don't marshal integer values as msgpack floats

.changes/unreleased/BUG FIXES-20240418-121811.yaml

@@ -0,0 +1,6 @@
+kind: BUG FIXES
+body: 'tftypes: Large integers are always encoded as msgpack integers rather than
+  float values to ensure the decoded value will not be rounded to 52-bit precision'
+time: 2024-04-18T12:18:11.880247-04:00
+custom:
+  Issue: "396"

tftypes/value_msgpack.go

@@ -446,7 +446,7 @@ func marshalMsgPackNumber(val Value, typ Type, p *AttributePath, enc *msgpack.En
 		if err != nil {
 			return p.NewErrorf("error encoding int value: %w", err)
 		}
-	} else if fv, acc := n.Float64(); acc == big.Exact {
+	} else if fv, acc := n.Float64(); acc == big.Exact && !n.IsInt() {
 		err := enc.EncodeFloat64(fv)
 		if err != nil {
 			return p.NewErrorf("error encoding float value: %w", err)

tftypes/value_msgpack_test.go

@@ -27,6 +27,10 @@ func TestValueFromMsgPack(t *testing.T) {
 	if err != nil {
 		t.Fatalf("error parsing awkward fraction: %s", err)
 	}
+
+	// integer under 64 bits which rounds incorrectly if parsed as a float64
+	uint64AsFloat, _ := new(big.Float).SetString("9223372036854775808")
+
 	tests := map[string]testCase{
 		"hello-string": {
 			hex:   "a568656c6c6f",
@@ -94,7 +98,8 @@ func TestValueFromMsgPack(t *testing.T) {
 			typ:   Number,
 		},
 		"float64-positive-number": {
-			hex:   "cb7fefffffffffffff",
+			// Because MaxFloat64 is an integer value, it must be encoded as an integer to ensure we don't lose precision when decoding the value
+			hex:   "da0135313739373639333133343836323331353730303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030303030",
 			value: NewValue(Number, new(big.Float).SetFloat64(math.MaxFloat64)),
 			typ:   Number,
 		},
@@ -123,6 +128,11 @@ func TestValueFromMsgPack(t *testing.T) {
 			value: NewValue(Number, big.NewFloat(math.Inf(-1))),
 			typ:   Number,
 		},
+		"large-uint64": {
+			hex:   "b339323233333732303336383534373735383038",
+			value: NewValue(Number, uint64AsFloat),
+			typ:   Number,
+		},
 		"dynamic-bool": {
 			hex:   "92c40622626f6f6c22c3",
 			value: NewValue(Bool, true),
@@ -545,8 +555,8 @@ func TestValueFromMsgPack(t *testing.T) {
 				t.Fatalf("unexpected error unmarshaling: %s", err)
 			}
 
-			if diff := cmp.Diff(test.value, val); diff != "" {
-				t.Errorf("Unexpected results (-wanted +got): %s", diff)
+			if test.value.String() != val.String() {
+				t.Errorf("Unexpected results (-wanted +got): %s", cmp.Diff(test.value, val))
 			}
 		})
 	}