Add algo package and Dave Cheney's errors package

algo/doc.go

+/*
+ * Copyright 2016 Dgraph Labs, 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 algo contains algorithms such as merging, intersecting sorted lists.
+package algo

x/heap.go → algo/heap.go

 /*
- * Copyright 2015 DGraph Labs, Inc.
+ * Copyright 2016 Dgraph Labs, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,22 +14,23 @@
  * limitations under the License.
  */
 
-package x
+package algo
 
-type Elem struct {
-	Uid uint64
-	Idx int // channel index
+type elem struct {
+	val     uint64 // Value of this element.
+	listIdx int    // Which list this element comes from.
 }
 
-type Uint64Heap []Elem
+type uint64Heap []elem
 
-func (h Uint64Heap) Len() int           { return len(h) }
-func (h Uint64Heap) Less(i, j int) bool { return h[i].Uid < h[j].Uid }
-func (h Uint64Heap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
-func (h *Uint64Heap) Push(x interface{}) {
-	*h = append(*h, x.(Elem))
+func (h uint64Heap) Len() int           { return len(h) }
+func (h uint64Heap) Less(i, j int) bool { return h[i].val < h[j].val }
+func (h uint64Heap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
+func (h *uint64Heap) Push(x interface{}) {
+	*h = append(*h, x.(elem))
 }
-func (h *Uint64Heap) Pop() interface{} {
+
+func (h *uint64Heap) Pop() interface{} {
 	old := *h
 	n := len(old)
 	x := old[n-1]

x/heap_test.go → algo/heap_test.go

 /*
- * Copyright 2016 DGraph Labs, Inc.
+ * Copyright 2016 Dgraph Labs, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package x
+package algo
 
 import (
 	"container/heap"
@@ -22,46 +22,46 @@ import (
 )
 
 func TestPush(t *testing.T) {
-	h := &Uint64Heap{}
+	h := &uint64Heap{}
 	heap.Init(h)
 
-	e := Elem{Uid: 5}
+	e := elem{val: 5}
 	heap.Push(h, e)
-	e.Uid = 3
+	e.val = 3
 	heap.Push(h, e)
-	e.Uid = 4
+	e.val = 4
 	heap.Push(h, e)
 
 	if h.Len() != 3 {
 		t.Errorf("Expected len 3. Found: %v", h.Len())
 	}
-	if (*h)[0].Uid != 3 {
+	if (*h)[0].val != 3 {
 		t.Errorf("Expected min 3. Found: %+v", (*h)[0])
 	}
-	e.Uid = 10
+	e.val = 10
 	(*h)[0] = e
 	heap.Fix(h, 0)
-	if (*h)[0].Uid != 4 {
+	if (*h)[0].val != 4 {
 		t.Errorf("Expected min 4. Found: %+v", (*h)[0])
 	}
-	e.Uid = 11
+	e.val = 11
 	(*h)[0] = e
 	heap.Fix(h, 0)
-	if (*h)[0].Uid != 5 {
+	if (*h)[0].val != 5 {
 		t.Errorf("Expected min 5. Found: %+v", (*h)[0])
 	}
 
-	e = heap.Pop(h).(Elem)
-	if e.Uid != 5 {
+	e = heap.Pop(h).(elem)
+	if e.val != 5 {
 		t.Errorf("Expected min 5. Found %+v", e)
 	}
 
-	e = heap.Pop(h).(Elem)
-	if e.Uid != 10 {
+	e = heap.Pop(h).(elem)
+	if e.val != 10 {
 		t.Errorf("Expected min 10. Found: %+v", e)
 	}
-	e = heap.Pop(h).(Elem)
-	if e.Uid != 11 {
+	e = heap.Pop(h).(elem)
+	if e.val != 11 {
 		t.Errorf("Expected min 11. Found: %+v", e)
 	}
 

