feat: add in-memory cache for DNS

This adds "stale-while-revalidate" in-memory cache for DNS. Instead of
calling `tokio::net::lookup_host` we use previous result of
`tokio::net::lookup_host` immediately and spawn revalidation task in the
background. This way all lookups after the first successful one return
immediately.

Most of the time results returned by resolvers are the same anyway, but
with this cache we avoid waiting 60 second timeout if DNS request is
lost. Common reason result may be different is round-robin DNS load
balancing and switching from IPv4 to IPv6 network. For round-robin DNS
we don't break load balancing but simply use a different result, and for
IPv6 we anyway likely have a result in persistent cache and can use IPv4
otherwise.

Especially frequent should be the case when you send a message over SMTP
and SMTP connection is stale (older than 60 s), so we open a new one.
With this change new connection will be set up faster as you don't need
to wait for DNS resolution, so message will be sent faster.

src/net/dns.rs

@@ -1,4 +1,44 @@
 //! DNS resolution and cache.
+//!
+//! DNS cache in Delta Chat has two layers:
+//! in-memory cache and persistent `dns_cache` SQL table.
+//!
+//! In-memory cache is using a "stale-while-revalidate" strategy.
+//! If there is a cached value, it is returned immediately
+//! and revalidation task is started in the background
+//! to replace old cached IP addresses with new ones.
+//! If there is no cached value yet,
+//! lookup only finishes when `lookup_host` returns first results.
+//! In-memory cache is shared between all accounts
+//! and is never stored on the disk.
+//! It can be thought of as an extension
+//! of the system resolver.
+//!
+//! Persistent `dns_cache` SQL table is used to collect
+//! all IP addresses ever seen for the hostname
+//! together with the timestamp
+//! of the last time IP address has been seen.
+//! Note that this timestamp reflects the time
+//! IP address was returned by the in-memory cache
+//! rather than the underlying system resolver.
+//! Unused entries are removed after 30 days
+//! (`CACHE_TTL` constant) to avoid having
+//! old non-working IP addresses in the cache indefinitely.
+//!
+//! When Delta Chat needs an IP address for the host,
+//! it queries in-memory cache for the next result
+//! and merges the list of IP addresses
+//! with the list of IP addresses from persistent cache.
+//! Resulting list is constructed
+//! by taking the first two results from the resolver
+//! followed up by persistent cache results
+//! and terminated by the rest of resolver results.
+//!
+//! Persistent cache results are sorted
+//! by the time of the most recent successful connection
+//! using the result. For results that have never been
+//! used for successful connection timestamp of
+//! retrieving them from in-memory cache is used.
 
 use anyhow::{Context as _, Result};
 use std::collections::HashMap;
@@ -42,33 +82,110 @@ pub(crate) async fn prune_dns_cache(context: &Context) -> Result<()> {
     Ok(())
 }
 
-/// Looks up the hostname and updates DNS cache
-/// on success.
+/// Map from hostname to IP addresses.
+///
+/// NOTE: sync RwLock is used, so it must not be held across `.await`
+/// to avoid deadlocks.
+/// See
+/// <https://docs.rs/tokio/1.40.0/tokio/sync/struct.Mutex.html#which-kind-of-mutex-should-you-use>
+/// and
+/// <https://stackoverflow.com/questions/63712823/why-do-i-get-a-deadlock-when-using-tokio-with-a-stdsyncmutex>.
+static LOOKUP_HOST_CACHE: Lazy<parking_lot::RwLock<HashMap<String, Vec<IpAddr>>>> =
+    Lazy::new(Default::default);
+
+/// Wrapper for `lookup_host` that returns IP addresses.
+async fn lookup_ips(host: impl tokio::net::ToSocketAddrs) -> Result<impl Iterator<Item = IpAddr>> {
+    Ok(lookup_host(host)
+        .await
+        .context("DNS lookup failure")?
+        .map(|addr| addr.ip()))
+}
+
+async fn lookup_host_with_memory_cache(
+    context: &Context,
+    hostname: &str,
+    port: u16,
+) -> Result<Vec<IpAddr>> {
+    let stale_result = {
+        let rwlock_read_guard = LOOKUP_HOST_CACHE.read();
+        rwlock_read_guard.get(hostname).cloned()
+    };
+    if let Some(stale_result) = stale_result {
+        // Revalidate the cache in the background.
+        {
+            let context = context.clone();
+            let hostname = hostname.to_string();
+            tokio::spawn(async move {
+                match lookup_ips((hostname.clone(), port)).await {
+                    Ok(res) => {
+                        LOOKUP_HOST_CACHE.write().insert(hostname, res.collect());
+                    }
+                    Err(err) => {
+                        warn!(
+                            context,
+                            "Failed to revalidate results for {hostname:?}: {err:#}."
+                        );
+                    }
+                }
+            });
+        }
+
+        info!(
+            context,
+            "Using memory-cached DNS resolution for {hostname}."
+        );
+        Ok(stale_result)
+    } else {
+        info!(
+            context,
+            "No memory-cached DNS resolution for {hostname} available, waiting for the resolver."
+        );
+        let res: Vec<IpAddr> = lookup_ips((hostname, port)).await?.collect();
+
+        // Insert initial result into the cache.
+        //
+        // There may already be a result from a parallel
+        // task stored, overwriting it is not a problem.
+        LOOKUP_HOST_CACHE
+            .write()
+            .insert(hostname.to_string(), res.clone());
+        Ok(res)
+    }
+}
+
+/// Looks up the hostname and updates
+/// persistent DNS cache on success.
 async fn lookup_host_and_update_cache(
     context: &Context,
     hostname: &str,
     port: u16,
     now: i64,
 ) -> Result<Vec<SocketAddr>> {
-    let res: Vec<SocketAddr> = timeout(super::TIMEOUT, lookup_host((hostname, port)))
-        .await
-        .context("DNS lookup timeout")?
-        .context("DNS lookup failure")?
-        .collect();
+    let res: Vec<IpAddr> = timeout(
+        super::TIMEOUT,
+        lookup_host_with_memory_cache(context, hostname, port),
+    )
+    .await
+    .context("DNS lookup timeout")?
+    .context("DNS lookup with memory cache failure")?;
 
-    for addr in &res {
-        let ip_string = addr.ip().to_string();
+    for ip in &res {
+        let ip_string = ip.to_string();
         if ip_string == hostname {
             // IP address resolved into itself, not interesting to cache.
             continue;
         }
 
-        info!(context, "Resolved {hostname}:{port} into {addr}.");
+        info!(context, "Resolved {hostname} into {ip}.");
 
         // Update the cache.
         update_cache(context, hostname, &ip_string, now).await?;
     }
 
+    let res = res
+        .into_iter()
+        .map(|ip| SocketAddr::new(ip, port))
+        .collect();
     Ok(res)
 }