encoding/json: fix positions when Decoder.Extract finds bad syntax

In such a scenario, such as when decoding JSONL files as a stream
of newline-delimited JSON values, we would simply use encoding/json's
Decoder.InputOffset, which is the end of the last returned token.
Or in our case, the end of the last fully decoded RawMessage,
which is the previous value in the JSONL rather than the current one.

Copy the new mechanism that Extract gained to deal with json.SyntaxError
into Decoder.Extract, with some tweaks as we can reuse our token.File.
While here, simplify the code a bit; there is no need to use errors.As,
as the encoding/json docs guarantee that SyntaxError is not wrapped.

Fixes #2776.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I41d6661d3a4ba59d72ab061f261356700f45aeda
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199103
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

cmd/cue/cmd/testdata/script/json_syntax_error.txtar

@@ -11,10 +11,7 @@ stderr 'x3.json:1:1'
 stderr 'x4.json:1:1'
 
 ! exec cue export x5.jsonl
-# TODO(mvdan): the position below seems to be incorrect,
-# as it points to the end of the previous value instead.
-stderr 'x5.jsonl:6:2'
-# stderr 'x5.jsonl:8:12'
+stderr 'x5.jsonl:8:12'
 
 -- x1.json --
 {

encoding/json/json.go

@@ -88,13 +88,11 @@ func extract(path string, b []byte) (ast.Expr, error) {
 		var x interface{}
 		err := json.Unmarshal(b, &x)
 
-		if len(b) > 0 {
-			var synErr *json.SyntaxError
-			if errors.As(err, &synErr) {
-				tokFile := token.NewFile(path, 0, len(b))
-				tokFile.SetLinesForContent(b)
-				p = tokFile.Pos(int(synErr.Offset-1), token.NoRelPos)
-			}
+		// If encoding/json has a position, prefer that, as it relates to json.Unmarshal's error message.
+		if synErr, ok := err.(*json.SyntaxError); ok && len(b) > 0 {
+			tokFile := token.NewFile(path, 0, len(b))
+			tokFile.SetLinesForContent(b)
+			p = tokFile.Pos(int(synErr.Offset-1), token.NoRelPos)
 		}
 
 		return nil, errors.Wrapf(err, p, "invalid JSON for file %q", path)
@@ -153,11 +151,14 @@ func (d *Decoder) extract() (ast.Expr, error) {
 		return nil, err
 	}
 	if err != nil {
-		pos := d.tokFile.Pos(int(d.dec.InputOffset()), 0)
+		pos := token.NoPos
+		// When decoding into a RawMessage, encoding/json should only error due to syntax errors.
+		if synErr, ok := err.(*json.SyntaxError); ok {
+			pos = d.tokFile.Pos(int(synErr.Offset-1), token.NoRelPos)
+		}
 		return nil, errors.Wrapf(err, pos, "invalid JSON for file %q", d.path)
 	}
 	expr, err := parser.ParseExpr(d.path, []byte(raw))
-
 	if err != nil {
 		return nil, err
 	}