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
```

Cargo.lock

@@ -3570,6 +3570,7 @@ dependencies = [
  "serde",
  "serde_json",
  "svg_fmt",
+ "thiserror",
  "tokio",
  "tokio-util",
  "toml_edit",

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;

libs/utils/src/hex.rs

@@ -19,13 +19,13 @@
 /// // right: [0x68; 1]
 /// # fn serialize_something() -> Vec<u8> { "hello world".as_bytes().to_vec() }
 /// ```
-#[derive(PartialEq)]
-pub struct Hex<'a>(pub &'a [u8]);
+pub struct Hex<S>(pub S);
 
-impl std::fmt::Debug for Hex<'_> {
+impl<S: AsRef<[u8]>> std::fmt::Debug for Hex<S> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "[")?;
-        for (i, c) in self.0.chunks(16).enumerate() {
+        let chunks = self.0.as_ref().chunks(16);
+        for (i, c) in chunks.enumerate() {
             if i > 0 && !c.is_empty() {
                 writeln!(f, ", ")?;
             }
@@ -36,6 +36,15 @@ impl std::fmt::Debug for Hex<'_> {
                 write!(f, "0x{b:02x}")?;
             }
         }
-        write!(f, "; {}]", self.0.len())
+        write!(f, "; {}]", self.0.as_ref().len())
+    }
+}
+
+impl<R: AsRef<[u8]>, L: AsRef<[u8]>> PartialEq<Hex<R>> for Hex<L> {
+    fn eq(&self, other: &Hex<R>) -> bool {
+        let left = self.0.as_ref();
+        let right = other.0.as_ref();
+
+        left == right
     }
 }

pageserver/ctl/Cargo.toml

@@ -17,6 +17,7 @@ pageserver = { path = ".." }
 pageserver_api.workspace = true
 remote_storage = { path = "../../libs/remote_storage" }
 postgres_ffi.workspace = true
+thiserror.workspace = true
 tokio.workspace = true
 tokio-util.workspace = true
 toml_edit.workspace = true

pageserver/ctl/src/key.rs

@@ -0,0 +1,477 @@
+use anyhow::Context;
+use clap::Parser;
+use pageserver_api::{
+    key::Key,
+    reltag::{BlockNumber, RelTag, SlruKind},
+    shard::{ShardCount, ShardStripeSize},
+};
+use std::str::FromStr;
+
+#[derive(Parser)]
+pub(super) struct DescribeKeyCommand {
+    /// Key material in one of the forms: hex, span attributes captured from log, reltag blocknum
+    input: Vec<String>,
+
+    /// The number of shards to calculate what Keys placement would be.
+    #[arg(long)]
+    shard_count: Option<CustomShardCount>,
+
+    /// The sharding stripe size.
+    ///
+    /// The default is hardcoded. It makes no sense to provide this without providing
+    /// `--shard-count`.
+    #[arg(long, requires = "shard_count")]
+    stripe_size: Option<u32>,
+}
+
+/// Sharded shard count without unsharded count, which the actual ShardCount supports.
+#[derive(Clone, Copy)]
+pub(super) struct CustomShardCount(std::num::NonZeroU8);
+
+#[derive(Debug, thiserror::Error)]
+pub(super) enum InvalidShardCount {
+    #[error(transparent)]
+    ParsingFailed(#[from] std::num::ParseIntError),
+    #[error("too few shards")]
+    TooFewShards,
+}
+
+impl FromStr for CustomShardCount {
+    type Err = InvalidShardCount;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let inner: std::num::NonZeroU8 = s.parse()?;
+        if inner.get() < 2 {
+            Err(InvalidShardCount::TooFewShards)
+        } else {
+            Ok(CustomShardCount(inner))
+        }
+    }
+}
+
+impl From<CustomShardCount> for ShardCount {
+    fn from(value: CustomShardCount) -> Self {
+        ShardCount::new(value.0.get())
+    }
+}
+
+impl DescribeKeyCommand {
+    pub(super) fn execute(self) {
+        let DescribeKeyCommand {
+            input,
+            shard_count,
+            stripe_size,
+        } = self;
+
+        let material = KeyMaterial::try_from(input.as_slice()).unwrap();
+        let kind = material.kind();
+        let key = Key::from(material);
+
+        println!("parsed from {kind}: {key}:");
+        println!();
+        println!("{key:?}");
+
+        macro_rules! kind_query {
+            ($name:ident) => {{
+                let s: &'static str = stringify!($name);
+                let s = s.strip_prefix("is_").unwrap_or(s);
+                let s = s.strip_suffix("_key").unwrap_or(s);
+
+                #[allow(clippy::needless_borrow)]
+                (s, pageserver_api::key::$name(key))
+            }};
+        }
+
+        // the current characterization is a mess of these boolean queries and separate
+        // "recognization". I think it accurately represents how strictly we model the Key
+        // right now, but could of course be made less confusing.
