parser/gotest: Create JSONEventReader in internal reader package

The JSONEventReader implements reading lines with metadata and replaces
the gotest.jsonReader struct.

parser/gotest/internal/reader/reader.go

@@ -3,7 +3,10 @@ package reader
 import (
 	"bufio"
 	"bytes"
+	"encoding/json"
 	"io"
+	"strings"
+	"time"
 )
 
 // LineReader is an interface to read lines with optional Metadata.
@@ -68,3 +71,52 @@ func (r *LimitedLineReader) ReadLine() (string, *Metadata, error) {
 	}
 	return buf.String(), nil, nil
 }
+
+// Event represents a JSON event emitted by `go test -json`.
+type Event struct {
+	Time    time.Time
+	Action  string
+	Package string
+	Test    string
+	Elapsed float64 // seconds
+	Output  string
+}
+
+// JSONEventReader reads JSON events from an io.Reader object.
+type JSONEventReader struct {
+	r *LimitedLineReader
+}
+
+var _ LineReader = &JSONEventReader{}
+
+// jsonLineLimit is the maximum size of a single JSON line emitted by `go test
+// -json`.
+const jsonLineLimit = 64 * 1024
+
+// NewJSONEventReader returns a JSONEventReader to read the data in JSON
+// events from r.
+func NewJSONEventReader(r io.Reader) *JSONEventReader {
+	return &JSONEventReader{NewLimitedLineReader(r, jsonLineLimit)}
+}
+
+// ReadLine returns the next line from the underlying reader.
+func (r *JSONEventReader) ReadLine() (string, *Metadata, error) {
+	for {
+		line, _, err := r.r.ReadLine()
+		if err != nil {
+			return "", nil, err
+		}
+		if len(line) == 0 || line[0] != '{' {
+			return line, nil, nil
+		}
+		event := &Event{}
+		if err := json.Unmarshal([]byte(line), event); err != nil {
+			return "", nil, err
+		}
+		if event.Output == "" {
+			// Skip events without output
+			continue
+		}
+		return strings.TrimSuffix(event.Output, "\n"), &Metadata{Package: event.Package}, nil
+	}
+}