algo/sorted.go

+/*
+ * Copyright 2016 Dgraph Labs, 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 algo
+
+import (
+	"container/heap"
+
+	"github.com/dgraph-io/dgraph/task"
+)
+
+// Uint64Lists is a list of Uint64List. We need this because []X is not equal to
+// []interface{} and we do not want to incur a O(n) cost for a trivial conversion.
+type Uint64Lists interface {
+	Get(int) Uint64List // Returns i-th list.
+	Size() int
+}
+
+// Uint64List is a list of uint64s.
+type Uint64List interface {
+	Get(int) uint64
+	Size() int
+}
+
+// UIDList is a list of UIDs.
+type UIDList struct{ task.UidList }
+
+// Get returns i-th element.
+func (ul *UIDList) Get(i int) uint64 { return ul.Uids(i) }
+
+// Size returns size of UID list.
+func (ul *UIDList) Size() int { return ul.UidsLength() }
+
+// UIDLists is a list of UIDList.
+type UIDLists []*UIDList
+
+// Get returns the i-th list.
+func (ul UIDLists) Get(i int) Uint64List { return ul[i] }
+
+// Size returns number of lists.
+func (ul UIDLists) Size() int { return len(ul) }
+
+// MergeSorted merges sorted uint64 lists. Only unique numbers are returned.
+// In the future, we might have another interface for the output.
+func MergeSorted(lists Uint64Lists) []uint64 {
+	n := lists.Size()
+	if n == 0 {
+		return []uint64{}
+	}
+
+	h := &uint64Heap{}
+	heap.Init(h)
+
+	for i := 0; i < n; i++ {
+		l := lists.Get(i)
+		if l.Size() > 0 {
+			heap.Push(h, elem{
+				val:     l.Get(0),
+				listIdx: i,
+			})
+		}
+	}
+
+	// Our final output. Give it some capacity.
+	output := make([]uint64, 0, 100)
+	// idx[i] is the element we are looking at for lists[i].
+	idx := make([]int, n)
+	var last uint64   // Last element added to sorted / final output.
+	for h.Len() > 0 { // While heap is not empty.
+		me := (*h)[0] // Peek at the top element in heap.
+		if len(output) == 0 || me.val != last {
+			output = append(output, me.val) // Add if unique.
+			last = me.val
+		}
+		l := lists.Get(me.listIdx)
+		if idx[me.listIdx] >= l.Size()-1 {
+			heap.Pop(h)
+		} else {
+			idx[me.listIdx]++
+			val := l.Get(idx[me.listIdx])
+			(*h)[0].val = val
+			heap.Fix(h, 0) // Faster than Pop() followed by Push().
+		}
+	}
+	return output
+}
+
+// IntersectSorted returns intersection of uint64 lists.
+func IntersectSorted(lists Uint64Lists) []uint64 {
+	n := lists.Size()
+	if n == 0 {
+		return []uint64{}
+	}
+
+	// Scan through the smallest list. Denote as A.
+	// For each x in A,
+	//   For each other list B,
+	//     Keep popping elements until we get a y >= x.
+	//     If y > x, we want to skip x. Break out of loop for B.
+	//   If we reach here, append our output by x.
+	// We also remove all duplicates.
+	var minLen, minLenIndex int
+	// minLen array is lists[minLenIndex].
+	for i := 0; i < n; i++ {
+		l := lists.Get(i)
+		if l.Size() < minLen {
+			minLen = l.Size()
+			minLenIndex = i
+		}
+	}
+
+	// Our final output. Give it some capacity.
+	output := make([]uint64, 0, minLenIndex)
+	// idx[i] is the element we are looking at for lists[i].
+	idx := make([]int, n)
+	shortList := lists.Get(minLenIndex)
+	for i := 0; i < shortList.Size(); i++ {
+		val := shortList.Get(i)
+		if i > 0 && val == shortList.Get(i-1) {
+			continue // Avoid duplicates.
+		}
+
+		var skip bool            // Should we skip val in output?
+		for j := 0; j < n; j++ { // For each other list in lists.
+			if j == minLenIndex {
+				// No point checking yourself.
+				continue
+			}
+			l := lists.Get(j)
+			k := idx[j]
+			for ; k < l.Size() && l.Get(k) < val; k++ {
+			}
+			idx[j] = k
+			if l.Get(k) > val {
+				skip = true
+				break
+			}
+			// Otherwise, l[k] = val and we continue checking other lists.
+		}
+		if !skip {
+			output = append(output, val)
+		}
+	}
+	return output
+}

