fix(metrics): Parse custom units

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+**Bug Fixes**:
+
+- Parse custom units with length < 15 without crashing. ([#1312](https://github.com/getsentry/relay/pull/1312))
+
 ## 22.6.0
 
 **Compatibility:** This version of Relay requires Sentry server `22.6.0` or newer.

relay-common/src/constants.rs

@@ -478,29 +478,31 @@ impl fmt::Display for FractionUnit {
     }
 }
 
+const CUSTOM_UNIT_MAX_SIZE: usize = 15;
+
 /// Custom user-defined units without builtin conversion.
 #[derive(Clone, Copy, Eq, PartialEq, Hash)]
-pub struct CustomUnit([u8; 15]);
+pub struct CustomUnit([u8; CUSTOM_UNIT_MAX_SIZE]);
 
 impl CustomUnit {
     /// Parses a `CustomUnit` from a string.
     pub fn parse(s: &str) -> Result<Self, ParseMetricUnitError> {
-        if s.len() > 15 || !s.is_ascii() {
+        if s.len() > CUSTOM_UNIT_MAX_SIZE || !s.is_ascii() {
             return Err(ParseMetricUnitError(()));
         }
 
-        let mut unit = Self(Default::default());
-        unit.0.copy_from_slice(s.as_bytes());
+        let mut unit = Self([0; CUSTOM_UNIT_MAX_SIZE]);
+        unit.0[..s.len()].copy_from_slice(s.as_bytes());
         unit.0.make_ascii_lowercase();
         Ok(unit)
     }
 
     /// Returns the string representation of this unit.
     #[inline]
     pub fn as_str(&self) -> &str {
-        // Safety: The string is already validated to be of length 32 and valid ASCII when
-        // constructing `ProjectKey`.
-        unsafe { std::str::from_utf8_unchecked(&self.0) }
+        // Safety: The string is already validated to be valid ASCII when
+        // parsing `CustomUnit`.
+        unsafe { std::str::from_utf8_unchecked(&self.0).trim_end_matches('\0') }
     }
 }
 
@@ -628,3 +630,21 @@ impl schemars::JsonSchema for MetricUnit {
         String::json_schema(gen)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::CustomUnit;
+
+    #[test]
+    fn test_custom_unit_parse() {
+        assert_eq!("foo", CustomUnit::parse("Foo").unwrap().as_str());
+        assert_eq!(
+            "0123456789abcde",
+            CustomUnit::parse("0123456789abcde").unwrap().as_str()
+        );
+        assert!(matches!(
+            CustomUnit::parse("this_is_a_unit_that_is_too_long"),
+            Err(_)
+        ));
+    }
+}