HACK: Track goroutine IDs to determine when we write by the wrong thread

To use: set `DENDRITE_TRACE_SQL=1` then grep for `unsafe`

internal/sqlutil/writer_exclusive.go

@@ -3,6 +3,10 @@ package sqlutil
 import (
 	"database/sql"
 	"errors"
+	"fmt"
+	"runtime"
+	"strconv"
+	"strings"
 
 	"go.uber.org/atomic"
 )
@@ -12,8 +16,9 @@ import (
 // contend on database locks in, e.g. SQLite. Only one task will run
 // at a time on a given ExclusiveWriter.
 type ExclusiveWriter struct {
-	running atomic.Bool
-	todo    chan transactionWriterTask
+	running  atomic.Bool
+	todo     chan transactionWriterTask
+	writerID int
 }
 
 func NewExclusiveWriter() Writer {
@@ -30,6 +35,15 @@ type transactionWriterTask struct {
 	wait chan error
 }
 
+func (w *ExclusiveWriter) Safe() string {
+	a := goid()
+	b := w.writerID
+	if a == b {
+		return ""
+	}
+	return fmt.Sprintf("%v != %v", a, b)
+}
+
 // Do queues a task to be run by a TransactionWriter. The function
 // provided will be ran within a transaction as supplied by the
 // txn parameter if one is supplied, and if not, will take out a
@@ -60,6 +74,7 @@ func (w *ExclusiveWriter) run() {
 	if !w.running.CAS(false, true) {
 		return
 	}
+	w.writerID = goid()
 	defer w.running.Store(false)
 	for task := range w.todo {
 		if task.db != nil && task.txn != nil {
@@ -74,3 +89,14 @@ func (w *ExclusiveWriter) run() {
 		close(task.wait)
 	}
 }
+
+func goid() int {
+	var buf [64]byte
+	n := runtime.Stack(buf[:], false)
+	idField := strings.Fields(strings.TrimPrefix(string(buf[:n]), "goroutine "))[0]
+	id, err := strconv.Atoi(idField)
+	if err != nil {
+		panic(fmt.Sprintf("cannot get goroutine id: %v", err))
+	}
+	return id
+}