algo/sorted_test.go

+/*
+ * Copyright 2016 Dgraph Labs, 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 algo
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/dgraph-io/dgraph/task"
+	"github.com/google/flatbuffers/go"
+)
+
+// TODO(jchiu): Use some test lib or build our own in future.
+func arrayCompare(a []uint64, b []uint64) error {
+	if len(a) != len(b) {
+		return fmt.Errorf("Size mismatch %d vs %d", len(a), len(b))
+	}
+	for i := 0; i < len(a); i++ {
+		if a[i] != b[i] {
+			return fmt.Errorf("Element mismatch at index %d", i)
+		}
+	}
+	return nil
+}
+
+// plainUintLists is the simplest possible Uint64Lists.
+type plainUintLists []plainUintList
+
+// Size returns number of lists.
+func (s plainUintLists) Size() int { return len(s) }
+
+// Get returns the i-th list.
+func (s plainUintLists) Get(i int) Uint64List { return s[i] }
+
+// plainUintList is the simplest possible Uint64List.
+type plainUintList []uint64
+
+// Size returns size of list.
+func (s plainUintList) Size() int { return len(s) }
+
+// Get returns i-th element of list.
+func (s plainUintList) Get(i int) uint64 { return (s)[i] }
+
+func TestMergeSorted1(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{1, 3, 6, 8, 10},
+		plainUintList{2, 4, 5, 7, 15},
+	}
+	expected := []uint64{1, 2, 3, 4, 5, 6, 7, 8, 10, 15}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted2(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{1, 3, 6, 8, 10},
+		plainUintList{},
+	}
+	expected := []uint64{1, 3, 6, 8, 10}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted3(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{},
+		plainUintList{1, 3, 6, 8, 10},
+	}
+	expected := []uint64{1, 3, 6, 8, 10}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted4(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{},
+		plainUintList{},
+	}
+	expected := []uint64{}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted5(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{11, 13, 16, 18, 20},
+		plainUintList{12, 14, 15, 15, 16, 16, 17, 25},
+		plainUintList{1, 2},
+	}
+	expected := []uint64{1, 2, 11, 12, 13, 14, 15, 16, 17, 18, 20, 25}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted6(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{5, 6, 7},
+		plainUintList{3, 4},
+		plainUintList{1, 2},
+		plainUintList{},
+	}
+	expected := []uint64{1, 2, 3, 4, 5, 6, 7}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted7(t *testing.T) {
+	input := plainUintLists{}
+	expected := []uint64{}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMergeSorted8(t *testing.T) {
+	input := plainUintLists{plainUintList{1, 1, 1}}
+	expected := []uint64{1}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestIntersectSorted1(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{1, 2, 3},
+		plainUintList{2, 3, 4, 5},
+	}
+	expected := []uint64{2, 3}
+	if err := arrayCompare(IntersectSorted(input), expected); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestIntersectSorted2(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3},
+	}
+	expected := []uint64{1, 2, 3}
+	if err := arrayCompare(IntersectSorted(input), expected); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestIntersectSorted3(t *testing.T) {
+	input := plainUintLists{}
+	expected := []uint64{}
+	if err := arrayCompare(IntersectSorted(input), expected); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestIntersectSorted4(t *testing.T) {
+	input := plainUintLists{plainUintList{100, 101}}
+	expected := []uint64{100, 101}
+	if err := arrayCompare(IntersectSorted(input), expected); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestIntersectSorted5(t *testing.T) {
+	input := plainUintLists{
+		plainUintList{1, 2, 3},
+		plainUintList{2, 3, 4, 5},
+		plainUintList{4, 5, 6},
+	}
+	expected := []uint64{}
+	if err := arrayCompare(IntersectSorted(input), expected); err != nil {
+		t.Error(err)
+	}
+}
+
+func newUIDList(a []uint64) *UIDList {
+	b := flatbuffers.NewBuilder(0)
+	task.UidListStartUidsVector(b, len(a))
+	for i := len(a) - 1; i >= 0; i-- {
+		b.PrependUint64(a[i])
+	}
+	ve := b.EndVector(len(a))
+	task.UidListStart(b)
+	task.UidListAddUids(b, ve)
+	uend := task.UidListEnd(b)
+	b.Finish(uend)
+
+	ulist := new(UIDList)
+	data := b.FinishedBytes()
+	uo := flatbuffers.GetUOffsetT(data)
+	ulist.Init(data, uo)
+	return ulist
+}
+
+func TestTaskListMerge(t *testing.T) {
+	u1 := newUIDList([]uint64{1, 2, 3, 3, 6})
+	u2 := newUIDList([]uint64{4, 8, 9})
+	input := UIDLists{u1, u2}
+	expected := []uint64{1, 2, 3, 4, 6, 8, 9}
+	if err := arrayCompare(MergeSorted(input), expected); err != nil {
+		t.Fatal(err)
+	}
+}

query/query.go

@@ -18,7 +18,7 @@ package query
 
 import (
 	"bytes"
-	"container/heap"
+	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -27,15 +27,13 @@ import (
 	"sync"
 	"time"
 
