kvdb+etcd: remove retry goroutine from the STM

This commit removes the retry goroutine from the STM as the retry loop
is only running when the STM transaction is encapsulated in Update/View
whereas for self-standing transactions we use a different approach.
By removing the goroutine we won't catch panics thrown that are supposed
to be catched outside of the STM.

channeldb/kvdb/etcd/stm.go

@@ -229,64 +229,41 @@ func makeSTM(cli *v3.Client, manual bool, so ...STMOptionFunc) *stm {
 // errors and handling commit. The loop will quit on every error except
 // CommitError which is used to indicate a necessary retry.
 func runSTM(s *stm, apply func(STM) error) error {
-	out := make(chan error, 1)
+	var (
+		retries int
+		stats   CommitStats
+		err     error
+	)
 
-	go func() {
-		var (
-			retries int
-			stats   CommitStats
-		)
-
-		defer func() {
-			// Recover DatabaseError panics so
-			// we can return them.
-			if r := recover(); r != nil {
-				e, ok := r.(DatabaseError)
-				if !ok {
-					// Unknown panic.
-					panic(r)
-				}
-
-				// Return the error.
-				out <- e.Unwrap()
-			}
-		}()
-
-		var err error
-
-		// In a loop try to apply and commit and roll back
-		// if the database has changed (CommitError).
-		for {
-			// Abort STM if there was an application error.
-			if err = apply(s); err != nil {
-				break
-			}
-
-			stats, err = s.commit()
-
-			// Re-apply only upon commit error
-			// (meaning the database was changed).
-			if _, ok := err.(CommitError); !ok {
-				// Anything that's not a CommitError
-				// aborts the STM run loop.
-				break
-			}
-
-			// Rollback before trying to re-apply.
-			s.Rollback()
-			retries++
+	// In a loop try to apply and commit and roll back
+	// if the database has changed (CommitError).
+	for {
+		// Abort STM if there was an application error.
+		if err = apply(s); err != nil {
+			break
 		}
 
-		if s.options.commitStatsCallback != nil {
-			stats.Retries = retries
-			s.options.commitStatsCallback(err == nil, stats)
+		stats, err = s.commit()
+
+		// Re-apply only upon commit error
+		// (meaning the database was changed).
+		if _, ok := err.(CommitError); !ok {
+			// Anything that's not a CommitError
+			// aborts the STM run loop.
+			break
 		}
 
-		// Return the error to the caller.
-		out <- err
-	}()
+		// Rollback before trying to re-apply.
+		s.Rollback()
+		retries++
+	}
 
-	return <-out
+	if s.options.commitStatsCallback != nil {
+		stats.Retries = retries
+		s.options.commitStatsCallback(err == nil, stats)
+	}
+
+	return err
 }
 
 // add inserts a txn response to the read set. This is useful when the txn
@@ -367,18 +344,10 @@ func (s *stm) fetch(key string, opts ...v3.OpOption) ([]KV, error) {
 		s.options.ctx, key, append(opts, s.getOpts...)...,
 	)
 	if err != nil {
-		dbErr := DatabaseError{
+		return nil, DatabaseError{
 			msg: "stm.fetch() failed",
 			err: err,
 		}
-
-		// Do not panic when executing a manual transaction.
-		if s.manual {
-			return nil, dbErr
-		}
-
-		// Panic when executing inside the STM runloop.
-		panic(dbErr)
 	}
 
 	// Set revison and serializable options upon first fetch