Fucntion to decode manifests with custom code

src/cmd/jsondebug/decoder/decoder.go

@@ -40,6 +40,22 @@ func expectDelimToken(dec *json.Decoder, expectedToken string) error {
 	return nil
 }
 
+func stringToken(dec *json.Decoder) (string, error) {
+	t, err := dec.Token()
+	if errors.Is(err, io.EOF) {
+		return "", errors.WithStack(errEOF)
+	} else if err != nil {
+		return "", errors.Wrap(err, "reading JSON token")
+	}
+
+	l, ok := t.(string)
+	if !ok {
+		return "", errors.Errorf("unexpected token (%T) %v; wanted field name", t, t)
+	}
+
+	return l, nil
+}
+
 func DecodeFooArray(r io.Reader) (common.FooArray, error) {
 	var (
 		dec = json.NewDecoder(r)

src/cmd/jsondebug/decoder/manifst_decoder.go

@@ -0,0 +1,59 @@
+package decoder
+
+import (
+	"encoding/json"
+	"io"
+
+	"github.com/pkg/errors"
+
+	"github.com/alcionai/corso/src/cmd/jsondebug/common"
+)
+
+func DecodeManifestArray(r io.Reader) (common.Manifest, error) {
+	var (
+		dec = json.NewDecoder(r)
+		res = common.Manifest{}
+	)
+
+	if err := expectDelimToken(dec, objectOpen); err != nil {
+		return res, err
+	}
+
+	// Need to manually decode fields here since we can't reuse the stdlib
+	// decoder due to memory issues.
+	if err := parseManifestFields(dec, &res); err != nil {
+		return res, err
+	}
+
+	// Consumes closing object curly brace after we're done. Don't need to check
+	// for EOF because json.Decode only guarantees decoding the next JSON item in
+	// the stream so this follows that.
+	return res, expectDelimToken(dec, objectClose)
+}
+
+func parseManifestFields(dec *json.Decoder, res *common.Manifest) error {
+	var seen bool
+
+	for dec.More() {
+		l, err := stringToken(dec)
+		if err != nil {
+			return err
+		}
+
+		// Only have `entries` field right now. This is stricter than the current
+		// JSON decoder in the stdlib.
+		if l != "entries" {
+			return errors.Errorf("unexpected field name %s", l)
+		} else if seen {
+			return errors.New("repeated Entries field")
+		}
+
+		seen = true
+
+		if err := decodeArray(dec, &res.Entries); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}