improve media key decoding logs

On my HS I observed 5 instances of keys with the following format:

* MXC bytes.
* A 0xFF byte.
* 4 bytes where the width and height are supposed to be, which are
  supposed to be 8 bytes in length.
* 3 consecutive 0xFF bytes. This means that the `content-type` and
  `content-disposition` sections both parse as the empty string, and
  there's an extra separator at the end too.
* Extra bytes, all of which were `image/png`.

The 4 bytes where the width and height are supposed to be were one of:

* 003ED000
* 003EE000
* 003EF001

Which seems to have some kind of pattern to it...

After much digging, we have absolutely no idea what could've caused
this. Cursed.

src/database/key_value/media.rs

@@ -19,24 +19,28 @@ struct MediaFileKeyParts {
 impl TryFrom<&MediaFileKey> for MediaFileKeyParts {
     type Error = Error;
 
+    #[tracing::instrument(
+        err,
+        fields(key = utils::u8_slice_to_hex(key.as_bytes())),
+    )]
     fn try_from(key: &MediaFileKey) -> Result<MediaFileKeyParts> {
         let mut parts = key.as_bytes().split(|&b| b == 0xFF);
 
         let mxc_bytes = parts
             .next()
-            .ok_or_else(|| Error::bad_database("Media ID in db is invalid."))?;
+            .ok_or_else(|| Error::BadDatabase("Media ID in db is invalid."))?;
         let mxc = utils::string_from_bytes(mxc_bytes)
             .map_err(|_| {
-                Error::bad_database("Media MXC URI in db is invalid unicode.")
+                Error::BadDatabase("Media MXC URI in db is invalid unicode.")
             })?
             .into();
 
         let thumbnail_size_bytes = parts
             .next()
-            .ok_or_else(|| Error::bad_database("Media ID in db is invalid."))?;
+            .ok_or_else(|| Error::BadDatabase("Media ID in db is invalid."))?;
         let thumbnail_size_bytes: &[u8; 8] =
             thumbnail_size_bytes.try_into().map_err(|_| {
-                Error::bad_database("Media ID thumbnail size in db is invalid")
+                Error::BadDatabase("Media ID thumbnail size in db is invalid")
             })?;
         let width = u32::from_be_bytes(
             thumbnail_size_bytes[..4].try_into().expect("should be 4 bytes"),
@@ -49,7 +53,7 @@ impl TryFrom<&MediaFileKey> for MediaFileKeyParts {
             .next()
             .map(|bytes| {
                 utils::string_from_bytes(bytes).map_err(|_| {
-                    Error::bad_database(
+                    Error::BadDatabase(
                         "Content type in mediaid_file is invalid unicode.",
                     )
                 })
@@ -58,14 +62,14 @@ impl TryFrom<&MediaFileKey> for MediaFileKeyParts {
 
         let content_disposition_bytes = parts
             .next()
-            .ok_or_else(|| Error::bad_database("Media ID in db is invalid."))?;
+            .ok_or_else(|| Error::BadDatabase("Media ID in db is invalid."))?;
 
         let content_disposition = if content_disposition_bytes.is_empty() {
             None
         } else {
             Some(utils::string_from_bytes(content_disposition_bytes).map_err(
                 |_| {
-                    Error::bad_database(
+                    Error::BadDatabase(
                         "Content Disposition in mediaid_file is invalid \
                          unicode.",
                     )

src/utils.rs

@@ -4,6 +4,7 @@ pub(crate) mod on_demand_hashmap;
 use std::{
     borrow::Cow,
     cmp, fmt,
+    fmt::Write,
     str::FromStr,
     time::{SystemTime, UNIX_EPOCH},
 };
@@ -366,6 +367,17 @@ pub(crate) fn curlify<T>(req: &http::Request<T>) -> Option<String> {
     )
 }
 
+/// Format a u8 slice as an uppercase hex string
+///
+/// The output does not contain a leading `0x` nor any non-hex characters (e.g.
+/// whitespace or commas do not appear in the output).
+pub(crate) fn u8_slice_to_hex(slice: &[u8]) -> String {
+    slice.iter().fold(String::new(), |mut acc, x| {
+        write!(acc, "{x:X}").expect("in-memory write should succeed");
+        acc
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use crate::utils::dbg_truncate_str;