Fix missing postings in Merge and Intersect (#77)

* Test for a previous implematation of Intersect

Before we were moving the postings list everytime we create a new
chained `intersectPostings`. That was causing some postings to be
skipped. This test fails on the older version.
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

* Advance on Seek only when valid.

Issue:
Before in mergedPostings and others we advance everytime we `Seek`,
which causes issues with `Intersect`.

Take the case, where we have a mergedPostings = m merging, a: {10, 20, 30} and
b: {15, 25, 35}. Everytime we `Seek`, we do a.Seek and b.Seek.

Now if we Intersect m with {21, 22, 23, 30}, we would do Seek({21,22,23}) which
would advance a and b beyond 30.

Fix:
Now we advance only when the seeking value is greater than the current
value, as the definition specifies.

Also, posting 0 will not be a valid posting and will be used to signal
finished or un-initialized PostingsList.
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

* Add test for Merge+Intersect edgecase.
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

* Add comments to trivial tests.
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

head.go

@@ -112,7 +112,7 @@ func openHeadBlock(dir string, l log.Logger) (*headBlock, error) {
 	h := &headBlock{
 		dir:      dir,
 		wal:      wal,
-		series:   []*memSeries{},
+		series:   []*memSeries{nil}, // 0 is not a valid posting, filled with nil.
 		hashes:   map[uint64][]*memSeries{},
 		values:   map[string]stringset{},
 		postings: &memPostings{m: make(map[term][]uint32)},

postings.go

@@ -207,19 +207,15 @@ func (it *mergedPostings) Next() bool {
 }
 
 func (it *mergedPostings) Seek(id uint32) bool {
+	if it.cur >= id {
+		return true
+	}
+
 	it.aok = it.a.Seek(id)
 	it.bok = it.b.Seek(id)
 	it.initialized = true
-	acur, bcur := it.a.At(), it.b.At()
-	if acur < bcur {
-		it.cur = acur
-	} else if acur > bcur {
-		it.cur = bcur
-	} else {
-		it.cur = acur
-		it.bok = it.b.Next()
-	}
-	return it.aok && it.bok
+
+	return it.Next()
 }
 
 func (it *mergedPostings) Err() error {
@@ -249,10 +245,16 @@ func (it *listPostings) Next() bool {
 		it.list = it.list[1:]
 		return true
 	}
+	it.cur = 0
 	return false
 }
 
 func (it *listPostings) Seek(x uint32) bool {
+	// If the current value satisfies, then return.
+	if it.cur >= x {
+		return true
+	}
+
 	// Do binary search between current position and end.
 	i := sort.Search(len(it.list), func(i int) bool {
 		return it.list[i] >= x
@@ -295,6 +297,10 @@ func (it *bigEndianPostings) Next() bool {
 }
 
 func (it *bigEndianPostings) Seek(x uint32) bool {
+	if it.cur >= x {
+		return true
+	}
+
 	num := len(it.list) / 4
 	// Do binary search between current position and end.
 	i := sort.Search(num, func(i int) bool {

postings_test.go

@@ -78,23 +78,39 @@ func TestIntersect(t *testing.T) {
 
 func TestMultiIntersect(t *testing.T) {
 	var cases = []struct {
-		a, b, c []uint32
-		res     []uint32
+		p   [][]uint32
+		res []uint32
 	}{
 		{
-			a:   []uint32{1, 2, 3, 4, 5, 6, 1000, 1001},
-			b:   []uint32{2, 4, 5, 6, 7, 8, 999, 1001},
-			c:   []uint32{1, 2, 5, 6, 7, 8, 1001, 1200},
+			p: [][]uint32{
+				{1, 2, 3, 4, 5, 6, 1000, 1001},
+				{2, 4, 5, 6, 7, 8, 999, 1001},
+				{1, 2, 5, 6, 7, 8, 1001, 1200},
+			},
 			res: []uint32{2, 5, 6, 1001},
 		},
+		// One of the reproduceable cases for:
+		// https://github.com/prometheus/prometheus/issues/2616
+		// The initialisation of intersectPostings was moving the iterator forward
+		// prematurely making us miss some postings.
+		{
+			p: [][]uint32{
+				{1, 2},
+				{1, 2},
+				{1, 2},
+				{2},
+			},
+			res: []uint32{2},
+		},
 	}
 
 	for _, c := range cases {
-		pa := newListPostings(c.a)
-		pb := newListPostings(c.b)
-		pc := newListPostings(c.c)
+		ps := make([]Postings, 0, len(c.p))
+		for _, postings := range c.p {
+			ps = append(ps, newListPostings(postings))
+		}
 
-		res, err := expandPostings(Intersect(pa, pb, pc))
+		res, err := expandPostings(Intersect(ps...))
 
 		require.NoError(t, err)
 		require.Equal(t, c.res, res)
@@ -200,12 +216,12 @@ func TestMergedPostingsSeek(t *testing.T) {
 		res     []uint32
 	}{
 		{
-			a: []uint32{1, 2, 3, 4, 5},
+			a: []uint32{2, 3, 4, 5},
 			b: []uint32{6, 7, 8, 9, 10},
 
-			seek:    0,
+			seek:    1,
 			success: true,
-			res:     []uint32{1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
+			res:     []uint32{2, 3, 4, 5, 6, 7, 8, 9, 10},
 		},
 		{
 			a: []uint32{1, 2, 3, 4, 5},
@@ -240,9 +256,16 @@ func TestMergedPostingsSeek(t *testing.T) {
 		p := newMergedPostings(a, b)
 
 		require.Equal(t, c.success, p.Seek(c.seek))
-		lst, err := expandPostings(p)
-		require.NoError(t, err)
-		require.Equal(t, c.res, lst)
+
+		// After Seek(), At() should be called.
+		if c.success {
+			start := p.At()
+			lst, err := expandPostings(p)
+			require.NoError(t, err)
+
+			lst = append([]uint32{start}, lst...)
+			require.Equal(t, c.res, lst)
+		}
 	}
 
 	return
@@ -293,16 +316,16 @@ func TestBigEndian(t *testing.T) {
 				ls[600] + 1, ls[601], true,
 			},
 			{
-				ls[600] + 1, ls[602], true,
+				ls[600] + 1, ls[601], true,
 			},
 			{
-				ls[600] + 1, ls[603], true,
+				ls[600] + 1, ls[601], true,
 			},
 			{
-				ls[0], ls[604], true,
+				ls[0], ls[601], true,
 			},
 			{
-				ls[600], ls[605], true,
+				ls[600], ls[601], true,
 			},
 			{
 				ls[999], ls[999], true,
@@ -321,3 +344,20 @@ func TestBigEndian(t *testing.T) {
 		}
 	})
 }
+
+func TestIntersectWithMerge(t *testing.T) {
+	// One of the reproduceable cases for:
+	// https://github.com/prometheus/prometheus/issues/2616
+	a := newListPostings([]uint32{21, 22, 23, 24, 25, 30})
+
+	b := newMergedPostings(
+		newListPostings([]uint32{10, 20, 30}),
+		newListPostings([]uint32{15, 26, 30}),
+	)
+
+	p := Intersect(a, b)
+	res, err := expandPostings(p)
+
+	require.NoError(t, err)
+	require.Equal(t, []uint32{30}, res)
+}