From 4e742159af79cf0f6a14da630a362a0344a5a121 Mon Sep 17 00:00:00 2001
From: Keuin <keuinx@gmail.com>
Date: Thu, 15 Sep 2022 00:58:10 +0800
Subject: Make copy and write IO operations async.

---
 common/copy.go | 150 +++++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 118 insertions(+), 32 deletions(-)

(limited to 'common')

diff --git a/common/copy.go b/common/copy.go
index 27bc9ad..02e33a5 100644
--- a/common/copy.go
+++ b/common/copy.go
@@ -6,62 +6,148 @@ Copied from https://ixday.github.io/post/golang-cancel-copy/
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"os"
+	"sync"
 )
 
 // CopyToFileWithBuffer copies data from io.Reader to os.File with given buffer and read chunk size.
+// The reader and writer may not be synchronized if any error occurs.
+// A block contains one or more chunks.
+// Every syscall reads at most one chunk.
+// Every disk-write writes at most one block.
+// The buffer is a ring contains one or more blocks.
 // The reader and file won't be closed.
 // If syncFile is set, the file will be synced after every read.
+// ringSize: how many blocks are in the buffer ring.
 func CopyToFileWithBuffer(
 	ctx context.Context,
 	out *os.File,
 	in io.Reader,
 	buffer []byte,
-	chunkSize int,
 	syncFile bool,
+	ringSize uint,
 ) (written int64, err error) {
-	bufSize := len(buffer)
-	off := 0   // offset to the end of data in buffer
-	nRead := 0 // how many bytes were read in the last read
-	defer func() {
-		if off+nRead > 0 {
-			// write unwritten data in buffer
-			nWrite, _ := out.Write(buffer[:off+nRead])
-			written += int64(nWrite)
-			if syncFile {
-				_ = out.Sync()
+	var blkSize uint
+	bufSize := uint(len(buffer))
+	if bufSize%ringSize != 0 {
+		err = fmt.Errorf("len(buffer) %% ringSize != 0")
+	} else {
+		blkSize = bufSize / ringSize
+	}
+
+	chWriteQue := make(chan uint, ringSize) // buffer write task queue
+	chReadQue := make(chan uint, ringSize)  // buffer read task queue
+
+	// when reader and writer are stopped, this channel must have 2 elements
+	chResult := make(chan error, 2)
+
+	type task struct {
+		Offset uint
+		Length uint
+	}
+
+	chLastWrite := make(chan task, 1)
+
+	rwCtx, cancelReadWrite := context.WithCancel(ctx)
+
+	// wait reader and writer to finish
+	wg := sync.WaitGroup{}
+
+	// buffer reader
+	// buffer -> file
+	go func() {
+		var err error
+		wg.Add(1)
+		defer wg.Done()
+		defer func() {
+			chResult <- err
+		}()
+		for {
+			select {
+			case <-rwCtx.Done():
+				return
+			default:
+				// get the next available block to read from
+				// this block is fully written in the buffer
+				off := <-chReadQue
+
+				// read the entire block to file
+				n, err2 := out.Write(buffer[off : off+blkSize])
+				if err2 != nil {
+					// failed to write to the file
+					// we can do nothing more than stop reading and writing
+					err = fmt.Errorf("write error (%v byte written in this call): %w", n, err2)
+					cancelReadWrite()
+					return
+				}
 			}
 		}
 	}()
 
-	for {
-		select {
-		case <-ctx.Done():
-			err = ctx.Err()
-			return
-		default:
-			nRead, err = in.Read(buffer[off:Min[int](off+chunkSize, bufSize)])
-			if err != nil {
-				return
-			}
-			off += nRead
-			if off == bufSize {
-				// buffer is full
-				var nWritten int
-				nWritten, err = out.Write(buffer)
-				if err != nil {
-					return
+	// buffer writer
+	// reader -> buffer
+	go func() {
+		var err error
+		wg.Add(1)
+		defer wg.Done()
+		defer func() {
+			chResult <- err
+		}()
+		for {
+			select {
+			case <-rwCtx.Done():
+				chLastWrite <- task{
+					Offset: 0,
+					Length: 0,
 				}
-				if syncFile {
-					err = out.Sync()
+				return
+			default:
+				// get the next free block for writing
+				off := <-chWriteQue // byte offset
+
+				// fully fill the block
+				nWritten := uint(0) // bytes written in this block currently
+				for nWritten < blkSize {
+					n, err := in.Read(buffer[off+nWritten : off+blkSize])
+					nWritten += uint(n)
 					if err != nil {
+						// if we can't fully fill current block (e.g. EOF or IO error),
+						// set nLeft to make the main goroutine handle that
+						cancelReadWrite()
+						err = fmt.Errorf("reader failed: %w", err)
+						chLastWrite <- task{
+							Offset: off,
+							Length: nWritten, // bytes of valid data in the last incomplete block
+						}
 						return
 					}
 				}
-				written += int64(nWritten)
-				off = 0
 			}
 		}
+	}()
+
+	// init write tasks, all buffer blocks are available for writing initially
+	for i := uint(0); i < bufSize; i += blkSize {
+		chWriteQue <- i
+	}
+
+	wg.Wait()
+
+	for i := 0; i < 2; i++ {
+		if err2 := <-chResult; err2 != nil {
+			err = err2
+			return
+		}
+	}
+
+	// write the remaining data
+	last := <-chLastWrite
+	if last.Length > 0 {
+		var n int
+		n, err = out.Write(buffer[last.Offset : last.Offset+last.Length])
+		written += int64(n)
 	}
+	return
 }
-- 
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