chore(pageserver): shrink aux keyspace to 0x60-0x7F (#7502)

extracted from https://github.com/neondatabase/neon/pull/7468, part of
https://github.com/neondatabase/neon/issues/7462.

In the page server, we use i128 (instead of u128) to do the integer
representation of the key, which indicates that the highest bit of the
key should not be 1. This constraints our keyspace to <= 0x7F.

Also fix the bug of `to_i128` that dropped the highest 4b. Now we keep
3b of them, dropping the sign bit.

And on that, we shrink the metadata keyspace to 0x60-0x7F for now, and
once we add support for u128, we can have a larger metadata keyspace.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/pageserver_api/src/key.rs

@@ -4,7 +4,6 @@ use bytes::BufMut;
 use postgres_ffi::relfile_utils::{FSM_FORKNUM, VISIBILITYMAP_FORKNUM};
 use postgres_ffi::{Oid, TransactionId};
 use serde::{Deserialize, Serialize};
-use std::ops::RangeInclusive;
 use std::{fmt, ops::Range};
 
 use crate::reltag::{BlockNumber, RelTag, SlruKind};
@@ -30,24 +29,25 @@ pub const KEY_SIZE: usize = 18;
 /// See [`Key::to_i128`] for more information on the encoding.
 pub const METADATA_KEY_SIZE: usize = 16;
 
-/// The key prefix start range for the metadata keys. All keys with the first byte >= 0x80 is a metadata key.
-pub const METADATA_KEY_BEGIN_PREFIX: u8 = 0x80;
+/// The key prefix start range for the metadata keys. All keys with the first byte >= 0x40 is a metadata key.
+pub const METADATA_KEY_BEGIN_PREFIX: u8 = 0x60;
+pub const METADATA_KEY_END_PREFIX: u8 = 0x7F;
 
 /// The (reserved) key prefix of relation sizes.
-pub const RELATION_SIZE_PREFIX: u8 = 0x81;
+pub const RELATION_SIZE_PREFIX: u8 = 0x61;
 
 /// The key prefix of AUX file keys.
-pub const AUX_KEY_PREFIX: u8 = 0x82;
+pub const AUX_KEY_PREFIX: u8 = 0x62;
 
 /// Check if the key falls in the range of metadata keys.
 pub const fn is_metadata_key_slice(key: &[u8]) -> bool {
-    key[0] >= METADATA_KEY_BEGIN_PREFIX
+    key[0] >= METADATA_KEY_BEGIN_PREFIX && key[0] < METADATA_KEY_END_PREFIX
 }
 
 impl Key {
     /// Check if the key falls in the range of metadata keys.
     pub const fn is_metadata_key(&self) -> bool {
-        self.field1 >= METADATA_KEY_BEGIN_PREFIX
+        self.field1 >= METADATA_KEY_BEGIN_PREFIX && self.field1 < METADATA_KEY_END_PREFIX
     }
 
     /// Encode a metadata key to a storage key.
@@ -80,7 +80,7 @@ impl Key {
     }
 
     /// Get the range of metadata keys.
-    pub fn metadata_key_range() -> RangeInclusive<Self> {
+    pub fn metadata_key_range() -> Range<Self> {
         Key {
             field1: METADATA_KEY_BEGIN_PREFIX,
             field2: 0,
@@ -88,13 +88,32 @@ impl Key {
             field4: 0,
             field5: 0,
             field6: 0,
-        }..=Key {
-            field1: u8::MAX,
-            field2: u16::MAX as u32,
-            field3: u32::MAX,
-            field4: u32::MAX,
-            field5: u8::MAX,
-            field6: u32::MAX,
+        }..Key {
+            field1: METADATA_KEY_END_PREFIX,
+            field2: 0,
+            field3: 0,
+            field4: 0,
+            field5: 0,
+            field6: 0,
+        }
+    }
+
+    /// Get the range of aux keys.
+    pub fn metadata_aux_key_range() -> Range<Self> {
+        Key {
+            field1: AUX_KEY_PREFIX,
+            field2: 0,
+            field3: 0,
+            field4: 0,
+            field5: 0,
+            field6: 0,
+        }..Key {
+            field1: AUX_KEY_PREFIX + 1,
+            field2: 0,
+            field3: 0,
+            field4: 0,
+            field5: 0,
+            field6: 0,
         }
     }
 
@@ -103,7 +122,7 @@ impl Key {
     /// we can assume that only some predefined namespace OIDs are used which can fit in u16
     pub fn to_i128(&self) -> i128 {
         assert!(self.field2 < 0xFFFF || self.field2 == 0xFFFFFFFF || self.field2 == 0x22222222);
-        (((self.field1 & 0xf) as i128) << 120)
+        (((self.field1 & 0x7F) as i128) << 120)
             | (((self.field2 & 0xFFFF) as i128) << 104)
             | ((self.field3 as i128) << 72)
             | ((self.field4 as i128) << 40)
@@ -113,7 +132,7 @@ impl Key {
 
     pub const fn from_i128(x: i128) -> Self {
         Key {
-            field1: ((x >> 120) & 0xf) as u8,
+            field1: ((x >> 120) & 0x7F) as u8,
             field2: ((x >> 104) & 0xFFFF) as u32,
             field3: (x >> 72) as u32,
             field4: (x >> 40) as u32,