gvisor.dev/gvisor@v0.0.0-20240520182842-f9d4d51c7e0f/pkg/linewriter/linewriter.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package linewriter provides an io.Writer which calls an emitter on each line.
    16  package linewriter
    17  
    18  import (
    19  	"bytes"
    20  
    21  	"gvisor.dev/gvisor/pkg/sync"
    22  )
    23  
    24  // Writer is an io.Writer which buffers input, flushing
    25  // individual lines through an emitter function.
    26  type Writer struct {
    27  	// the mutex locks buf.
    28  	sync.Mutex
    29  
    30  	// buf holds the data we haven't emitted yet.
    31  	buf bytes.Buffer
    32  
    33  	// emit is used to flush individual lines.
    34  	emit func(p []byte)
    35  }
    36  
    37  // NewWriter creates a Writer which emits using emitter.
    38  // The emitter must not retain p. It may change after emitter returns.
    39  func NewWriter(emitter func(p []byte)) *Writer {
    40  	return &Writer{emit: emitter}
    41  }
    42  
    43  // Write implements io.Writer.Write.
    44  // It calls emit on each line of input, not including the newline.
    45  // Write may be called concurrently.
    46  func (w *Writer) Write(p []byte) (int, error) {
    47  	w.Lock()
    48  	defer w.Unlock()
    49  
    50  	total := 0
    51  	for len(p) > 0 {
    52  		emit := true
    53  		i := bytes.IndexByte(p, '\n')
    54  		if i < 0 {
    55  			// No newline, we will buffer everything.
    56  			i = len(p)
    57  			emit = false
    58  		}
    59  
    60  		n, err := w.buf.Write(p[:i])
    61  		if err != nil {
    62  			return total, err
    63  		}
    64  		total += n
    65  
    66  		p = p[i:]
    67  
    68  		if emit {
    69  			// Skip the newline, but still count it.
    70  			p = p[1:]
    71  			total++
    72  
    73  			w.emit(w.buf.Bytes())
    74  			w.buf.Reset()
    75  		}
    76  	}
    77  
    78  	return total, nil
    79  }