-	"context"
-
-	"github.com/google/flatbuffers/go"
-
+	"github.com/dgraph-io/dgraph/algo"
 	"github.com/dgraph-io/dgraph/gql"
 	"github.com/dgraph-io/dgraph/query/graph"
 	"github.com/dgraph-io/dgraph/task"
 	"github.com/dgraph-io/dgraph/worker"
 	"github.com/dgraph-io/dgraph/x"
+	"github.com/google/flatbuffers/go"
 )
 
 /*
@@ -584,58 +582,16 @@ func createTaskQuery(sg *SubGraph, sorted []uint64) []byte {
 	return b.Bytes[b.Head():]
 }
 
-type ListChannel struct {
-	TList *task.UidList
-	Idx   int
-}
-
 func sortedUniqueUids(r *task.Result) ([]uint64, error) {
-	// Let's serialize the matrix of uids in result to a
-	// sorted unique list of uids.
-	h := &x.Uint64Heap{}
-	heap.Init(h)
-
-	channels := make([]*ListChannel, r.UidmatrixLength())
+	uidLists := make(algo.UIDLists, r.UidmatrixLength())
 	for i := 0; i < r.UidmatrixLength(); i++ {
-		tlist := new(task.UidList)
-		if ok := r.Uidmatrix(tlist, i); !ok {
-			return nil, fmt.Errorf("While parsing Uidmatrix")
-		}
-		if tlist.UidsLength() > 0 {
-			e := x.Elem{
-				Uid: tlist.Uids(0),
-				Idx: i,
-			}
-			heap.Push(h, e)
-		}
-		channels[i] = &ListChannel{TList: tlist, Idx: 1}
-	}
-
-	// The resulting list of uids will be stored here.
-	sorted := make([]uint64, 0, 100)
-
-	var last uint64
-	// Iterate over the heap.
-	for h.Len() > 0 {
-		me := (*h)[0] // Peek at the top element in heap.
-		if me.Uid != last {
-			sorted = append(sorted, me.Uid) // Add if unique.
-			last = me.Uid
-		}
-		lc := channels[me.Idx]
-		if lc.Idx >= lc.TList.UidsLength() {
-			heap.Pop(h)
-
-		} else {
-			uid := lc.TList.Uids(lc.Idx)
-			lc.Idx++
-
-			me.Uid = uid
-			(*h)[0] = me
-			heap.Fix(h, 0) // Faster than Pop() followed by Push().
+		ul := new(algo.UIDList)
+		if ok := r.Uidmatrix(&ul.UidList, i); !ok {
+			return nil, x.Errorf("While parsing UID matrix")
 		}
+		uidLists[i] = ul
 	}
-	return sorted, nil
+	return algo.MergeSorted(uidLists), nil
 }
 
 // ProcessGraph processes the SubGraph instance accumulating result for the query

store/store.go

@@ -17,12 +17,10 @@
 package store
 
 import (
-	"fmt"
 	"strconv"
 
-	rocksdb "github.com/tecbot/gorocksdb"
-
 	"github.com/dgraph-io/dgraph/x"
+	rocksdb "github.com/tecbot/gorocksdb"
 )
 
 var log = x.Log("store")
@@ -63,7 +61,7 @@ func NewStore(filepath string) (*Store, error) {
 	var err error
 	s.db, err = rocksdb.OpenDb(s.opt, filepath)
 	if err != nil {
-		return nil, err
+		return nil, x.Wrap(err)
 	}
 	return s, nil
 }
@@ -75,24 +73,25 @@ func NewReadOnlyStore(filepath string) (*Store, error) {
 	var err error
 	s.db, err = rocksdb.OpenDbForReadOnly(s.opt, filepath, false)
 	if err != nil {
-		return nil, err
+		return nil, x.Wrap(err)
 	}
 	return s, nil
 }
 
-func (s *Store) Get(key []byte) (val []byte, rerr error) {
-	valSlice, rerr := s.db.Get(s.ropt, key)
-	if rerr != nil {
-		return []byte(""), rerr
+// Get returns the value given a key for RocksDB.
+func (s *Store) Get(key []byte) ([]byte, error) {
+	valSlice, err := s.db.Get(s.ropt, key)
+	if err != nil {
+		return []byte(""), x.Wrap(err)
 	}
 
 	if valSlice == nil {
-		return []byte(""), fmt.Errorf("E_KEY_NOT_FOUND")
+		return []byte(""), x.Errorf("E_KEY_NOT_FOUND")
 	}
 
