transport/timestamp_generator: Added MonotonicTimestampGenerator

Added TimestampGenerator trait and MonotonicTimestampGenerator based on c++ driver's implementation

Author: smoczy123

scylla/src/transport/mod.rs

@@ -18,6 +18,7 @@ pub mod retry_policy;
 pub mod session;
 pub mod session_builder;
 pub mod speculative_execution;
+pub mod timestamp_generator;
 pub mod topology;
 
 pub use crate::frame::{Authenticator, Compression};

scylla/src/transport/timestamp_generator.rs

@@ -0,0 +1,76 @@
+use std::{
+    sync::atomic::AtomicI64,
+    time::{SystemTime, UNIX_EPOCH},
+};
+
+use std::sync::atomic::Ordering;
+use tracing::warn;
+
+pub(crate) trait TimestampGenerator {
+    fn next(&self) -> i64;
+}
+
+pub struct MonotonicTimestampGenerator {
+    last: AtomicI64,
+    warning_threshold_us: i64,
+}
+
+impl MonotonicTimestampGenerator {
+    pub fn new_with_settings(warning_threshold_us: i64) -> Self {
+        MonotonicTimestampGenerator {
+            last: AtomicI64::new(0),
+            warning_threshold_us,
+        }
+    }
+    pub fn new() -> Self {
+        MonotonicTimestampGenerator::new_with_settings(1000000)
+    }
+
+    // This is guaranteed to return a monotonic timestamp. If clock skew is detected
+    // then this method will increment the last timestamp.
+    fn compute_next(&self, last: i64) -> i64 {
+        let current = SystemTime::now().duration_since(UNIX_EPOCH);
+        if let Ok(cur_time) = current {
+            let u_cur = cur_time.as_micros() as i64;
+            if u_cur > last {
+                return u_cur;
+            } else if last - u_cur > self.warning_threshold_us {
+                warn!(
+                    "Clock skew detected. The current time ({}) was {} \
+                    microseconds behind the last generated timestamp ({}). \
+                    The next generated timestamp will be artificially incremented \
+                    to guarantee monotonicity.",
+                    u_cur,
+                    last - u_cur,
+                    last
+                )
+            }
+        } else {
+            warn!("Clock skew detected. The current time was behind UNIX epoch.");
+        }
+
+        last + 1
+    }
+}
+
+impl Default for MonotonicTimestampGenerator {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl TimestampGenerator for MonotonicTimestampGenerator {
+    fn next(&self) -> i64 {
+        loop {
+            let last = self.last.load(Ordering::SeqCst);
+            let cur = self.compute_next(last);
+            if self
+                .last
+                .compare_exchange(last, cur, Ordering::SeqCst, Ordering::SeqCst)
+                .is_ok()
+            {
+                return cur;
+            }
+        }
+    }
+}