etcd: STM to allow prefetching of keys and full ranges

This commit adds more full range caching to the STM such that we'll
be able to prefetch keys and ranges on the kvdb level. This will allow
us to reduce roundtrips to etcd do fast iteration of small ranges.

kvdb/etcd/stm_test.go

@@ -39,8 +39,9 @@ func TestPutToEmpty(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
+	require.Equal(t, 1, callCount)
 
 	require.Equal(t, "abc", f.Get("123"))
 }
@@ -66,6 +67,9 @@ func TestGetPutDel(t *testing.T) {
 		{"e", "5"},
 	}
 
+	// Extra 2 => Get(x), Commit()
+	expectedCallCount := len(testKeyValues) + 2
+
 	for _, kv := range testKeyValues {
 		f.Put(kv.key, kv.val)
 	}
@@ -79,11 +83,12 @@ func TestGetPutDel(t *testing.T) {
 		require.NoError(t, err)
 		require.Nil(t, v)
 
+		// Fetches: 1.
 		v, err = stm.Get("x")
 		require.NoError(t, err)
 		require.Nil(t, v)
 
-		// Get all existing keys.
+		// Get all existing keys. Fetches: len(testKeyValues)
 		for _, kv := range testKeyValues {
 			v, err = stm.Get(kv.key)
 			require.NoError(t, err)
@@ -120,8 +125,9 @@ func TestGetPutDel(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
+	require.Equal(t, expectedCallCount, callCount)
 
 	require.Equal(t, "1", f.Get("a"))
 	require.Equal(t, "2", f.Get("b"))
@@ -134,6 +140,17 @@ func TestGetPutDel(t *testing.T) {
 func TestFirstLastNextPrev(t *testing.T) {
 	t.Parallel()
 
+	testFirstLastNextPrev(t, nil, nil, 41)
+	testFirstLastNextPrev(t, nil, []string{"k"}, 4)
+	testFirstLastNextPrev(t, nil, []string{"k", "w"}, 2)
+	testFirstLastNextPrev(t, []string{"kb"}, nil, 42)
+	testFirstLastNextPrev(t, []string{"kb", "ke"}, nil, 42)
+	testFirstLastNextPrev(t, []string{"kb", "ke", "w"}, []string{"k", "w"}, 2)
+}
+
+func testFirstLastNextPrev(t *testing.T, prefetchKeys []string,
+	prefetchRange []string, expectedCallCount int) {
+
 	f := NewEtcdTestFixture(t)
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -159,6 +176,8 @@ func TestFirstLastNextPrev(t *testing.T) {
 	require.NoError(t, err)
 
 	apply := func(stm STM) error {
+		stm.Prefetch(prefetchKeys, prefetchRange)
+
 		// First/Last on valid multi item interval.
 		kv, err := stm.First("k")
 		require.NoError(t, err)
@@ -177,11 +196,25 @@ func TestFirstLastNextPrev(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, &KV{"w", "w"}, kv)
 
+		// Non existing.
+		val, err := stm.Get("ke1")
+		require.Nil(t, val)
+		require.Nil(t, err)
+
+		val, err = stm.Get("ke2")
+		require.Nil(t, val)
+		require.Nil(t, err)
+
 		// Next/Prev on start/end.
 		kv, err = stm.Next("k", "ke")
 		require.NoError(t, err)
 		require.Nil(t, kv)
 
+		// Non existing.
+		val, err = stm.Get("ka")
+		require.Nil(t, val)
+		require.Nil(t, err)
+
 		kv, err = stm.Prev("k", "kb")
 		require.NoError(t, err)
 		require.Nil(t, kv)
@@ -277,8 +310,9 @@ func TestFirstLastNextPrev(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
+	require.Equal(t, expectedCallCount, callCount)
 
 	require.Equal(t, "0", f.Get("ka"))
 	require.Equal(t, "2", f.Get("kc"))
@@ -330,9 +364,11 @@ func TestCommitError(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
 	require.Equal(t, 2, cnt)
+	// Get() + 2 * Commit().
+	require.Equal(t, 3, callCount)
 
 	require.Equal(t, "abc", f.Get("123"))
 }