database/sql: add SetMaxOpenConns

Update #4805

Add the ability to set an open connection limit.
Fixed case where the Conn finalCloser was being called with db.mu locked.
Added seperate benchmarks for each path for Exec and Query.
Replaced slice based idle pool with list based idle pool.

R=bradfitz
CC=golang-dev
https://golang.org/cl/10726044

src/pkg/database/sql/fakedb_test.go

@@ -447,6 +447,10 @@ func (c *fakeConn) Prepare(query string) (driver.Stmt, error) {
 		return c.prepareCreate(stmt, parts)
 	case "INSERT":
 		return c.prepareInsert(stmt, parts)
+	case "NOSERT":
+		// Do all the prep-work like for an INSERT but don't actually insert the row.
+		// Used for some of the concurrent tests.
+		return c.prepareInsert(stmt, parts)
 	default:
 		stmt.Close()
 		return nil, errf("unsupported command type %q", cmd)
@@ -497,13 +501,20 @@ func (s *fakeStmt) Exec(args []driver.Value) (driver.Result, error) {
 		}
 		return driver.ResultNoRows, nil
 	case "INSERT":
-		return s.execInsert(args)
+		return s.execInsert(args, true)
+	case "NOSERT":
+		// Do all the prep-work like for an INSERT but don't actually insert the row.
+		// Used for some of the concurrent tests.
+		return s.execInsert(args, false)
 	}
 	fmt.Printf("EXEC statement, cmd=%q: %#v\n", s.cmd, s)
 	return nil, fmt.Errorf("unimplemented statement Exec command type of %q", s.cmd)
 }
 
-func (s *fakeStmt) execInsert(args []driver.Value) (driver.Result, error) {
+// When doInsert is true, add the row to the table.
+// When doInsert is false do prep-work and error checking, but don't
+// actually add the row to the table.
+func (s *fakeStmt) execInsert(args []driver.Value, doInsert bool) (driver.Result, error) {
 	db := s.c.db
 	if len(args) != s.placeholders {
 		panic("error in pkg db; should only get here if size is correct")
@@ -518,7 +529,10 @@ func (s *fakeStmt) execInsert(args []driver.Value) (driver.Result, error) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
-	cols := make([]interface{}, len(t.colname))
+	var cols []interface{}
+	if doInsert {
+		cols = make([]interface{}, len(t.colname))
+	}
 	argPos := 0
 	for n, colname := range s.colName {
 		colidx := t.columnIndex(colname)
@@ -532,10 +546,14 @@ func (s *fakeStmt) execInsert(args []driver.Value) (driver.Result, error) {
 		} else {
 			val = s.colValue[n]
 		}
-		cols[colidx] = val
+		if doInsert {
+			cols[colidx] = val
+		}
 	}
 
-	t.rows = append(t.rows, &row{cols: cols})
+	if doInsert {
+		t.rows = append(t.rows, &row{cols: cols})
+	}
 	return driver.RowsAffected(1), nil
 }