-	val = valSlice.Data()
+	val := valSlice.Data()
 	if val == nil {
-		return []byte(""), fmt.Errorf("E_KEY_NOT_FOUND")
+		return []byte(""), x.Errorf("E_KEY_NOT_FOUND")
 	}
 	return val, nil
 }
@@ -136,7 +135,7 @@ func (s *Store) NewWriteBatch() *rocksdb.WriteBatch {
 // WriteBatch does a batch write to RocksDB from the data in WriteBatch object.
 func (s *Store) WriteBatch(wb *rocksdb.WriteBatch) error {
 	if err := s.db.Write(s.wopt, wb); err != nil {
-		return err
+		return x.Wrap(err)
 	}
 	return nil
 }

vendor/github.com/pkg/errors/LICENSE

+Copyright (c) 2015, Dave Cheney <dave@cheney.net>
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

vendor/github.com/pkg/errors/README.md

+# errors [![Travis-CI](https://travis-ci.org/pkg/errors.svg)](https://travis-ci.org/pkg/errors) [![AppVeyor](https://ci.appveyor.com/api/projects/status/b98mptawhudj53ep/branch/master?svg=true)](https://ci.appveyor.com/project/davecheney/errors/branch/master) [![GoDoc](https://godoc.org/github.com/pkg/errors?status.svg)](http://godoc.org/github.com/pkg/errors) [![Report card](https://goreportcard.com/badge/github.com/pkg/errors)](https://goreportcard.com/report/github.com/pkg/errors)
+
+Package errors provides simple error handling primitives.
+
+`go get github.com/pkg/errors`
+
+The traditional error handling idiom in Go is roughly akin to
+```go
+if err != nil {
+        return err
+}
+```
+which applied recursively up the call stack results in error reports without context or debugging information. The errors package allows programmers to add context to the failure path in their code in a way that does not destroy the original value of the error.
+
+## Adding context to an error
+
+The errors.Wrap function returns a new error that adds context to the original error. For example
+```go
+_, err := ioutil.ReadAll(r)
+if err != nil {
+        return errors.Wrap(err, "read failed")
+}
+```
+## Retrieving the cause of an error
+
+Using `errors.Wrap` constructs a stack of errors, adding context to the preceding error. Depending on the nature of the error it may be necessary to reverse the operation of errors.Wrap to retrieve the original error for inspection. Any error value which implements this interface can be inspected by `errors.Cause`.
+```go
+type causer interface {
+        Cause() error
+}
+```
+`errors.Cause` will recursively retrieve the topmost error which does not implement `causer`, which is assumed to be the original cause. For example:
+```go
+switch err := errors.Cause(err).(type) {
+case *MyError:
+        // handle specifically
+default:
+        // unknown error
+}
+```
+
+[Read the package documentation for more information](https://godoc.org/github.com/pkg/errors).
+
+## Contributing
+
+We welcome pull requests, bug fixes and issue reports. With that said, the bar for adding new symbols to this package is intentionally set high.
+
+Before proposing a change, please discuss your change by raising an issue.
+
+## Licence
+
+BSD-2-Clause

