pagectl: key command for dumping what we know about the key (#7890)

What we know about the key via added `pagectl key $key` command:
- debug formatting
- shard placement when `--shard-count` is specified
- different boolean queries in `key.rs`
- aux files v2

Example:

```
$ cargo run -qp pagectl -- key 000000063F00004005000060270000100E2C
parsed from hex: 000000063F00004005000060270000100E2C:

Key { field1: 0, field2: 1599, field3: 16389, field4: 24615, field5: 0, field6: 1052204 }
rel_block:         true
rel_vm_block:      false
rel_fsm_block:     false
slru_block:        false
inherited:         true
rel_size:          false
slru_segment_size: false
recognized kind:   None
```

libs/pageserver_api/src/key.rs

@@ -381,10 +381,15 @@ pub fn rel_size_to_key(rel: RelTag) -> Key {
         field3: rel.dbnode,
         field4: rel.relnode,
         field5: rel.forknum,
-        field6: 0xffffffff,
+        field6: 0xffff_ffff,
     }
 }
 
+#[inline(always)]
+pub fn is_rel_size_key(key: &Key) -> bool {
+    key.field1 == 0 && key.field6 == u32::MAX
+}
+
 #[inline(always)]
 pub fn rel_key_range(rel: RelTag) -> Range<Key> {
     Key {
@@ -422,6 +427,25 @@ pub fn slru_dir_to_key(kind: SlruKind) -> Key {
     }
 }
 
+#[inline(always)]
+pub fn slru_dir_kind(key: &Key) -> Option<Result<SlruKind, u32>> {
+    if key.field1 == 0x01
+        && key.field3 == 0
+        && key.field4 == 0
+        && key.field5 == 0
+        && key.field6 == 0
+    {
+        match key.field2 {
+            0 => Some(Ok(SlruKind::Clog)),
+            1 => Some(Ok(SlruKind::MultiXactMembers)),
+            2 => Some(Ok(SlruKind::MultiXactOffsets)),
+            x => Some(Err(x)),
+        }
+    } else {
+        None
+    }
+}
+
 #[inline(always)]
 pub fn slru_block_to_key(kind: SlruKind, segno: u32, blknum: BlockNumber) -> Key {
     Key {
@@ -450,10 +474,18 @@ pub fn slru_segment_size_to_key(kind: SlruKind, segno: u32) -> Key {
         field3: 1,
         field4: segno,
         field5: 0,
-        field6: 0xffffffff,
+        field6: 0xffff_ffff,
     }
 }
 
+pub fn is_slru_segment_size_key(key: &Key) -> bool {
+    key.field1 == 0x01
+        && key.field2 < 0x03
+        && key.field3 == 0x01
+        && key.field5 == 0
+        && key.field6 == u32::MAX
+}
+
 #[inline(always)]
 pub fn slru_segment_key_range(kind: SlruKind, segno: u32) -> Range<Key> {
     let field2 = match kind {

libs/pageserver_api/src/reltag.rs

@@ -3,7 +3,7 @@ use std::cmp::Ordering;
 use std::fmt;
 
 use postgres_ffi::pg_constants::GLOBALTABLESPACE_OID;
-use postgres_ffi::relfile_utils::forknumber_to_name;
+use postgres_ffi::relfile_utils::{forkname_to_number, forknumber_to_name, MAIN_FORKNUM};
 use postgres_ffi::Oid;
 
 ///
@@ -68,6 +68,57 @@ impl fmt::Display for RelTag {
     }
 }
 
+#[derive(Debug, thiserror::Error)]
+pub enum ParseRelTagError {
+    #[error("invalid forknum")]
+    InvalidForknum(#[source] std::num::ParseIntError),
+    #[error("missing triplet member {}", .0)]
+    MissingTripletMember(usize),
+    #[error("invalid triplet member {}", .0)]
+    InvalidTripletMember(usize, #[source] std::num::ParseIntError),
+}
+
+impl std::str::FromStr for RelTag {
+    type Err = ParseRelTagError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        use ParseRelTagError::*;
+
+        // FIXME: in postgres logs this separator is dot
+        // Example:
+        //     could not read block 2 in rel 1663/208101/2620.1 from page server at lsn 0/2431E6F0
+        // with a regex we could get this more painlessly
+        let (triplet, forknum) = match s.split_once('_').or_else(|| s.split_once('.')) {
+            Some((t, f)) => {
+                let forknum = forkname_to_number(Some(f));
+                let forknum = if let Ok(f) = forknum {
+                    f
+                } else {
+                    f.parse::<u8>().map_err(InvalidForknum)?
+                };
+
+                (t, Some(forknum))
+            }
+            None => (s, None),
+        };
+
+        let mut split = triplet
+            .splitn(3, '/')
+            .enumerate()
+            .map(|(i, s)| s.parse::<u32>().map_err(|e| InvalidTripletMember(i, e)));
+        let spcnode = split.next().ok_or(MissingTripletMember(0))??;
+        let dbnode = split.next().ok_or(MissingTripletMember(1))??;
+        let relnode = split.next().ok_or(MissingTripletMember(2))??;
+
+        Ok(RelTag {
+            spcnode,
+            forknum: forknum.unwrap_or(MAIN_FORKNUM),
+            dbnode,
+            relnode,
+        })
+    }
+}
+
 impl RelTag {
     pub fn to_segfile_name(&self, segno: u32) -> String {
         let mut name = if self.spcnode == GLOBALTABLESPACE_OID {

libs/pageserver_api/src/shard.rs

@@ -428,6 +428,12 @@ impl<'de> Deserialize<'de> for TenantShardId {
 #[derive(Clone, Copy, Serialize, Deserialize, Eq, PartialEq, Debug)]
 pub struct ShardStripeSize(pub u32);
 
+impl Default for ShardStripeSize {
+    fn default() -> Self {
+        DEFAULT_STRIPE_SIZE
+    }
+}
+
 /// Layout version: for future upgrades where we might change how the key->shard mapping works
 #[derive(Clone, Copy, Serialize, Deserialize, Eq, PartialEq, Debug)]
 pub struct ShardLayout(u8);
@@ -713,6 +719,25 @@ fn key_to_shard_number(count: ShardCount, stripe_size: ShardStripeSize, key: &Ke
     ShardNumber((hash % count.0 as u32) as u8)
 }
 
+/// For debugging, while not exposing the internals.
+#[derive(Debug)]
+#[allow(unused)] // used by debug formatting by pagectl
+struct KeyShardingInfo {
+    shard0: bool,
+    shard_number: ShardNumber,
+}
+
+pub fn describe(
+    key: &Key,
+    shard_count: ShardCount,
+    stripe_size: ShardStripeSize,
+) -> impl std::fmt::Debug {
+    KeyShardingInfo {
+        shard0: key_is_shard0(key),
+        shard_number: key_to_shard_number(shard_count, stripe_size, key),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use utils::Hex;