fix(db): Fix root cause of RocksDB misbehavior (#301)

# What ❔

Fixes (hopefully) the root cause of recent RocksDB misbehavior. After
recent refactoring, it improperly cancels background compactions /
flushes each time a `RocksDB` instance is dropped, which leads to
compactions / flushes being constantly interrupted. (After refactoring,
`RocksDB` instances are `Clone`eable and act essentially as `Arc`
wrappers; a new instance is created and dropped, in particular, [when
processing](https://github.com/matter-labs/zksync-era/blob/ec41692063909742005acd656694a8f0838fcf33/core/lib/zksync_core/src/metadata_calculator/updater.rs#L385)
each chunk of L1 batches.) A proper solution, implemented by this PR, is
to cancel background work when dropping an internal, non-`Arc`'d DB
instance.

## Why ❔

That's a bug that makes RocksDB behave *very* unstably, as witnessed by
recent issues with it.

## Checklist

- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [x] Code has been formatted via `zk fmt` and `zk lint`.

---------

Co-authored-by: Igor Borodin <hatemosphere@protonmail.com>

core/lib/storage/src/db.rs

@@ -203,6 +203,16 @@ impl RocksDBInner {
     }
 }
 
+impl Drop for RocksDBInner {
+    fn drop(&mut self) {
+        tracing::debug!(
+            "Canceling background compactions / flushes for DB `{}`",
+            self.db_name
+        );
+        self.db.cancel_all_background_work(true);
+    }
+}
+
 /// Configuration for retries when RocksDB writes are stalled.
 #[derive(Debug, Clone, Copy)]
 pub struct StalledWritesRetries {
@@ -448,13 +458,21 @@ impl<CF: NamedColumnFamily> RocksDB<CF> {
 
         let raw_batch_bytes = raw_batch.data().to_vec();
         let mut retries = self.stalled_writes_retries.intervals();
+        let mut stalled_write_reported = false;
+        let started_at = Instant::now();
         loop {
             match self.write_inner(raw_batch) {
-                Ok(()) => return Ok(()),
+                Ok(()) => {
+                    if stalled_write_reported {
+                        METRICS.observe_stalled_write_duration(CF::DB_NAME, started_at.elapsed());
+                    }
+                    return Ok(());
+                }
                 Err(err) => {
                     let is_stalled_write = StalledWritesRetries::is_write_stall_error(&err);
-                    if is_stalled_write {
-                        METRICS.report_stalled_write(CF::DB_NAME);
+                    if is_stalled_write && !stalled_write_reported {
+                        METRICS.observe_stalled_write(CF::DB_NAME);
+                        stalled_write_reported = true;
                     } else {
                         return Err(err);
                     }
@@ -552,12 +570,6 @@ impl RocksDB<()> {
     }
 }
 
-impl<CF> Drop for RocksDB<CF> {
-    fn drop(&mut self) {
-        self.inner.db.cancel_all_background_work(true);
-    }
-}
-
 /// Empty struct used to register rocksdb instance
 #[derive(Debug)]
 struct RegistryEntry;

core/lib/storage/src/metrics.rs

@@ -6,6 +6,7 @@ use vise::{Buckets, Collector, Counter, EncodeLabelSet, Family, Gauge, Histogram
 use std::{
     collections::HashMap,
     sync::{Mutex, Weak},
+    time::Duration,
 };
 
 use crate::db::RocksDBInner;
@@ -41,18 +42,33 @@ pub(crate) struct RocksdbMetrics {
     /// Size of a serialized `WriteBatch` written to a RocksDB instance.
     #[metrics(buckets = BYTE_SIZE_BUCKETS)]
     write_batch_size: Family<DbLabel, Histogram<usize>>,
-    /// Number of stalled writes for a RocksDB instance.
+    /// Number of independent stalled writes for a RocksDB instance.
+    // The counter is similar for the counter in `stalled_write_duration` histogram, but is reported earlier
+    // (immediately when stalled write is encountered, rather than when it's resolved).
     write_stalled: Family<DbLabel, Counter>,
+    /// Total duration of a stalled writes instance for a RocksDB instance. Naturally, this only reports
+    /// stalled writes that were resolved in time (otherwise, the stall error is propagated, which
+    /// leads to a panic).
+    #[metrics(buckets = Buckets::LATENCIES, unit = Unit::Seconds)]
+    stalled_write_duration: Family<DbLabel, Histogram<Duration>>,
 }
 
 impl RocksdbMetrics {
     pub(crate) fn report_batch_size(&self, db: &'static str, batch_size: usize) {
         self.write_batch_size[&db.into()].observe(batch_size);
     }
 
-    pub(crate) fn report_stalled_write(&self, db: &'static str) {
+    pub(crate) fn observe_stalled_write(&self, db: &'static str) {
         self.write_stalled[&db.into()].inc();
     }
+
+    pub(crate) fn observe_stalled_write_duration(
+        &self,
+        db: &'static str,
+        stall_duration: Duration,
+    ) {
+        self.stalled_write_duration[&db.into()].observe(stall_duration);
+    }
 }
 
 #[vise::register]