Make smeared timestamp creation non-async

Using atomic operations instead,
so create_smeared_timestamp() can be used in sync functions,
such as SQL transactions.

src/timesmearing.rs

@@ -0,0 +1,193 @@
+//! # Time smearing.
+//!
+//! As e-mails typically only use a second-based-resolution for timestamps,
+//! the order of two mails sent withing one second is unclear.
+//! This is bad e.g. when forwarding some messages from a chat -
+//! these messages will appear at the recipient easily out of order.
+//!
+//! We work around this issue by not sending out two mails with the same timestamp.
+//! For this purpose, in short, we track the last timestamp used in `last_smeared_timestamp`
+//! when another timestamp is needed in the same second, we use `last_smeared_timestamp+1`
+//! after some moments without messages sent out,
+//! `last_smeared_timestamp` is again in sync with the normal time.
+//!
+//! However, we do not do all this for the far future,
+//! but at max `MAX_SECONDS_TO_LEND_FROM_FUTURE`
+
+use std::cmp::{max, min};
+use std::sync::atomic::{AtomicI64, Ordering};
+
+pub(crate) const MAX_SECONDS_TO_LEND_FROM_FUTURE: i64 = 5;
+
+/// Smeared timestamp generator.
+#[derive(Debug)]
+pub struct SmearedTimestamp {
+    /// Next timestamp available for allocation.
+    smeared_timestamp: AtomicI64,
+}
+
+impl SmearedTimestamp {
+    /// Creates a new smeared timestamp generator.
+    pub fn new() -> Self {
+        Self {
+            smeared_timestamp: AtomicI64::new(0),
+        }
+    }
+
+    /// Allocates `count` unique timestamps.
+    ///
+    /// Returns the first allocated timestamp.
+    pub fn create_n(&self, now: i64, count: i64) -> i64 {
+        let mut prev = self.smeared_timestamp.load(Ordering::Relaxed);
+        loop {
+            // Advance the timestamp if it is in the past,
+            // but keep `count - 1` timestamps from the past if possible.
+            let t = max(prev, now - count + 1);
+
+            // Rewind the time back if there is no room
+            // to allocate `count` timestamps without going too far into the future.
+            // Not going too far into the future
+            // is more important than generating unique timestamps.
+            let first = min(t, now + MAX_SECONDS_TO_LEND_FROM_FUTURE - count + 1);
+
+            // Allocate `count` timestamps by advancing the current timestamp.
+            let next = first + count;
+
+            if let Err(x) = self.smeared_timestamp.compare_exchange_weak(
+                prev,
+                next,
+                Ordering::Relaxed,
+                Ordering::Relaxed,
+            ) {
+                prev = x;
+            } else {
+                return first;
+            }
+        }
+    }
+
+    /// Creates a single timestamp.
+    pub fn create(&self, now: i64) -> i64 {
+        self.create_n(now, 1)
+    }
+
+    /// Returns the current smeared timestamp.
+    pub fn current(&self) -> i64 {
+        self.smeared_timestamp.load(Ordering::Relaxed)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::SystemTime;
+
+    use super::*;
+    use crate::test_utils::TestContext;
+    use crate::tools::{create_smeared_timestamp, create_smeared_timestamps, smeared_time, time};
+
+    #[test]
+    fn test_smeared_timestamp() {
+        let smeared_timestamp = SmearedTimestamp::new();
+        let now = time();
+
+        assert_eq!(smeared_timestamp.current(), 0);
+
+        for i in 0..MAX_SECONDS_TO_LEND_FROM_FUTURE {
+            assert_eq!(smeared_timestamp.create(now), now + i);
+        }
+        assert_eq!(
+            smeared_timestamp.create(now),
+            now + MAX_SECONDS_TO_LEND_FROM_FUTURE
+        );
+        assert_eq!(
+            smeared_timestamp.create(now),
+            now + MAX_SECONDS_TO_LEND_FROM_FUTURE
+        );
+
+        // System time rewinds back by 1000 seconds.
+        let now = now - 1000;
+        assert_eq!(
+            smeared_timestamp.create(now),
+            now + MAX_SECONDS_TO_LEND_FROM_FUTURE
+        );
+        assert_eq!(
+            smeared_timestamp.create(now),
+            now + MAX_SECONDS_TO_LEND_FROM_FUTURE
+        );
+        assert_eq!(
+            smeared_timestamp.create(now + 1),
+            now + MAX_SECONDS_TO_LEND_FROM_FUTURE + 1
+        );
+        assert_eq!(smeared_timestamp.create(now + 100), now + 100);
+        assert_eq!(smeared_timestamp.create(now + 100), now + 101);
+        assert_eq!(smeared_timestamp.create(now + 100), now + 102);
+    }
+
+    #[test]
+    fn test_create_n_smeared_timestamps() {
+        let smeared_timestamp = SmearedTimestamp::new();
+        let now = time();
+
+        // Create a single timestamp to initialize the generator.
+        assert_eq!(smeared_timestamp.create(now), now);
+
+        // Wait a minute.
+        let now = now + 60;
+
+        // Simulate forwarding 7 messages.
+        let forwarded_messages = 7;
+
+        // We have not sent anything for a minute,
+        // so we can take the current timestamp and take 6 timestamps from the past.
+        assert_eq!(smeared_timestamp.create_n(now, forwarded_messages), now - 6);
+
+        assert_eq!(smeared_timestamp.current(), now + 1);
+
+        // Wait 4 seconds.
+        // Now we have 3 free timestamps in the past.
+        let now = now + 4;
+
+        assert_eq!(smeared_timestamp.current(), now - 3);
+
+        // Forward another 7 messages.
+        // We can only lend 3 timestamps from the past.
+        assert_eq!(smeared_timestamp.create_n(now, forwarded_messages), now - 3);
+
+        // We had to borrow 3 timestamps from the future
+        // because there were not enough timestamps in the past.
+        assert_eq!(smeared_timestamp.current(), now + 4);
+
+        // Forward another 7 messages.
+        // We cannot use more than 5 timestamps from the future,
+        // so we use 5 timestamps from the future,
+        // the current timestamp and one timestamp from the past.
+        assert_eq!(smeared_timestamp.create_n(now, forwarded_messages), now - 1);
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_create_smeared_timestamp() {
+        let t = TestContext::new().await;
+        assert_ne!(create_smeared_timestamp(&t), create_smeared_timestamp(&t));
+        assert!(
+            create_smeared_timestamp(&t)
+                >= SystemTime::now()
+                    .duration_since(SystemTime::UNIX_EPOCH)
+                    .unwrap()
+                    .as_secs() as i64
+        );
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_create_smeared_timestamps() {
+        let t = TestContext::new().await;
+        let count = MAX_SECONDS_TO_LEND_FROM_FUTURE - 1;
+        let start = create_smeared_timestamps(&t, count as usize);
+        let next = smeared_time(&t);
+        assert!((start + count - 1) < next);
+
+        let count = MAX_SECONDS_TO_LEND_FROM_FUTURE + 30;
+        let start = create_smeared_timestamps(&t, count as usize);
+        let next = smeared_time(&t);
+        assert!((start + count - 1) < next);
+    }
+}