Fix panics when query file metadata, if timestamps predate the epoch

It's possible for a file to have a timestamp that predates the Epoch.
But Rust's std::time::Duration object cannot be negative.  So
cap-primitives must carefully handle any negative timestamps it gets.

Fixes #328

cap-primitives/src/rustix/fs/metadata_ext.rs

@@ -285,7 +285,14 @@ impl MetadataExt {
 
 #[allow(clippy::similar_names)]
 fn system_time_from_rustix(sec: i64, nsec: u64) -> Option<SystemTime> {
-    SystemClock::UNIX_EPOCH.checked_add(Duration::new(u64::try_from(sec).unwrap(), nsec as _))
+    if sec >= 0 {
+        SystemClock::UNIX_EPOCH.checked_add(Duration::new(u64::try_from(sec).unwrap(), nsec as _))
+    } else {
+        SystemClock::UNIX_EPOCH
+            .checked_sub(Duration::new(u64::try_from(-sec).unwrap(), 0))
+            .map(|t| t.checked_add(Duration::new(0, nsec as u32)))
+            .flatten()
+    }
 }
 
 impl rustix::fs::MetadataExt for MetadataExt {
@@ -396,3 +403,15 @@ impl rustix::fs::MetadataExt for MetadataExt {
         self.ctim
     }
 }
+
+/// It should be possible to represent times before the Epoch.
+/// https://github.com/bytecodealliance/cap-std/issues/328
+#[test]
+fn negative_time() {
+    let system_time = system_time_from_rustix(-1, 1).unwrap();
+    let d = SystemClock::UNIX_EPOCH.duration_since(system_time).unwrap();
+    assert_eq!(d.as_secs(), 0);
+    if !cfg!(emulate_second_only_system) {
+        assert_eq!(d.subsec_nanos(), 999999999);
+    }
+}

cap-primitives/src/time/system_time.rs

@@ -37,10 +37,20 @@ impl SystemTime {
     #[inline]
     pub fn from_std(std: time::SystemTime) -> Self {
         if cfg!(emulate_second_only_system) {
-            let duration = std.duration_since(time::SystemTime::UNIX_EPOCH).unwrap();
-            let secs = time::Duration::from_secs(duration.as_secs());
-            Self {
-                std: time::SystemTime::UNIX_EPOCH.checked_add(secs).unwrap(),
+            match std.duration_since(time::SystemTime::UNIX_EPOCH) {
+                Ok(duration) => {
+                    let secs = time::Duration::from_secs(duration.as_secs());
+                    Self {
+                        std: time::SystemTime::UNIX_EPOCH.checked_add(secs).unwrap(),
+                    }
+                }
+                Err(_) => {
+                    let duration = time::SystemTime::UNIX_EPOCH.duration_since(std).unwrap();
+                    let secs = time::Duration::from_secs(duration.as_secs());
+                    Self {
+                        std: time::SystemTime::UNIX_EPOCH.checked_sub(secs).unwrap(),
+                    }
+                }
             }
         } else {
             Self { std }