[#2252] fstree: Allow concurrent writes

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -7,6 +7,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"syscall"
 
@@ -230,23 +231,63 @@ func (t *FSTree) Put(prm common.PutPrm) (common.PutRes, error) {
 		prm.RawData = t.Compress(prm.RawData)
 	}
 
-	tmpPath := p + "#"
+	// Here is a situation:
-	err := t.writeFile(tmpPath, prm.RawData)
+	// Feb 09 13:10:37 buky neofs-node[32445]: 2023-02-09T13:10:37.161Z        info        log/log.go:13        local object storage operation        {"shard_id": "SkT8BfjouW6t93oLuzQ79s", "address": "7NxFz4SruSi8TqXacr2Ae22nekMhgYk1sfkddJo9PpWk/5enyUJGCyU1sfrURDnHEjZFdbGqANVhayYGfdSqtA6wA", "op": "PUT", "type": "fstree", "storage_id": ""}
+	// Feb 09 13:10:37 buky neofs-node[32445]: 2023-02-09T13:10:37.183Z        info        log/log.go:13        local object storage operation        {"shard_id": "SkT8BfjouW6t93oLuzQ79s", "address": "7NxFz4SruSi8TqXacr2Ae22nekMhgYk1sfkddJo9PpWk/5enyUJGCyU1sfrURDnHEjZFdbGqANVhayYGfdSqtA6wA", "op": "metabase PUT"}
+	// Feb 09 13:10:37 buky neofs-node[32445]: 2023-02-09T13:10:37.862Z        debug        policer/check.go:231        shortage of object copies detected        {"component": "Object Policer", "object": "7NxFz4SruSi8TqXacr2Ae22nekMhgYk1sfkddJo9PpWk/5enyUJGCyU1sfrURDnHEjZFdbGqANVhayYGfdSqtA6wA", "shortage": 1}
+	// Feb 09 13:10:37 buky neofs-node[32445]: 2023-02-09T13:10:37.862Z        debug        shard/get.go:124        object is missing in write-cache        {"shard_id": "SkT8BfjouW6t93oLuzQ79s", "addr": "7NxFz4SruSi8TqXacr2Ae22nekMhgYk1sfkddJo9PpWk/5enyUJGCyU1sfrURDnHEjZFdbGqANVhayYGfdSqtA6wA", "skip_meta": false}
+	//
+	// 1. We put an object on node 1.
+	// 2. Relentless policer sees that it has only 1 copy and tries to PUT it to node 2.
+	// 3. PUT operation started by client at (1) also puts an object here.
+	// 4. Now we have concurrent writes and one of `Rename` calls will return `no such file` error.
+	//    Even more than that, concurrent writes can corrupt data.
+	//
+	// So here is a solution:
+	// 1. Write a file to 'name + 1'.
+	// 2. If it exists, retry with temporary name being 'name + 2'.
+	// 3. Set some reasonable number of attempts.
+	//
+	// It is a bit kludgey, but I am unusually proud about having found this out after
+	// hours of research on linux kernel, dirsync mount option and ext4 FS, turned out
+	// to be so hecking simple.
+	// In a very rare situation we can have multiple partially written copies on disk,
+	// this will be fixed in another issue (we should remove garbage on start).
+	const retryCount = 5
+	for i := 0; i < retryCount; i++ {
+		tmpPath := p + "#" + strconv.FormatUint(uint64(i), 10)
+		err := t.writeAndRename(tmpPath, p, prm.RawData)
+		if err != syscall.EEXIST || i == retryCount-1 {
+			return common.PutRes{StorageID: []byte{}}, err
+		}
+	}
+
+	// unreachable, but precaution never hurts, especially 1 day before release.
+	return common.PutRes{StorageID: []byte{}}, fmt.Errorf("couldn't read file after %d retries", retryCount)
+}
+
+// writeAndRename opens tmpPath exclusively, writes data to it and renames it to p.
+func (t *FSTree) writeAndRename(tmpPath, p string, data []byte) error {
+	err := t.writeFile(tmpPath, data)
 	if err != nil {
 		var pe *fs.PathError
-		if errors.As(err, &pe) && pe.Err == syscall.ENOSPC {
+		if errors.As(err, &pe) {
+			switch pe.Err {
+			case syscall.ENOSPC:
 				err = common.ErrNoSpace
 				_ = os.RemoveAll(tmpPath)
+			case syscall.EEXIST:
+				return syscall.EEXIST
+			}
 		}
 	} else {
 		err = os.Rename(tmpPath, p)
 	}
-
+	return err
-	return common.PutRes{StorageID: []byte{}}, err
 }
 
 func (t *FSTree) writeFlags() int {
-	flags := os.O_WRONLY | os.O_CREATE | os.O_TRUNC
+	flags := os.O_WRONLY | os.O_CREATE | os.O_TRUNC | os.O_EXCL
 	if t.noSync {
 		return flags
 	}