fix(NIP-12): correctly search for mixed-case hex-like tags

Only lowercase and even-length tag values are stored as binary BLOBs.
Previously there was an error which search results from being returned
if the tag value was mixed-case and could be interpreted as hex.

A new database migration has been created to repair the `tag` table
for existing relays.

fixes: https://todo.sr.ht/~gheartsfield/nostr-rs-relay/37

src/utils.rs

@@ -13,3 +13,21 @@ pub fn unix_time() -> u64 {
 pub fn is_hex(s: &str) -> bool {
     s.chars().all(|x| char::is_ascii_hexdigit(&x))
 }
+
+/// Check if a string contains only lower-case hex chars.
+pub fn is_lower_hex(s: &str) -> bool {
+    s.chars().all(|x| {
+        (char::is_ascii_lowercase(&x) || char::is_ascii_digit(&x)) && char::is_ascii_hexdigit(&x)
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn lower_hex() {
+        let hexstr = "abcd0123";
+        assert_eq!(is_lower_hex(hexstr), true);
+    }
+}