wal_decoder: fix compact key protobuf encoding (#10074)

## Problem

Protobuf doesn't support 128 bit integers, so we encode the keys as two
64 bit integers. Issue is that when we split the 128 bit compact key we
use signed 64 bit integers to represent the two halves. This may result
in a negative lower half when relnode is larger than `0x00800000`. When
we convert the lower half to an i128 we get a negative `CompactKey`.

## Summary of Changes

Use unsigned integers when encoding into Protobuf.

## Deployment

* Prod: We disabled the interpreted proto, so no compat concerns.
* Staging: Disable the interpreted proto, do one release, and then
release the fixed version.
We do this because a negative int32 will convert to a large uint32 value
and could give
a key in the actual pageserver space. In production we would around this
by adding new
fields to the proto and deprecating the old ones, but we can make our
lives easy here.
* Pre-prod: Same as staging

libs/pageserver_api/src/key.rs

@@ -24,7 +24,7 @@ pub struct Key {
 
 /// When working with large numbers of Keys in-memory, it is more efficient to handle them as i128 than as
 /// a struct of fields.
-#[derive(Clone, Copy, Hash, PartialEq, Eq, Ord, PartialOrd, Serialize, Deserialize)]
+#[derive(Clone, Copy, Hash, PartialEq, Eq, Ord, PartialOrd, Serialize, Deserialize, Debug)]
 pub struct CompactKey(i128);
 
 /// The storage key size.

libs/wal_decoder/proto/interpreted_wal.proto

@@ -37,7 +37,7 @@ message ValueMeta {
 }
 
 message CompactKey {
-  int64 high = 1;
-  int64 low = 2;
+  uint64 high = 1;
+  uint64 low = 2;
 }
 

libs/wal_decoder/src/wire_format.rs

@@ -236,8 +236,8 @@ impl From<ValueMeta> for proto::ValueMeta {
 impl From<CompactKey> for proto::CompactKey {
     fn from(value: CompactKey) -> Self {
         proto::CompactKey {
-            high: (value.raw() >> 64) as i64,
-            low: value.raw() as i64,
+            high: (value.raw() >> 64) as u64,
+            low: value.raw() as u64,
         }
     }
 }
@@ -354,3 +354,64 @@ impl From<proto::CompactKey> for CompactKey {
         (((value.high as i128) << 64) | (value.low as i128)).into()
     }
 }
+
+#[test]
+fn test_compact_key_with_large_relnode() {
+    use pageserver_api::key::Key;
+
+    let inputs = vec![
+        Key {
+            field1: 0,
+            field2: 0x100,
+            field3: 0x200,
+            field4: 0,
+            field5: 0x10,
+            field6: 0x5,
+        },
+        Key {
+            field1: 0,
+            field2: 0x100,
+            field3: 0x200,
+            field4: 0x007FFFFF,
+            field5: 0x10,
+            field6: 0x5,
+        },
+        Key {
+            field1: 0,
+            field2: 0x100,
+            field3: 0x200,
+            field4: 0x00800000,
+            field5: 0x10,
+            field6: 0x5,
+        },
+        Key {
+            field1: 0,
+            field2: 0x100,
+            field3: 0x200,
+            field4: 0x00800001,
+            field5: 0x10,
+            field6: 0x5,
+        },
+        Key {
+            field1: 0,
+            field2: 0xFFFFFFFF,
+            field3: 0xFFFFFFFF,
+            field4: 0xFFFFFFFF,
+            field5: 0x0,
+            field6: 0x0,
+        },
+    ];
+
+    for input in inputs {
+        assert!(input.is_valid_key_on_write_path());
+        let compact = input.to_compact();
+        let proto: proto::CompactKey = compact.into();
+        let from_proto: CompactKey = proto.into();
+
+        assert_eq!(
+            compact, from_proto,
+            "Round trip failed for key with relnode={:#x}",
+            input.field4
+        );
+    }
+}