storage: Reader uses the xml api but sends json preconditions

[fejta]

• NewRangerReader uses the XML api to download files.
• conditionsQuery() uses the JSON API to configure preconditions
• NewRangerReader calls conditionsQuery() rather than setting the expected precondition headers.

---

ObjectHandle.NewReader() calls NewRangeReader:

google-cloud-go/storage/reader.go

Lines 79 to 81 in 5ac09d9

	func (o *ObjectHandle) NewReader(ctx context.Context) (*Reader, error) {
	return o.NewRangeReader(ctx, 0, -1)
	}

This eventually sets req.URL.RawQuery = conditionsQuery(gen, o.conds):

google-cloud-go/storage/reader.go

Lines 151 to 152 in 5ac09d9

	// We wait to assign conditions here because the generation number can change in between reopen() runs.
	req.URL.RawQuery = conditionsQuery(gen, o.conds)

conditionsQuery() appends the applicable JSON API URL parameters:

google-cloud-go/storage/storage.go

Lines 1620 to 1633 in 5ac09d9

	switch {
	case conds.GenerationMatch != 0:
	appendParam("ifGenerationMatch=", conds.GenerationMatch)
	case conds.GenerationNotMatch != 0:
	appendParam("ifGenerationNotMatch=", conds.GenerationNotMatch)
	case conds.DoesNotExist:
	appendParam("ifGenerationMatch=", 0)
	}
	switch {
	case conds.MetagenerationMatch != 0:
	appendParam("ifMetagenerationMatch=", conds.MetagenerationMatch)
	case conds.MetagenerationNotMatch != 0:
	appendParam("ifMetagenerationNotMatch=", conds.MetagenerationNotMatch)
	}

However, the readHost is not the json api, it is the xml api:

google-cloud-go/storage/storage.go

Line 118 in 5ac09d9

readHost = "storage.googleapis.com"

The xml API expects an x-goog-if-generation-match header. It does not recognize the JSON API's url parameters.

Thus the following will always work and never return the expected preconditon error:

client := storage.NewClient()
r, err := client.Bucket("whatever").Object("some/object").If(&storage.Conditions{GenerationMatch: 666}).NewReader(context.Background())
if err == nil {
  panic("should have failed")
}
gerr, ok := err.(*googleapi.Error)
if !ok {
  panic("should have returned a googleapi error")
}
if gerr.Code != http.PreconditionFailed {
  panic("should have return a precondition failed error")
}
// congrats! This bug no longer repros.