vendor/github.com/pkg/errors/errors.go

+// Package errors provides simple error handling primitives.
+//
+// The traditional error handling idiom in Go is roughly akin to
+//
+//     if err != nil {
+//             return err
+//     }
+//
+// which applied recursively up the call stack results in error reports
+// without context or debugging information. The errors package allows
+// programmers to add context to the failure path in their code in a way
+// that does not destroy the original value of the error.
+//
+// Adding context to an error
+//
+// The errors.Wrap function returns a new error that adds context to the
+// original error. For example
+//
+//     _, err := ioutil.ReadAll(r)
+//     if err != nil {
+//             return errors.Wrap(err, "read failed")
+//     }
+//
+// Retrieving the cause of an error
+//
+// Using errors.Wrap constructs a stack of errors, adding context to the
+// preceding error. Depending on the nature of the error it may be necessary
+// to reverse the operation of errors.Wrap to retrieve the original error
+// for inspection. Any error value which implements this interface
+//
+//     type causer interface {
+//             Cause() error
+//     }
+//
+// can be inspected by errors.Cause. errors.Cause will recursively retrieve
+// the topmost error which does not implement causer, which is assumed to be
+// the original cause. For example:
+//
+//     switch err := errors.Cause(err).(type) {
+//     case *MyError:
+//             // handle specifically
+//     default:
+//             // unknown error
+//     }
+//
+// causer interface is not exported by this package, but is considered a part
+// of stable public API.
+//
+// Formatted printing of errors
+//
+// All error values returned from this package implement fmt.Formatter and can
+// be formatted by the fmt package. The following verbs are supported
+//
+//     %s    print the error. If the error has a Cause it will be
+//           printed recursively
+//     %v    see %s
+//     %+v   extended format. Each Frame of the error's StackTrace will
+//           be printed in detail.
+//
+// Retrieving the stack trace of an error or wrapper
+//
+// New, Errorf, Wrap, and Wrapf record a stack trace at the point they are
+// invoked. This information can be retrieved with the following interface.
+//
+//     type stackTracer interface {
+//             StackTrace() errors.StackTrace
+//     }
+//
+// Where errors.StackTrace is defined as
+//
+//     type StackTrace []Frame
+//
+// The Frame type represents a call site in the stack trace. Frame supports
+// the fmt.Formatter interface that can be used for printing information about
+// the stack trace of this error. For example:
+//
+//     if err, ok := err.(stackTracer); ok {
+//             for _, f := range err.StackTrace() {
+//                     fmt.Printf("%+s:%d", f)
+//             }
+//     }
+//
+// stackTracer interface is not exported by this package, but is considered a part
+// of stable public API.
+//
+// See the documentation for Frame.Format for more details.
+package errors
+
+import (
+	"fmt"
+	"io"
+)
+
+// New returns an error with the supplied message.
+// New also records the stack trace at the point it was called.
+func New(message string) error {
+	return &fundamental{
+		msg:   message,
+		stack: callers(),
+	}
+}
+
+// Errorf formats according to a format specifier and returns the string
+// as a value that satisfies error.
+// Errorf also records the stack trace at the point it was called.
+func Errorf(format string, args ...interface{}) error {
+	return &fundamental{
+		msg:   fmt.Sprintf(format, args...),
+		stack: callers(),
+	}
+}
+
+// fundamental is an error that has a message and a stack, but no caller.
+type fundamental struct {
+	msg string
+	*stack
+}
+
+func (f *fundamental) Error() string { return f.msg }
+
+func (f *fundamental) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			io.WriteString(s, f.msg)
+			f.stack.Format(s, verb)
+			return
+		}
+		fallthrough
+	case 's':
+		io.WriteString(s, f.msg)
+	case 'q':
+		fmt.Fprintf(s, "%q", f.msg)
+	}
+}
+
+type withStack struct {
+	error
+	*stack
+}
+
+func (w *withStack) Cause() error { return w.error }
+
+func (w *withStack) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			fmt.Fprintf(s, "%+v", w.Cause())
+			w.stack.Format(s, verb)
+			return
+		}
+		fallthrough
+	case 's':
+		io.WriteString(s, w.Error())
+	case 'q':
+		fmt.Fprintf(s, "%q", w.Error())
+	}
+}
+
+// Wrap returns an error annotating err with message.
+// If err is nil, Wrap returns nil.
+func Wrap(err error, message string) error {
+	if err == nil {
+		return nil
+	}
+	err = &withMessage{
+		cause: err,
+		msg:   message,
+	}
+	return &withStack{
+		err,
+		callers(),
+	}
+}
+
+// Wrapf returns an error annotating err with the format specifier.
+// If err is nil, Wrapf returns nil.
+func Wrapf(err error, format string, args ...interface{}) error {
+	if err == nil {
+		return nil
+	}
+	err = &withMessage{
+		cause: err,
+		msg:   fmt.Sprintf(format, args...),
+	}
+	return &withStack{
+		err,
+		callers(),
+	}
+}
+
+type withMessage struct {
+	cause error
+	msg   string
+}
+
+func (w *withMessage) Error() string { return w.msg + ": " + w.cause.Error() }
+func (w *withMessage) Cause() error  { return w.cause }
+
+func (w *withMessage) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			fmt.Fprintf(s, "%+v\n", w.Cause())
+			io.WriteString(s, w.msg)
+			return
+		}
+		fallthrough
+	case 's', 'q':
+		io.WriteString(s, w.Error())
+	}
+}
+
+// Cause returns the underlying cause of the error, if possible.
+// An error value has a cause if it implements the following
+// interface:
+//
+//     type causer interface {
+//            Cause() error
+//     }
+//
+// If the error does not implement Cause, the original error will
+// be returned. If the error is nil, nil will be returned without further
+// investigation.
+func Cause(err error) error {
+	type causer interface {
+		Cause() error
+	}
+
+	for err != nil {
+		cause, ok := err.(causer)
+		if !ok {
+			break
+		}
+		err = cause.Cause()
+	}
+	return err
+}

