XL: listOnlineDisks should use modTime instead of version. (#2166)

This change is needed to make reading from objects future proof
in-terms of handling online disks. Our current counter is not
based on affirmative knowledge and relies on arithmetic sequence
which can lead to bugs.

Using modTime simplifies the understanding of `xl.json` and future
tooling / debugging of the format.

docs/backend/xl/xl.json

@@ -42,7 +42,6 @@
         ],
     },
     "stat": {
-        "version": 0,
         "modTime": "2016-05-24T00:09:40.122390255Z",
         "size": 14824084
     },

xl-v1-healing.go

@@ -16,33 +16,65 @@
 
 package main
 
-import "sync"
+import (
+	"sync"
+	"time"
+)
 
-// Get the highest integer from a given integer slice.
-func highestInt(intSlice []int64, highestInt int64) (highestInteger int64) {
-	highestInteger = highestInt
-	for _, integer := range intSlice {
-		if highestInteger < integer {
-			highestInteger = integer
-			break
+// commonTime returns a maximally occurring time from a list of time.
+func commonTime(modTimes []time.Time) (modTime time.Time) {
+	var maxima int // Counter for remembering max occurrence of elements.
+	timeOccurenceMap := make(map[time.Time]int)
+	// Ignore the uuid sentinel and count the rest.
+	for _, time := range modTimes {
+		if time == timeSentinel {
+			continue
+		}
+		timeOccurenceMap[time]++
+	}
+	// Find the common cardinality from previously collected
+	// occurrences of elements.
+	for time, count := range timeOccurenceMap {
+		if count > maxima {
+			maxima = count
+			modTime = time
 		}
 	}
-	return highestInteger
+	// Return the collected common uuid.
+	return modTime
 }
 
-// Extracts objects versions from xlMetaV1 slice and returns version slice.
-func listObjectVersions(partsMetadata []xlMetaV1, errs []error) (versions []int64) {
-	versions = make([]int64, len(partsMetadata))
+var timeSentinel = time.Unix(0, 0).UTC()
+
+// Boot uuids upto disk count, setting the value to UUID sentinel.
+func bootModtimes(diskCount int) []time.Time {
+	modTimes := make([]time.Time, diskCount)
+	// Boots up all the uuids.
+	for i := range modTimes {
+		modTimes[i] = timeSentinel
+	}
+	return modTimes
+}
+
+// Extracts list of times from xlMetaV1 slice and returns, skips
+// slice elements which have errors. As a special error
+// errFileNotFound is treated as a initial good condition.
+func listObjectModtimes(partsMetadata []xlMetaV1, errs []error) (modTimes []time.Time) {
+	modTimes = bootModtimes(len(partsMetadata))
+	// Set a new time value, specifically set when
+	// error == errFileNotFound (this is needed when this is a
+	// fresh PutObject).
+	timeNow := time.Now().UTC()
 	for index, metadata := range partsMetadata {
 		if errs[index] == nil {
-			versions[index] = metadata.Stat.Version
+			// Once the file is found, save the uuid saved on disk.
+			modTimes[index] = metadata.Stat.ModTime
 		} else if errs[index] == errFileNotFound {
-			versions[index] = 1
-		} else {
-			versions[index] = -1
+			// Once the file is not found then the epoch is current time.
+			modTimes[index] = timeNow
 		}
 	}
-	return versions
+	return modTimes
 }
 
 // Reads all `xl.json` metadata as a xlMetaV1 slice.
@@ -100,23 +132,22 @@ func (xl xlObjects) shouldHeal(onlineDisks []StorageAPI) (heal bool) {
 // - slice returing readable disks.
 // - xlMetaV1
 // - bool value indicating if healing is needed.
-// - error if any.
-func (xl xlObjects) listOnlineDisks(partsMetadata []xlMetaV1, errs []error) (onlineDisks []StorageAPI, version int64, err error) {
+func (xl xlObjects) listOnlineDisks(partsMetadata []xlMetaV1, errs []error) (onlineDisks []StorageAPI, modTime time.Time) {
 	onlineDisks = make([]StorageAPI, len(xl.storageDisks))
 
-	// List all the file versions from partsMetadata list.
-	versions := listObjectVersions(partsMetadata, errs)
+	// List all the file commit ids from parts metadata.
+	modTimes := listObjectModtimes(partsMetadata, errs)
 
