cow.go

  var cowPool sync.Pool                                                                                                                                                                                     
                                                                                                                                                                                                            
  func acquireCow(r io.Reader) *cow {                                                                                                                                                                       
      if c, ok := cowPool.Get().(*cow); ok {                                                                                                                                                                
          c.r = r                                                                                                                                                                                           
          return c                                                                                                                                                                                          
      }                                                                                                                                                                                                     
      return &cow{r: r}                                                                                                                                                                                     
  }                                                                                                                                                                                                         
                                                                                                                                                                                                            
  func releaseCow(c *cow) {                                                                                                                                                                                 
      cowPool.Put(c)                                                                                                                                                                                        
  }                                                                                                                                                                                                         
                                                                                                                                                                                                            
  type cow struct {                                                                                                                                                                                         
      r  io.Reader                                                                                                                                                                                          
      n  int                                                                                                                                                                                                
      sz int                                                                                                                                                                                                
      ps [1]uintptr                                                                                                                                                                                         
  }                                                                                                                                                                                                         
                                                                                                                                                                                                            
  func (r *cow) Read(p []byte) (int, error) {                                                                                                                                                               
      if r.sz > 0 {                                                                                                                                                                                         
          r.move()                                                                                                                                                                                          
      }                                                                                                                                                                                                     
      return r.r.Read(p)                                                                                                                                                                                    
  }                                                                                                                                                                                                         
                                                                                                                                                                                                            
  func (r *cow) move() {                                                                                                                                                                                    
      cp := make([]byte, r.sz)                                                                                                                                                                              
      for i, ptr := range r.ps {                                                                                                                                                                            
          p := (*[]byte)(unsafe.Pointer(ptr))                                                                                                                                                               
          n := copy(cp, *p)                                                                                                                                                                                 
          *p = cp[:n]                                                                                                                                                                                       
          cp = cp[n:]                                                                                                                                                                                       
          r.ps[i] = 0                                                                                                                                                                                       
      }                                                                                                                                                                                                     
      r.n = 0                                                                                                                                                                                               
      r.sz = 0                                                                                                                                                                                              
  }                                                                                                                                                                                                         
                                                                                                                                                                                                            
  func (r *cow) add(p uintptr, n int) {                                                                                                                                                                     
      r.ps[r.n] = p                                                                                                                                                                                         
      r.n++                                                                                                                                                                                                 
      r.sz += n                                                                                                                                                                                             
  }                                                                                                                                                                                                         
                                                                                                                                                                                                            
  func (r *cow) del(p uintptr, n int) {                                                                                                                                                                     
      for i := 0; i < r.n; i++ {                                                                                                                                                                            
          if r.ps[i] == p {                                                                                                                                                                                 
              r.n--                                                                                                                                                                                         
              r.sz -= n                                                                                                                                                                                     
              r.ps[i] = r.ps[r.n]                                                                                                                                                                           
              r.ps[r.n] = 0                                                                                                                                                                                 
              return                                                                                                                                                                                        
          }                                                                                                                                                                                                 
      }                                                                                                                                                                                                     
  }