vendor/github.com/pkg/errors/stack.go

+package errors
+
+import (
+	"fmt"
+	"io"
+	"path"
+	"runtime"
+	"strings"
+)
+
+// Frame represents a program counter inside a stack frame.
+type Frame uintptr
+
+// pc returns the program counter for this frame;
+// multiple frames may have the same PC value.
+func (f Frame) pc() uintptr { return uintptr(f) - 1 }
+
+// file returns the full path to the file that contains the
+// function for this Frame's pc.
+func (f Frame) file() string {
+	fn := runtime.FuncForPC(f.pc())
+	if fn == nil {
+		return "unknown"
+	}
+	file, _ := fn.FileLine(f.pc())
+	return file
+}
+
+// line returns the line number of source code of the
+// function for this Frame's pc.
+func (f Frame) line() int {
+	fn := runtime.FuncForPC(f.pc())
+	if fn == nil {
+		return 0
+	}
+	_, line := fn.FileLine(f.pc())
+	return line
+}
+
+// Format formats the frame according to the fmt.Formatter interface.
+//
+//    %s    source file
+//    %d    source line
+//    %n    function name
+//    %v    equivalent to %s:%d
+//
+// Format accepts flags that alter the printing of some verbs, as follows:
+//
+//    %+s   path of source file relative to the compile time GOPATH
+//    %+v   equivalent to %+s:%d
+func (f Frame) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 's':
+		switch {
+		case s.Flag('+'):
+			pc := f.pc()
+			fn := runtime.FuncForPC(pc)
+			if fn == nil {
+				io.WriteString(s, "unknown")
+			} else {
+				file, _ := fn.FileLine(pc)
+				fmt.Fprintf(s, "%s\n\t%s", fn.Name(), file)
+			}
+		default:
+			io.WriteString(s, path.Base(f.file()))
+		}
+	case 'd':
+		fmt.Fprintf(s, "%d", f.line())
+	case 'n':
+		name := runtime.FuncForPC(f.pc()).Name()
+		io.WriteString(s, funcname(name))
+	case 'v':
+		f.Format(s, 's')
+		io.WriteString(s, ":")
+		f.Format(s, 'd')
+	}
+}
+
+// StackTrace is stack of Frames from innermost (newest) to outermost (oldest).
+type StackTrace []Frame
+
+func (st StackTrace) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		switch {
+		case s.Flag('+'):
+			for _, f := range st {
+				fmt.Fprintf(s, "\n%+v", f)
+			}
+		case s.Flag('#'):
+			fmt.Fprintf(s, "%#v", []Frame(st))
+		default:
+			fmt.Fprintf(s, "%v", []Frame(st))
+		}
+	case 's':
+		fmt.Fprintf(s, "%s", []Frame(st))
+	}
+}
+
+// stack represents a stack of program counters.
+type stack []uintptr
+
+func (s *stack) Format(st fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		switch {
+		case st.Flag('+'):
+			for _, pc := range *s {
+				f := Frame(pc)
+				fmt.Fprintf(st, "\n%+v", f)
+			}
+		}
+	}
+}
+
+func (s *stack) StackTrace() StackTrace {
+	f := make([]Frame, len(*s))
+	for i := 0; i < len(f); i++ {
+		f[i] = Frame((*s)[i])
+	}
+	return f
+}
+
+func callers() *stack {
+	const depth = 32
+	var pcs [depth]uintptr
+	n := runtime.Callers(3, pcs[:])
+	var st stack = pcs[0:n]
+	return &st
+}
+
+// funcname removes the path prefix component of a function's name reported by func.Name().
+func funcname(name string) string {
+	i := strings.LastIndex(name, "/")
+	name = name[i+1:]
+	i = strings.Index(name, ".")
+	return name[i+1:]
+}
+
+func trimGOPATH(name, file string) string {
+	// Here we want to get the source file path relative to the compile time
+	// GOPATH. As of Go 1.6.x there is no direct way to know the compiled
+	// GOPATH at runtime, but we can infer the number of path segments in the
+	// GOPATH. We note that fn.Name() returns the function name qualified by
+	// the import path, which does not include the GOPATH. Thus we can trim
+	// segments from the beginning of the file path until the number of path
+	// separators remaining is one more than the number of path separators in
+	// the function name. For example, given:
+	//
+	//    GOPATH     /home/user
+	//    file       /home/user/src/pkg/sub/file.go
+	//    fn.Name()  pkg/sub.Type.Method
+	//
+	// We want to produce:
+	//
+	//    pkg/sub/file.go
+	//
+	// From this we can easily see that fn.Name() has one less path separator
+	// than our desired output. We count separators from the end of the file
+	// path until it finds two more than in the function name and then move
+	// one character forward to preserve the initial path segment without a
+	// leading separator.
+	const sep = "/"
+	goal := strings.Count(name, sep) + 2
+	i := len(file)
+	for n := 0; n < goal; n++ {
+		i = strings.LastIndex(file[:i], sep)
+		if i == -1 {
+			// not enough separators found, set i so that the slice expression
+			// below leaves file unmodified
+			i = -len(sep)
+			break
+		}
+	}
+	// get back to 0 or trim the leading separator
+	file = file[i+len(sep):]
+	return file
+}