-	// Get highest object version.
-	highestVersion := highestInt(versions, int64(1))
+	// Reduce list of UUIDs to a single common value.
+	modTime = commonTime(modTimes)
 
-	// Pick online disks with version set to highestVersion.
-	for index, version := range versions {
-		if version == highestVersion {
+	// Create a new online disks slice, which have common uuid.
+	for index, t := range modTimes {
+		if t == modTime {
 			onlineDisks[index] = xl.storageDisks[index]
 		} else {
 			onlineDisks[index] = nil
 		}
 	}
-	return onlineDisks, highestVersion, nil
+	return onlineDisks, modTime
 }

xl-v1-healing_test.go

@@ -0,0 +1,83 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+// validates functionality provided to find most common
+// time occurrence from a list of time.
+func TestCommonTime(t *testing.T) {
+	// List of test cases for common modTime.
+	testCases := []struct {
+		times []time.Time
+		time  time.Time
+	}{
+		{
+			// 1. Tests common times when slice has varying time elements.
+			[]time.Time{
+				time.Unix(0, 1).UTC(),
+				time.Unix(0, 2).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 2).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 1).UTC(),
+			}, time.Unix(0, 3).UTC(),
+		},
+		{
+			// 2. Tests common time obtained when all elements are equal.
+			[]time.Time{
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+			}, time.Unix(0, 3).UTC(),
+		},
+		{
+			// 3. Tests common time obtained when elements have a mixture
+			// of sentinel values.
+			[]time.Time{
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 2).UTC(),
+				time.Unix(0, 1).UTC(),
+				time.Unix(0, 3).UTC(),
+				time.Unix(0, 4).UTC(),
+				time.Unix(0, 3).UTC(),
+				timeSentinel,
+				timeSentinel,
+				timeSentinel,
+			}, time.Unix(0, 3).UTC(),
+		},
+	}
+
+	// Tests all the testcases, and validates them against expected
+	// common modtime. Tests fail if modtime does not match.
+	for i, testCase := range testCases {
+		// Obtain a common mod time from modTimes slice.
+		ctime := commonTime(testCase.times)
+		if testCase.time != ctime {
+			t.Fatalf("Test case %d, expect to pass but failed. Wanted modTime: %s, got modTime: %s\n", i+1, testCase.time, ctime)
+		}
+	}
+}

xl-v1-metadata.go

@@ -87,7 +87,6 @@ func pickValidErasureInfo(eInfos []erasureInfo) erasureInfo {
 type statInfo struct {
 	Size    int64     `json:"size"`    // Size of the object `xl.json`.
 	ModTime time.Time `json:"modTime"` // ModTime of the object `xl.json`.
-	Version int64     `json:"version"` // Version of the object `xl.json`, useful to calculate quorum.
 }
 
 // A xlMetaV1 represents `xl.json` metadata header.

xl-v1-multipart.go

@@ -266,7 +266,6 @@ func (xl xlObjects) newMultipartUpload(bucket string, object string, meta map[st
 		meta["content-type"] = contentType
 	}
 	xlMeta.Stat.ModTime = time.Now().UTC()
-	xlMeta.Stat.Version = 1
 	xlMeta.Meta = meta
 
 	// This lock needs to be held for any changes to the directory contents of ".minio/multipart/object/"
@@ -355,13 +354,14 @@ func (xl xlObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 	nsMutex.RUnlock(minioMetaBucket, uploadIDPath)
 
 	// List all online disks.
-	onlineDisks, _, err := xl.listOnlineDisks(partsMetadata, errs)
-	if err != nil {
-		return "", toObjectErr(err, bucket, object)
-	}
+	onlineDisks, _ := xl.listOnlineDisks(partsMetadata, errs)
+
+	// Pick one from the first valid metadata.
+	xlMeta := pickValidXLMeta(partsMetadata)
 
 	// Need a unique name for the part being written in minioMetaBucket to
 	// accommodate concurrent PutObjectPart requests
+
 	partSuffix := fmt.Sprintf("part.%d", partID)
 	tmpSuffix := getUUID()
 	tmpPartPath := path.Join(tmpMetaPrefix, tmpSuffix)
@@ -434,8 +434,7 @@ func (xl xlObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 	}
 
