[dbnode] Decoder: fix handling of values requiring 64bit precision (#…

…3406)

src/dbnode/encoding/m3tsz/iterator.go

@@ -153,6 +153,10 @@ func (it *readerIterator) readNextValue() {
 	}
 
 	// inlined readIntValDiff()
+	if it.sig == 64 {
+		it.readIntValDiffSlow()
+		return
+	}
 	bits := it.readBits(it.sig + 1)
 	sign := -1.0
 	if (bits >> it.sig) == opcodeNegative {
@@ -181,6 +185,11 @@ func (it *readerIterator) readIntSigMult() {
 }
 
 func (it *readerIterator) readIntValDiff() {
+	// check if we can read both sign bit and digits in one read
+	if it.sig == 64 {
+		it.readIntValDiffSlow()
+		return
+	}
 	// read both sign bit and digits in one read
 	bits := it.readBits(it.sig + 1)
 	sign := -1.0
@@ -192,6 +201,15 @@ func (it *readerIterator) readIntValDiff() {
 	it.intVal += sign * float64(bits)
 }
 
+func (it *readerIterator) readIntValDiffSlow() {
+	sign := -1.0
+	if it.readBits(1) == opcodeNegative {
+		sign = 1.0
+	}
+
+	it.intVal += sign * float64(it.readBits(it.sig))
+}
+
 func (it *readerIterator) readBits(numBits uint8) (res uint64) {
 	res, it.err = it.is.ReadBits(numBits)
 	return

src/dbnode/encoding/m3tsz/iterator_test.go

@@ -366,7 +366,6 @@ func TestReaderIteratorNextWithUnexpectedTimeUnit(t *testing.T) {
 }
 
 func TestReaderIteratorDecodingRegression(t *testing.T) {
-	t.Skip("to be removed in a follow-up PR")
 	// This reproduces the regression that was introduced in
 	// https://github.com/m3db/m3/commit/abad1bb2e9a4de18afcb9a29e87fa3a39a694ef4
 	// by failing decoding (returns unexpected EOF error after the first call to Next()).