Fix panic in DateTime::from_secs_f64 (#3806)

## Motivation and Context
- #3805 

## Description
Recover from floating point values extremely close to whole numbers by
incrementing the epoch seconds value.

## Testing
- proptest

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

.changelog/1724779868.md

@@ -0,0 +1,9 @@
+---
+applies_to: ["client", "server"]
+authors: ["rcoh"]
+references: ["smithy-rs#3805"]
+breaking: false
+new_feature: false
+bug_fix: true
+---
+Fix bug in `DateTime::from_secs_f64` where certain floating point values could lead to a panic.

rust-runtime/aws-smithy-types/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "aws-smithy-types"
-version = "1.2.4"
+version = "1.2.5"
 authors = [
     "AWS Rust SDK Team <aws-sdk-rust@amazon.com>",
     "Russell Cohen <rcoh@amazon.com>",

rust-runtime/aws-smithy-types/src/date_time/mod.rs

@@ -110,13 +110,22 @@ impl DateTime {
     ///     DateTime::from_fractional_secs(1, 0.5),
     /// );
     /// ```
-    pub fn from_fractional_secs(epoch_seconds: i64, fraction: f64) -> Self {
-        let subsecond_nanos = (fraction * 1_000_000_000_f64) as u32;
+    pub fn from_fractional_secs(mut epoch_seconds: i64, fraction: f64) -> Self {
+        // Because of floating point issues, `fraction` can end up being 1.0 leading to
+        // a full second of subsecond nanos. In that case, rollover the subsecond into the second.
+        let mut subsecond_nanos = (fraction * 1_000_000_000_f64) as u32;
+        if subsecond_nanos == 1_000_000_000 {
+            epoch_seconds += 1;
+            subsecond_nanos = 0;
+        }
         DateTime::from_secs_and_nanos(epoch_seconds, subsecond_nanos)
     }
 
     /// Creates a `DateTime` from a number of seconds and sub-second nanos since the Unix epoch.
     ///
+    /// # Panics
+    /// This function will panic if `subsecond_nanos` is >= 1_000_000_000
+    ///
     /// # Example
     /// ```
     /// # use aws_smithy_types::DateTime;
@@ -680,6 +689,17 @@ mod test {
         assert!(fifth == fifth);
     }
 
+    /// https://github.com/smithy-lang/smithy-rs/issues/3805
+    #[test]
+    fn panic_in_fromsecs_f64() {
+        assert_eq!(DateTime::from_secs_f64(-1.0), DateTime::from_secs(-1));
+
+        assert_eq!(
+            DateTime::from_secs_f64(-1.95877825437922e-309),
+            DateTime::from_secs(0)
+        );
+    }
+
     const MIN_RFC_3339_MILLIS: i64 = -62135596800000;
     const MAX_RFC_3339_MILLIS: i64 = 253402300799999;
 
@@ -699,4 +719,11 @@ mod test {
             assert_eq!(left.cmp(&right), left_str.cmp(&right_str));
         }
     }
+
+    proptest! {
+        #[test]
+        fn from_secs_f64_proptest(secs: f64) {
+            let _date = DateTime::from_secs_f64(secs);
+        }
+    }
 }