database/sql: remove a distracting alloc, use atomic.Bool

This removes an allocation in Conn.grabConn that, while not super
important, was distracting me when optimizing code elsewhere.

While here, convert an atomic that was forgotten when this package was
earlier updated to use the new Go 1.19 typed atomics.

Change-Id: I4666256b4c0512e2162bd485c389130699f9d5ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/475415
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -1947,20 +1947,27 @@ type Conn struct {
 	// it's returned to the connection pool.
 	dc *driverConn
 
-	// done transitions from 0 to 1 exactly once, on close.
+	// done transitions from false to true exactly once, on close.
 	// Once done, all operations fail with ErrConnDone.
-	// Use atomic operations on value when checking value.
-	done int32
+	done atomic.Bool
+
+	// releaseConn is a cache of c.closemuRUnlockCondReleaseConn
+	// to save allocations in a call to grabConn.
+	releaseConnOnce  sync.Once
+	releaseConnCache releaseConn
 }
 
 // grabConn takes a context to implement stmtConnGrabber
 // but the context is not used.
 func (c *Conn) grabConn(context.Context) (*driverConn, releaseConn, error) {
-	if atomic.LoadInt32(&c.done) != 0 {
+	if c.done.Load() {
 		return nil, nil, ErrConnDone
 	}
+	c.releaseConnOnce.Do(func() {
+		c.releaseConnCache = c.closemuRUnlockCondReleaseConn
+	})
 	c.closemu.RLock()
-	return c.dc, c.closemuRUnlockCondReleaseConn, nil
+	return c.dc, c.releaseConnCache, nil
 }
 
 // PingContext verifies the connection to the database is still alive.
@@ -2084,7 +2091,7 @@ func (c *Conn) txCtx() context.Context {
 }
 
 func (c *Conn) close(err error) error {
-	if !atomic.CompareAndSwapInt32(&c.done, 0, 1) {
+	if !c.done.CompareAndSwap(false, true) {
 		return ErrConnDone
 	}
 

src/database/sql/sql_test.go

@@ -9,6 +9,8 @@ import (
 	"database/sql/driver"
 	"errors"
 	"fmt"
+	"internal/race"
+	"internal/testenv"
 	"math/rand"
 	"reflect"
 	"runtime"
@@ -4583,3 +4585,35 @@ func BenchmarkManyConcurrentQueries(b *testing.B) {
 		}
 	})
 }
+
+func TestGrabConnAllocs(t *testing.T) {
+	testenv.SkipIfOptimizationOff(t)
+	if race.Enabled {
+		t.Skip("skipping allocation test when using race detector")
+	}
+	c := new(Conn)
+	ctx := context.Background()
+	n := int(testing.AllocsPerRun(1000, func() {
+		_, release, err := c.grabConn(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		release(nil)
+	}))
+	if n > 0 {
+		t.Fatalf("Conn.grabConn allocated %v objects; want 0", n)
+	}
+}
+
+func BenchmarkGrabConn(b *testing.B) {
+	b.ReportAllocs()
+	c := new(Conn)
+	ctx := context.Background()
+	for i := 0; i < b.N; i++ {
+		_, release, err := c.grabConn(ctx)
+		if err != nil {
+			b.Fatal(err)
+		}
+		release(nil)
+	}
+}