database/sql: fix typo bug resulting in double-Prepare

Bug reported by Blake Mizerany found while writing
his new Postgres driver.

R=golang-dev, blake.mizerany
CC=golang-dev
https://golang.org/cl/5754057

src/pkg/database/sql/fakedb_test.go

@@ -82,6 +82,7 @@ type fakeConn struct {
 	mu          sync.Mutex
 	stmtsMade   int
 	stmtsClosed int
+	numPrepare  int
 }
 
 func (c *fakeConn) incrStat(v *int) {
@@ -339,6 +340,7 @@ func (c *fakeConn) prepareInsert(stmt *fakeStmt, parts []string) (driver.Stmt, e
 }
 
 func (c *fakeConn) Prepare(query string) (driver.Stmt, error) {
+	c.numPrepare++
 	if c.db == nil {
 		panic("nil c.db; conn = " + fmt.Sprintf("%#v", c))
 	}

src/pkg/database/sql/sql.go

@@ -700,7 +700,7 @@ func (s *Stmt) connStmt() (ci driver.Conn, releaseConn func(), si driver.Stmt, e
 	for _, v := range s.css {
 		// TODO(bradfitz): lazily clean up entries in this
 		// list with dead conns while enumerating
-		if _, match = s.db.connIfFree(cs.ci); match {
+		if _, match = s.db.connIfFree(v.ci); match {
 			cs = v
 			break
 		}

src/pkg/database/sql/sql_test.go

@@ -47,9 +47,19 @@ func closeDB(t *testing.T, db *DB) {
 	}
 }
 
+// numPrepares assumes that db has exactly 1 idle conn and returns
+// its count of calls to Prepare
+func numPrepares(t *testing.T, db *DB) int {
+	if n := len(db.freeConn); n != 1 {
+		t.Fatalf("free conns = %d; want 1", n)
+	}
+	return db.freeConn[0].(*fakeConn).numPrepare
+}
+
 func TestQuery(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)
+	prepares0 := numPrepares(t, db)
 	rows, err := db.Query("SELECT|people|age,name|")
 	if err != nil {
 		t.Fatalf("Query: %v", err)
@@ -83,7 +93,10 @@ func TestQuery(t *testing.T) {
 	// And verify that the final rows.Next() call, which hit EOF,
 	// also closed the rows connection.
 	if n := len(db.freeConn); n != 1 {
-		t.Errorf("free conns after query hitting EOF = %d; want 1", n)
+		t.Fatalf("free conns after query hitting EOF = %d; want 1", n)
+	}
+	if prepares := numPrepares(t, db) - prepares0; prepares != 1 {
+		t.Errorf("executed %d Prepare statements; want 1", prepares)
 	}
 }