RUST-2028 Fix Decimal128 panic when parsing strings w/o a char boundary at idx 34 (#496)

Author: arthurprs

src/decimal128.rs

@@ -333,6 +333,7 @@ pub enum ParseError {
     Overflow,
     Underflow,
     InexactRounding,
+    Unparseable,
 }
 
 impl fmt::Display for ParseError {
@@ -344,6 +345,7 @@ impl fmt::Display for ParseError {
             ParseError::Overflow => write!(f, "overflow"),
             ParseError::Underflow => write!(f, "underflow"),
             ParseError::InexactRounding => write!(f, "inexact rounding"),
+            ParseError::Unparseable => write!(f, "unparseable"),
         }
     }
 }
@@ -469,6 +471,12 @@ impl std::str::FromStr for ParsedDecimal128 {
 }
 
 fn round_decimal_str(s: &str, precision: usize) -> Result<&str, ParseError> {
+    // TODO: In 1.80+ there's split_at_checked to make sure the split doesn't
+    // panic if the index doesn't falls at a codepoint boundary, until then
+    // we can check it with s.is_char_boundary(precision)
+    if !s.is_char_boundary(precision) {
+        return Err(ParseError::Unparseable);
+    }
     let (pre, post) = s.split_at(precision);
     // Any nonzero trimmed digits mean it would be an imprecise round.
     if post.chars().any(|c| c != '0') {

src/raw/test/mod.rs

@@ -10,10 +10,22 @@ use crate::{
     Binary,
     Bson,
     DateTime,
+    Decimal128,
     Regex,
     Timestamp,
 };
 
+#[test]
+fn test_decimal128_doesnt_panic_on_bad_codepoint_boundary() {
+    use crate::decimal128::ParseError;
+    use std::str::FromStr;
+    // idx 34 (Coefficient::MAX_DIGITS) on this string isn't a valid codepoint boundary
+    assert!(matches!(
+        Decimal128::from_str("111111111111111111111111111111111❤"),
+        Err(ParseError::Unparseable)
+    ))
+}
+
 #[test]
 fn string_from_document() {
     let rawdoc = rawdoc! {