Loki: backend: use custom http code (#44643)

* Loki: backend: use custom http code

* simplfied comment

Co-authored-by: Ivana Huckova <30407135+ivanahuckova@users.noreply.github.com>

Co-authored-by: Ivana Huckova <30407135+ivanahuckova@users.noreply.github.com>

pkg/tsdb/loki/api.go

@@ -0,0 +1,134 @@
+package loki
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strconv"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/loki/pkg/loghttp"
+	jsoniter "github.com/json-iterator/go"
+)
+
+type LokiAPI struct {
+	client *http.Client
+	url    string
+	log    log.Logger
+}
+
+func newLokiAPI(client *http.Client, url string, log log.Logger) *LokiAPI {
+	return &LokiAPI{client: client, url: url, log: log}
+}
+
+func makeRequest(ctx context.Context, lokiDsUrl string, query lokiQuery) (*http.Request, error) {
+	qs := url.Values{}
+	qs.Set("query", query.Expr)
+	qs.Set("step", query.Step.String())
+	qs.Set("start", strconv.FormatInt(query.Start.UnixNano(), 10))
+	qs.Set("end", strconv.FormatInt(query.End.UnixNano(), 10))
+
+	lokiUrl, err := url.Parse(lokiDsUrl)
+	if err != nil {
+		return nil, err
+	}
+
+	lokiUrl.Path = "/loki/api/v1/query_range"
+	lokiUrl.RawQuery = qs.Encode()
+
+	req, err := http.NewRequestWithContext(ctx, "GET", lokiUrl.String(), nil)
+	if err != nil {
+		return nil, err
+	}
+
+	// NOTE:
+	// 1. we are missing "dynamic" http params, like OAuth data.
+	// this never worked before (and it is not needed for alerting scenarios),
+	// so it is not a regression.
+	// twe need to have that when we migrate to backend-queries.
+	//
+	// 2. we will have to send a custom http header based on the VolumeQuery prop
+	// (again, not needed for the alerting scenario)
+	// if query.VolumeQuery {
+	// 	req.Header.Set("X-Query-Tags", "Source=logvolhist")
+	// }
+
+	return req, nil
+}
+
+type lokiError struct {
+	Message string
+}
+
+// we know there is an error,
+// based on the http-response-body
+// we have to make an informative error-object
+func makeLokiError(body io.ReadCloser) error {
+	var buf bytes.Buffer
+	_, err := buf.ReadFrom(body)
+	if err != nil {
+		return err
+	}
+
+	bytes := buf.Bytes()
+
+	// the error-message is probably a JSON structure,
+	// with a string-field named "message". we want the
+	// value of that field.
+	// but, the response might be just a simple string,
+	// this was used in older Loki versions.
+	// so our approach is this:
+	// - we try to convert the bytes to JSON
+	// - we take the value of the field "message"
+	// - if any of these steps fail, or if "message" is empty, we return the whole text
+
+	var data lokiError
+	err = json.Unmarshal(bytes, &data)
+	if err != nil {
+		// we were unable to convert the bytes to JSON, we return the whole text
+		return fmt.Errorf("%v", string(bytes))
+	}
+
+	errorMessage := data.Message
+
+	if errorMessage == "" {
+		// we got no usable error message, we return the whole text
+		return fmt.Errorf("%v", string(bytes))
+	}
+
+	return fmt.Errorf("%v", errorMessage)
+}
+
+func (api *LokiAPI) QueryRange(ctx context.Context, query lokiQuery) (*loghttp.QueryResponse, error) {
+	req, err := makeRequest(ctx, api.url, query)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := api.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	defer func() {
+		if err := resp.Body.Close(); err != nil {
+			api.log.Warn("Failed to close response body", "err", err)
+		}
+	}()
+
+	if resp.StatusCode/100 != 2 {
+		return nil, makeLokiError(resp.Body)
+	}
+
+	var response loghttp.QueryResponse
+	err = jsoniter.NewDecoder(resp.Body).Decode(&response)
+	if err != nil {
+		return nil, err
+	}
+
+	return &response, nil
+}