+
+        let queries = [
+            ("rel_block", pageserver_api::key::is_rel_block_key(&key)),
+            kind_query!(is_rel_vm_block_key),
+            kind_query!(is_rel_fsm_block_key),
+            kind_query!(is_slru_block_key),
+            kind_query!(is_inherited_key),
+            ("rel_size", pageserver_api::key::is_rel_size_key(&key)),
+            (
+                "slru_segment_size",
+                pageserver_api::key::is_slru_segment_size_key(&key),
+            ),
+        ];
+
+        let recognized_kind = "recognized kind";
+        let metadata_key = "metadata key";
+        let shard_placement = "shard placement";
+
+        let longest = queries
+            .iter()
+            .map(|t| t.0)
+            .chain([recognized_kind, metadata_key, shard_placement])
+            .map(|s| s.len())
+            .max()
+            .unwrap();
+
+        let colon = 1;
+        let padding = 1;
+
+        for (name, is) in queries {
+            let width = longest - name.len() + colon + padding;
+            println!("{}{:width$}{}", name, ":", is);
+        }
+
+        let width = longest - recognized_kind.len() + colon + padding;
+        println!(
+            "{}{:width$}{:?}",
+            recognized_kind,
+            ":",
+            RecognizedKeyKind::new(key),
+        );
+
+        if let Some(shard_count) = shard_count {
+            // seeing the sharding placement might be confusing, so leave it out unless shard
+            // count was given.
+
+            let stripe_size = stripe_size.map(ShardStripeSize).unwrap_or_default();
+            println!(
+                "# placement with shard_count: {} and stripe_size: {}:",
+                shard_count.0, stripe_size.0
+            );
+            let width = longest - shard_placement.len() + colon + padding;
+            println!(
+                "{}{:width$}{:?}",
+                shard_placement,
+                ":",
+                pageserver_api::shard::describe(&key, shard_count.into(), stripe_size)
+            );
+        }
+    }
+}
+
+/// Hand-wavy "inputs we accept" for a key.
+#[derive(Debug)]
+pub(super) enum KeyMaterial {
+    Hex(Key),
+    String(SpanAttributesFromLogs),
+    Split(RelTag, BlockNumber),
+}
+
+impl KeyMaterial {
+    fn kind(&self) -> &'static str {
+        match self {
+            KeyMaterial::Hex(_) => "hex",
+            KeyMaterial::String(_) | KeyMaterial::Split(_, _) => "split",
+        }
+    }
+}
+
+impl From<KeyMaterial> for Key {
+    fn from(value: KeyMaterial) -> Self {
+        match value {
+            KeyMaterial::Hex(key) => key,
+            KeyMaterial::String(SpanAttributesFromLogs(rt, blocknum))
+            | KeyMaterial::Split(rt, blocknum) => {
+                pageserver_api::key::rel_block_to_key(rt, blocknum)
+            }
+        }
+    }
+}
+
+impl<S: AsRef<str>> TryFrom<&[S]> for KeyMaterial {
+    type Error = anyhow::Error;
+
+    fn try_from(value: &[S]) -> Result<Self, Self::Error> {
+        match value {
+            [] => anyhow::bail!(
+                "need 1..N positional arguments describing the key, try hex or a log line"
+            ),
+            [one] => {
+                let one = one.as_ref();
+
+                let key = Key::from_hex(one).map(KeyMaterial::Hex);
+
+                let attrs = SpanAttributesFromLogs::from_str(one).map(KeyMaterial::String);
+
+                match (key, attrs) {
+                    (Ok(key), _) => Ok(key),
+                    (_, Ok(s)) => Ok(s),
+                    (Err(e1), Err(e2)) => anyhow::bail!(
+                        "failed to parse {one:?} as hex or span attributes:\n- {e1:#}\n- {e2:#}"
+                    ),
+                }
+            }
+            more => {
+                // assume going left to right one of these is a reltag and then we find a blocknum
+                // this works, because we don't have plain numbers at least right after reltag in
+                // logs. for some definition of "works".
