database/sql: Avoid re-preparing statements when all connections are busy

Previously, if all connections were busy, we would always
re-prepare the statement on the connection we were assigned from
the pool.  That meant that if all connections were busy most of the
time, the number of prepared statements for each connection would
keep increasing over time.

Instead, after getting a free connection, check to see if the
statement has already been prepared on it, and reuse the statement
handle if so.

LGTM=bradfitz
R=golang-codereviews, gobot, bradfitz
CC=golang-codereviews
https://golang.org/cl/116930043

src/pkg/database/sql/sql.go

@@ -1326,15 +1326,12 @@ func (s *Stmt) connStmt() (ci *driverConn, releaseConn func(error), si driver.St
 		return ci, releaseConn, s.txsi.si, nil
 	}
 
-	var cs connStmt
-	match := false
 	for i := 0; i < len(s.css); i++ {
 		v := s.css[i]
 		_, err := s.db.connIfFree(v.dc)
 		if err == nil {
-			match = true
-			cs = v
-			break
+			s.mu.Unlock()
+			return v.dc, v.dc.releaseConn, v.si, nil
 		}
 		if err == errConnClosed {
 			// Lazily remove dead conn from our freelist.
@@ -1346,28 +1343,41 @@ func (s *Stmt) connStmt() (ci *driverConn, releaseConn func(error), si driver.St
 	}
 	s.mu.Unlock()
 
-	// Make a new conn if all are busy.
-	// TODO(bradfitz): or wait for one? make configurable later?
-	if !match {
-		dc, err := s.db.conn()
-		if err != nil {
-			return nil, nil, nil, err
-		}
-		dc.Lock()
-		si, err := dc.prepareLocked(s.query)
-		dc.Unlock()
-		if err != nil {
-			s.db.putConn(dc, err)
-			return nil, nil, nil, err
-		}
-		s.mu.Lock()
-		cs = connStmt{dc, si}
-		s.css = append(s.css, cs)
-		s.mu.Unlock()
+	// If all connections are busy, either wait for one to become available (if
+	// we've already hit the maximum number of open connections) or create a
+	// new one.
+	//
+	// TODO(bradfitz): or always wait for one? make configurable later?
+	dc, err := s.db.conn()
+	if err != nil {
+		return nil, nil, nil, err
 	}
 
-	conn := cs.dc
-	return conn, conn.releaseConn, cs.si, nil
+	// Do another pass over the list to see whether this statement has
+	// already been prepared on the connection assigned to us.
+	s.mu.Lock()
+	for _, v := range s.css {
+		if v.dc == dc {
+			s.mu.Unlock()
+			return dc, dc.releaseConn, v.si, nil
+		}
+	}
+	s.mu.Unlock()
+
+	// No luck; we need to prepare the statement on this connection
+	dc.Lock()
+	si, err = dc.prepareLocked(s.query)
+	dc.Unlock()
+	if err != nil {
+		s.db.putConn(dc, err)
+		return nil, nil, nil, err
+	}
+	s.mu.Lock()
+	cs := connStmt{dc, si}
+	s.css = append(s.css, cs)
+	s.mu.Unlock()
+
+	return dc, dc.releaseConn, si, nil
 }
 
 // Query executes a prepared query statement with the given arguments