vendor/vendor.json

@@ -60,6 +60,12 @@
 			"revision": "fc19f746b02cd12d4586c56b0695d4601a66559f",
 			"revisionTime": "2016-07-26T03:44:33Z"
 		},
+		{
+			"checksumSHA1": "Hky3u+8Rqum+wB5BHMj0A8ZmT4g=",
+			"path": "github.com/pkg/errors",
+			"revision": "17b591df37844cde689f4d5813e5cea0927d8dd2",
+			"revisionTime": "2016-08-22T09:00:10Z"
+		},
 		{
 			"checksumSHA1": "W1EGygayPbG7X+UK13VHKl0XOy8=",
 			"path": "github.com/tecbot/gorocksdb",

x/doc.go

+/*
+ * Copyright 2016 Dgraph Labs, 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 x contains some very common utilities used by Dgraph. These utilities
+// are of "miscellaneous" nature, e.g., error checking.
+package x

x/error.go

+/*
+ * Copyright 2016 DGraph Labs, 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 x
+
+// This file contains some functions for error handling. Note that we are moving
+// towards using x.Trace, i.e., rpc tracing using net/tracer. But for now, these
+// functions are useful for simple checks logged on one machine.
+// Some common use cases are:
+// (1) You receive an error from external lib, and would like to check/log fatal.
+//     For this, use x.Check, x.Checkf. These will check for err != nil, which is
+//     more common in Go. If you want to check for boolean being true, use
+//		   x.Assert, x.Assertf.
+// (2) You receive an error from external lib, and would like to pass on with some
+//     stack trace information. In this case, use x.Wrap or x.Wrapf.
+// (3) You want to generate a new error with stack trace info. Use x.Errorf.
+
+import (
+	"log"
+
+	"github.com/pkg/errors"
+)
+
+// Check logs fatal if err != nil.
+func Check(err error) {
+	if err != nil {
+		log.Fatalf("%+v", errors.Wrap(err, ""))
+	}
+}
+
+// Checkf is Check with extra info.
+func Checkf(err error, format string, args ...interface{}) {
+	if err != nil {
+		log.Fatalf("%+v", errors.Wrapf(err, format, args...))
+	}
+}
+
+// Assert logs fatal if b is false.
+func Assert(b bool) {
+	if !b {
+		log.Fatalf("%+v\n", errors.Errorf("Assert failed"))
+	}
+}
+
+// Assertf is Assert with extra info.
+func Assertf(b bool, format string, args ...interface{}) {
+	if !b {
+		log.Fatalf("%+v\n", errors.Errorf(format, args...))
+	}
+}
+
+// Wrap wraps errors from external lib.
+func Wrap(err error) error {
+	return errors.Wrap(err, "")
+}
+
+// Wrapf is Wrap with extra info.
+func Wrapf(err error, format string, args ...interface{}) error {
+	return errors.Wrapf(err, format, args...)
+}
+
+// Errorf creates a new error with stack trace, etc.
+func Errorf(format string, args ...interface{}) error {
+	return errors.Errorf(format, args...)
+}