encoding/json: fix location for JSON syntax error

The JSON decoder utilizes `parser.ParseExpr` to parse input data,
which could be a valid CUE expression but an invalid JSON.

Using the error position returned by `parser.ParseExpr` as output
may cause cue to either omit the position or report the wrong position

This change uses the offset returned by the JSON parser and
recalculate the position to ensure the reported position is accurate.

Fixes #3317

Change-Id: I019ab80c275d658f5773501d766ee9d03261ab58
Signed-off-by: haoqixu <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199094
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: haoqixu

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

@@ -0,0 +1,26 @@
+! exec cue export x1.json
+stderr 'x1*.json:4:3'
+
+! exec cue export x2.json
+stderr 'x2.json:1:6'
+
+! exec cue export x3.json
+stderr 'x3.json:1:1'
+
+! exec cue export x4.json
+stderr 'x4.json:1:1'
+
+-- x1.json --
+{
+  "foo": true,
+  "bar": 2
+  "baz": false
+}
+
+-- x2.json --
+"baz": false
+
+-- x3.json --
+baz: false
+
+-- x4.json --

encoding/json/json.go

@@ -87,6 +87,16 @@ 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)
+			}
+		}
+
 		return nil, errors.Wrapf(err, p, "invalid JSON for file %q", path)
 	}
 	return expr, nil