 	// Read metadata (again) associated with the object from all disks.
-	partsMetadata, errs = readAllXLMetadata(onlineDisks, minioMetaBucket,
-		uploadIDPath)
+	partsMetadata, errs = readAllXLMetadata(onlineDisks, minioMetaBucket, uploadIDPath)
 	if !isQuorum(errs, xl.writeQuorum) {
 		return "", toObjectErr(errXLWriteQuorum, bucket, object)
 	}
@@ -448,7 +447,6 @@ func (xl xlObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 	var checksums []checkSumInfo
 	for index, eInfo := range newEInfos {
 		if eInfo.IsValid() {
-
 			// Use a map to find union of checksums of parts that
 			// we concurrently written and committed before this
 			// part. N B For a different, concurrent upload of the
@@ -474,21 +472,13 @@ func (xl xlObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 	}
 
 	// Pick one from the first valid metadata.
-	xlMeta := pickValidXLMeta(partsMetadata)
+	xlMeta = pickValidXLMeta(partsMetadata)
 
 	// Get current highest version based on re-read partsMetadata.
-	_, higherVersion, err := xl.listOnlineDisks(partsMetadata, errs)
-	if err != nil {
-		return "", toObjectErr(err, bucket, object)
-	}
-
-	// Increment version only if we have online disks less than configured storage disks.
-	if diskCount(onlineDisks) < len(xl.storageDisks) {
-		higherVersion++
-	}
+	onlineDisks, _ = xl.listOnlineDisks(partsMetadata, errs)
 
 	// Once part is successfully committed, proceed with updating XL metadata.
-	xlMeta.Stat.Version = higherVersion
+	xlMeta.Stat.ModTime = time.Now().UTC()
 
 	// Add the current part.
 	xlMeta.AddObjectPart(partID, partSuffix, newMD5Hex, size)

xl-v1-object.go

@@ -73,15 +73,12 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 	}
 
 	// List all online disks.
-	onlineDisks, highestVersion, err := xl.listOnlineDisks(metaArr, errs)
-	if err != nil {
-		return toObjectErr(err, bucket, object)
-	}
+	onlineDisks, modTime := xl.listOnlineDisks(metaArr, errs)
 
 	// Pick latest valid metadata.
 	var xlMeta xlMetaV1
 	for _, meta := range metaArr {
-		if meta.IsValid() && meta.Stat.Version == highestVersion {
+		if meta.IsValid() && meta.Stat.ModTime == modTime {
 			xlMeta = meta
 			break
 		}
@@ -386,15 +383,7 @@ func (xl xlObjects) PutObject(bucket string, object string, size int64, data io.
 	}
 
 	// List all online disks.
-	onlineDisks, higherVersion, err := xl.listOnlineDisks(partsMetadata, errs)
-	if err != nil {
-		return "", toObjectErr(err, bucket, object)
-	}
-
-	// Increment version only if we have online disks less than configured storage disks.
-	if diskCount(onlineDisks) < len(xl.storageDisks) {
-		higherVersion++
-	}
+	onlineDisks, _ := xl.listOnlineDisks(partsMetadata, errs)
 
 	var mw io.Writer
 	// Initialize md5 writer.
@@ -513,7 +502,7 @@ func (xl xlObjects) PutObject(bucket string, object string, size int64, data io.
 	xlMeta.Meta = metadata
 	xlMeta.Stat.Size = size
 	xlMeta.Stat.ModTime = modTime
-	xlMeta.Stat.Version = higherVersion
+
 	// Add the final part.
 	xlMeta.AddObjectPart(1, "part.1", newMD5Hex, xlMeta.Stat.Size)
 

xl-v1-utils.go

@@ -78,14 +78,14 @@ func reduceErrs(errs []error) error {
 
 // Validates if we have quorum based on the errors with errDiskNotFound.
 func isQuorum(errs []error, minQuorumCount int) bool {
-	var diskFoundCount int
+	var errCount int
 	for _, err := range errs {
-		if err == errDiskNotFound {
+		if err == errDiskNotFound || err == errFaultyDisk || err == errDiskAccessDenied {
 			continue
 		}
-		diskFoundCount++
+		errCount++
 	}
-	return diskFoundCount >= minQuorumCount
+	return errCount >= minQuorumCount
 }
 
 // Similar to 'len(slice)' but returns  the actual elements count