tests(fuzz): Add parse_date fuzzing target (#1127)

Add a fuzzing target for parsing into a Timestamp and comparing the result with crate chrono.

fuzz/Cargo.toml

@@ -12,6 +12,8 @@ cargo-fuzz = true
 libfuzzer-sys = "0.4"
 tests = { path = "../tests" }
 protobuf = { path = "../protobuf" }
+prost-types = { path = "../prost-types" }
+chrono = "0.4"
 
 [[bin]]
 name = "proto3"
@@ -26,3 +28,10 @@ path = "fuzzers/proto2.rs"
 test = false
 doc = false
 bench = false
+
+[[bin]]
+name = "parse_date"
+path = "fuzzers/parse_date.rs"
+test = false
+doc = false
+bench = false

fuzz/fuzzers/parse_date.rs

@@ -0,0 +1,49 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| test_parse_date(data));
+
+fn test_parse_date(data: &[u8]) {
+    use std::str::from_utf8;
+    use std::str::FromStr;
+
+    // input must be text
+    let Ok(original_text) = from_utf8(data) else {
+        return;
+    };
+
+    // parse input as a datetime
+    let Ok(timestamp) = prost_types::Timestamp::from_str(original_text) else {
+        let chrono_parse = chrono::DateTime::parse_from_rfc3339(original_text);
+        assert!(
+            chrono_parse.is_err(),
+            "prost failed to parse time, but chrono does parse this time: {}",
+            original_text
+        );
+        return;
+    };
+
+    // roundtrip to and from string
+    let roundtrip_text = format!("{timestamp}");
+    assert_eq!(Ok(&timestamp), roundtrip_text.parse().as_ref());
+
+    // chrono can only parse year 0000 till 9999
+    if let Ok(chrono_time) = chrono::DateTime::parse_from_rfc3339(original_text) {
+        if chrono_time.timestamp_subsec_nanos() > 999_999_999 {
+            // prost ignores leap seconds, but chrono increases the nanos in that case
+            return;
+        }
+
+        assert_eq!(timestamp.seconds, chrono_time.timestamp());
+        assert_eq!(timestamp.nanos, chrono_time.timestamp_subsec_nanos() as i32);
+
+        // roundtrip using chrono
+        let chrono_text = chrono_time.to_utc().to_rfc3339();
+        assert_eq!(
+            roundtrip_text.strip_suffix("Z").unwrap(),
+            chrono_text.strip_suffix("+00:00").unwrap()
+        );
+        assert_eq!(Ok(&timestamp), chrono_text.parse().as_ref());
+    }
+}