timeline,
+    Lease,
+}
+
 struct SvgDraw<'a> {
     storage: &'a StorageModel,
     branches: &'a [String],
-    seg_to_branch: &'a [usize],
+    seg_to_branch: &'a [(usize, SvgBranchKind)],
     sizes: &'a [SegmentSizeResult],
 
     // layout
@@ -42,13 +49,18 @@ fn draw_legend(result: &mut String) -> anyhow::Result<()> {
         "<line x1=\"5\" y1=\"70\" x2=\"15\" y2=\"70\" stroke-width=\"1\" stroke=\"gray\" />"
     )?;
     writeln!(result, "<text x=\"20\" y=\"75\">WAL not retained</text>")?;
+    writeln!(
+        result,
+        "<line x1=\"10\" y1=\"85\" x2=\"10\" y2=\"95\" stroke-width=\"3\" stroke=\"blue\" />"
+    )?;
+    writeln!(result, "<text x=\"20\" y=\"95\">LSN lease</text>")?;
     Ok(())
 }
 
 pub fn draw_svg(
     storage: &StorageModel,
     branches: &[String],
-    seg_to_branch: &[usize],
+    seg_to_branch: &[(usize, SvgBranchKind)],
     sizes: &SizeResult,
 ) -> anyhow::Result<String> {
     let mut draw = SvgDraw {
@@ -100,7 +112,7 @@ impl<'a> SvgDraw<'a> {
 
         // Layout the timelines on Y dimension.
         // TODO
-        let mut y = 100.0;
+        let mut y = 120.0;
         let mut branch_y_coordinates = Vec::new();
         for _branch in self.branches {
             branch_y_coordinates.push(y);
@@ -109,7 +121,7 @@ impl<'a> SvgDraw<'a> {
 
         // Calculate coordinates for each point
         let seg_coordinates = std::iter::zip(segments, self.seg_to_branch)
-            .map(|(seg, branch_id)| {
+            .map(|(seg, (branch_id, _))| {
                 let x = (seg.lsn - min_lsn) as f32 / xscale;
                 let y = branch_y_coordinates[*branch_id];
                 (x, y)
@@ -175,6 +187,22 @@ impl<'a> SvgDraw<'a> {
 
         // draw a snapshot point if it's needed
         let (coord_x, coord_y) = self.seg_coordinates[seg_id];
+
+        let (_, kind) = &self.seg_to_branch[seg_id];
+        if kind == &SvgBranchKind::Lease {
+            let (x1, y1) = (coord_x, coord_y - 10.0);
+            let (x2, y2) = (coord_x, coord_y + 10.0);
+
+            let style = "stroke-width=\"3\" stroke=\"blue\"";
+
+            writeln!(
+                result,
+                "<line x1=\"{x1}\" y1=\"{y1}\" x2=\"{x2}\" y2=\"{y2}\" {style}>",
+            )?;
+            writeln!(result, "  <title>leased lsn at {}</title>", seg.lsn)?;
+            writeln!(result, "</line>")?;
+        }
+
         if self.sizes[seg_id].method == SegmentMethod::SnapshotHere {
             writeln!(
                 result,