Print checkpoint key contents with "pagectl print-layer-file" (#6541)

This was very useful in debugging the bugs fixed in #6410 and #6502.

There's a lot more we could do. This only adds the printing to delta
layers, not image layers, for example, and it might be useful to print
details of more record types. But this is a good start.

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -884,7 +884,7 @@ impl DeltaLayerInner {
 
         let keys = self.load_keys(ctx).await?;
 
-        async fn dump_blob(val: ValueRef<'_>, ctx: &RequestContext) -> anyhow::Result<String> {
+        async fn dump_blob(val: &ValueRef<'_>, ctx: &RequestContext) -> anyhow::Result<String> {
             let buf = val.reader.read_blob(val.blob_ref.pos(), ctx).await?;
             let val = Value::des(&buf)?;
             let desc = match val {
@@ -906,13 +906,32 @@ impl DeltaLayerInner {
 
         for entry in keys {
             let DeltaEntry { key, lsn, val, .. } = entry;
-            let desc = match dump_blob(val, ctx).await {
+            let desc = match dump_blob(&val, ctx).await {
                 Ok(desc) => desc,
                 Err(err) => {
                     format!("ERROR: {err}")
                 }
             };
             println!("  key {key} at {lsn}: {desc}");
+
+            // Print more details about CHECKPOINT records. Would be nice to print details
+            // of many other record types too, but these are particularly interesting, as
+            // have a lot of special processing for them in walingest.rs.
+            use pageserver_api::key::CHECKPOINT_KEY;
+            use postgres_ffi::CheckPoint;
+            if key == CHECKPOINT_KEY {
+                let buf = val.reader.read_blob(val.blob_ref.pos(), ctx).await?;
+                let val = Value::des(&buf)?;
+                match val {
+                    Value::Image(img) => {
+                        let checkpoint = CheckPoint::decode(&img)?;
+                        println!("   CHECKPOINT: {:?}", checkpoint);
+                    }
+                    Value::WalRecord(_rec) => {
+                        println!("   unexpected walrecord value for checkpoint key");
+                    }
+                }
+            }
         }
 
         Ok(())