+
+                let Some((reltag_at, reltag)) = more
+                    .iter()
+                    .map(AsRef::as_ref)
+                    .enumerate()
+                    .find_map(|(i, s)| {
+                        s.split_once("rel=")
+                            .map(|(_garbage, actual)| actual)
+                            .unwrap_or(s)
+                            .parse::<RelTag>()
+                            .ok()
+                            .map(|rt| (i, rt))
+                    })
+                else {
+                    anyhow::bail!("found no RelTag in arguments");
+                };
+
+                let Some(blocknum) = more
+                    .iter()
+                    .map(AsRef::as_ref)
+                    .skip(reltag_at)
+                    .find_map(|s| {
+                        s.split_once("blkno=")
+                            .map(|(_garbage, actual)| actual)
+                            .unwrap_or(s)
+                            .parse::<BlockNumber>()
+                            .ok()
+                    })
+                else {
+                    anyhow::bail!("found no blocknum in arguments");
+                };
+
+                Ok(KeyMaterial::Split(reltag, blocknum))
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+pub(super) struct SpanAttributesFromLogs(RelTag, BlockNumber);
+
+impl std::str::FromStr for SpanAttributesFromLogs {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        // accept the span separator but do not require or fail if either is missing
+        // "whatever{rel=1663/16389/24615 blkno=1052204 req_lsn=FFFFFFFF/FFFFFFFF}"
+        let (_, reltag) = s
+            .split_once("rel=")
+            .ok_or_else(|| anyhow::anyhow!("cannot find 'rel='"))?;
+        let reltag = reltag.split_whitespace().next().unwrap();
+
+        let (_, blocknum) = s
+            .split_once("blkno=")
+            .ok_or_else(|| anyhow::anyhow!("cannot find 'blkno='"))?;
+        let blocknum = blocknum.split_whitespace().next().unwrap();
+
+        let reltag = reltag
+            .parse()
+            .with_context(|| format!("parse reltag from {reltag:?}"))?;
+        let blocknum = blocknum
+            .parse()
+            .with_context(|| format!("parse blocknum from {blocknum:?}"))?;
+
+        Ok(Self(reltag, blocknum))
+    }
+}
+
+#[derive(Debug)]
+#[allow(dead_code)] // debug print is used
+enum RecognizedKeyKind {
+    DbDir,
+    ControlFile,
+    Checkpoint,
+    AuxFilesV1,
+    SlruDir(Result<SlruKind, u32>),
+    RelMap(RelTagish<2>),
+    RelDir(RelTagish<2>),
+    AuxFileV2(Result<AuxFileV2, utils::Hex<[u8; 16]>>),
+}
+
+#[derive(Debug, PartialEq)]
+#[allow(unused)]
+enum AuxFileV2 {
+    Recognized(&'static str, utils::Hex<[u8; 13]>),
+    OtherWithPrefix(&'static str, utils::Hex<[u8; 13]>),
+    Other(utils::Hex<[u8; 13]>),
+}
+
+impl RecognizedKeyKind {
+    fn new(key: Key) -> Option<Self> {
+        use RecognizedKeyKind::{
+            AuxFilesV1, Checkpoint, ControlFile, DbDir, RelDir, RelMap, SlruDir,
+        };
+
+        let slru_dir_kind = pageserver_api::key::slru_dir_kind(&key);
+
+        Some(match key {
+            pageserver_api::key::DBDIR_KEY => DbDir,
+            pageserver_api::key::CONTROLFILE_KEY => ControlFile,
+            pageserver_api::key::CHECKPOINT_KEY => Checkpoint,
+            pageserver_api::key::AUX_FILES_KEY => AuxFilesV1,
+            _ if slru_dir_kind.is_some() => SlruDir(slru_dir_kind.unwrap()),
+            _ if key.field1 == 0 && key.field4 == 0 && key.field5 == 0 && key.field6 == 0 => {
+                RelMap([key.field2, key.field3].into())
+            }
+            _ if key.field1 == 0 && key.field4 == 0 && key.field5 == 0 && key.field6 == 1 => {
+                RelDir([key.field2, key.field3].into())
+            }
+            _ if key.is_metadata_key() => RecognizedKeyKind::AuxFileV2(
+                AuxFileV2::new(key).ok_or_else(|| utils::Hex(key.to_i128().to_be_bytes())),
+            ),
+            _ => return None,
+        })
+    }
+}
+
+impl AuxFileV2 {
+    fn new(key: Key) -> Option<AuxFileV2> {
+        const EMPTY_HASH: [u8; 13] = {
+            let mut out = [0u8; 13];
+            let hash = pageserver::aux_file::fnv_hash(b"").to_be_bytes();
+            let mut i = 3;
+            while i < 16 {
+                out[i - 3] = hash[i];
+                i += 1;
+            }
+            out
+        };
+
+        let bytes = key.to_i128().to_be_bytes();
+        let hash = utils::Hex(<[u8; 13]>::try_from(&bytes[3..]).unwrap());
+
+        assert_eq!(EMPTY_HASH.len(), hash.0.len());
+
+        // TODO: we could probably find the preimages for the hashes
+
+        Some(match (bytes[1], bytes[2]) {
+            (1, 1) => AuxFileV2::Recognized("pg_logical/mappings/", hash),
+            (1, 2) => AuxFileV2::Recognized("pg_logical/snapshots/", hash),
+            (1, 3) if hash.0 == EMPTY_HASH => {
+                AuxFileV2::Recognized("pg_logical/replorigin_checkpoint", hash)
+            }
+            (2, 1) => AuxFileV2::Recognized("pg_replslot/", hash),
+            (1, 0xff) => AuxFileV2::OtherWithPrefix("pg_logical/", hash),
+            (0xff, 0xff) => AuxFileV2::Other(hash),
+            _ => return None,
+        })
+    }
+}
+
+/// Prefix of RelTag, currently only known use cases are the two item versions.
+///
+/// Renders like a reltag with `/`, nothing else.
+struct RelTagish<const N: usize>([u32; N]);
+
+impl<const N: usize> From<[u32; N]> for RelTagish<N> {
+    fn from(val: [u32; N]) -> Self {
+        RelTagish(val)
+    }
+}
+
+impl<const N: usize> std::fmt::Debug for RelTagish<N> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use std::fmt::Write as _;
+        let mut first = true;
+        self.0.iter().try_for_each(|x| {
+            if !first {
+                f.write_char('/')?;
+            }
+            first = false;
+            write!(f, "{}", x)
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use pageserver::aux_file::encode_aux_file_key;
+
+    use super::*;
+
+    #[test]
+    fn hex_is_key_material() {
+        let m = KeyMaterial::try_from(&["000000067F0000400200DF927900FFFFFFFF"][..]).unwrap();
+        assert!(matches!(m, KeyMaterial::Hex(_)), "{m:?}");
+    }
+
+    #[test]
+    fn single_positional_spanalike_is_key_material() {
+        // why is this needed? if you are checking many, then copypaste starts to appeal
+        let strings = [
+            (line!(), "2024-05-15T15:33:49.873906Z ERROR page_service_conn_main{peer_addr=A:B}:process_query{tenant_id=C timeline_id=D}:handle_pagerequests:handle_get_page_at_lsn_request{rel=1663/208101/2620_fsm blkno=2 req_lsn=0/238D98C8}: error reading relation or page version: Read error: could not find data for key 000000067F00032CE5000000000000000001 (shard ShardNumber(0)) at LSN 0/1D0A16C1, request LSN 0/238D98C8, ancestor 0/0"),
+            (line!(), "rel=1663/208101/2620_fsm blkno=2"),
+            (line!(), "rel=1663/208101/2620.1 blkno=2"),
+        ];
+
+        let mut first: Option<Key> = None;
+
+        for (line, example) in strings {
+            let m = KeyMaterial::try_from(&[example][..])
+                .unwrap_or_else(|e| panic!("failed to parse example from line {line}: {e:?}"));
+            let key = Key::from(m);
+            if let Some(first) = first {
+                assert_eq!(first, key);
+            } else {
+                first = Some(key);
+            }
+        }
+
+        // not supporting this is rather accidential, but I think the input parsing is lenient
+        // enough already
+        KeyMaterial::try_from(&["1663/208101/2620_fsm 2"][..]).unwrap_err();
+    }
+
+    #[test]
+    fn multiple_spanlike_args() {
+        let strings = [
+            (line!(), &["process_query{tenant_id=C", "timeline_id=D}:handle_pagerequests:handle_get_page_at_lsn_request{rel=1663/208101/2620_fsm", "blkno=2", "req_lsn=0/238D98C8}"][..]),
+            (line!(), &["rel=1663/208101/2620_fsm", "blkno=2"][..]),
+            (line!(), &["1663/208101/2620_fsm", "2"][..]),
+        ];
+
+        let mut first: Option<Key> = None;
+
+        for (line, example) in strings {
+            let m = KeyMaterial::try_from(example)
+                .unwrap_or_else(|e| panic!("failed to parse example from line {line}: {e:?}"));
+            let key = Key::from(m);
+            if let Some(first) = first {
+                assert_eq!(first, key);
+            } else {
+                first = Some(key);
+            }
+        }
+    }
+    #[test]
+    fn recognized_auxfiles() {
+        use AuxFileV2::*;
+
+        let empty = [
+            0x2e, 0x07, 0xbb, 0x01, 0x42, 0x62, 0xb8, 0x21, 0x75, 0x62, 0x95, 0xc5, 0x8d,
+        ];
+        let foobar = [
+            0x62, 0x79, 0x3c, 0x64, 0xbf, 0x6f, 0x0d, 0x35, 0x97, 0xba, 0x44, 0x6f, 0x18,
+        ];
+
+        #[rustfmt::skip]
+        let examples = [
+            (line!(), "pg_logical/mappings/foobar", Recognized("pg_logical/mappings/", utils::Hex(foobar))),
+            (line!(), "pg_logical/snapshots/foobar", Recognized("pg_logical/snapshots/", utils::Hex(foobar))),
+            (line!(), "pg_logical/replorigin_checkpoint", Recognized("pg_logical/replorigin_checkpoint", utils::Hex(empty))),
+            (line!(), "pg_logical/foobar", OtherWithPrefix("pg_logical/", utils::Hex(foobar))),
+            (line!(), "pg_replslot/foobar", Recognized("pg_replslot/", utils::Hex(foobar))),
+            (line!(), "foobar", Other(utils::Hex(foobar))),
+        ];
+
+        for (line, path, expected) in examples {
+            let key = encode_aux_file_key(path);
+            let recognized =
+                AuxFileV2::new(key).unwrap_or_else(|| panic!("line {line} example failed"));
+
+            assert_eq!(recognized, expected);
+        }
+
+        assert_eq!(
+            AuxFileV2::new(Key::from_hex("600000102000000000000000000000000000").unwrap()),
+            None,
+            "example key has one too few 0 after 6 before 1"
+        );
+    }
+}

pageserver/ctl/src/main.rs

@@ -6,6 +6,7 @@
 
 mod draw_timeline_dir;
 mod index_part;
+mod key;
 mod layer_map_analyzer;
 mod layers;
 
@@ -61,6 +62,8 @@ enum Commands {
     AnalyzeLayerMap(AnalyzeLayerMapCmd),
     #[command(subcommand)]
     Layer(LayerCmd),
+    /// Debug print a hex key found from logs
+    Key(key::DescribeKeyCommand),
 }
 
 /// Read and update pageserver metadata file
@@ -183,6 +186,7 @@ async fn main() -> anyhow::Result<()> {
                 .time_travel_recover(Some(&prefix), timestamp, done_if_after, &cancel)
                 .await?;
         }
+        Commands::Key(dkc) => dkc.execute(),
     };
     Ok(())
 }