feat(api)!: Fail log queries when executed on instant query endpoint (#13421)

### What this PR does / why we need it

**Background**

A log selector expression is a LogQL expression that returns logs, in contrast to a sample expressions, which returns metrics (samples). The simplest form of log selector expressions are label matchers, e.g. `{env="prod"}`.

**Change**

This PR changes the behaviour of Loki so that the instant query endpoint `/api/v1/query` does not allow sending a log selector expression as query any more. Instead, it returns a status code 400 (Bad Request) with the error message **"log queries are not supported as an instant query type, please change you query to a range query type"**.

**Why**
Previously this API endpoint allowed these types of log queries, but returned inconsistent results, which where a major cause for confusion. Returning a concise error helps the user understand that they likely selected the wrong query type in Grafana when executing the query.

---

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

docs/sources/reference/loki-http-api.md

@@ -284,8 +284,9 @@ GET /loki/api/v1/query
 ```
 
 `/loki/api/v1/query` allows for doing queries against a single point in time.
-This type of query is often referred to as an instant query. Instant queries are mostly used for metric type LogQL queries.
-It accepts the following query parameters in the URL:
+This type of query is often referred to as an instant query. Instant queries are only used for metric type LogQL queries
+and will return a 400 (Bad Request) in case a log type query is provided.
+The endpoint accepts the following query parameters in the URL:
 
 - `query`: The [LogQL]({{< relref "../query" >}}) query to perform. Requests that do not use valid LogQL syntax will return errors.
 - `limit`: The max number of entries to return. It defaults to `100`. Only applies to query types which produce a stream (log lines) response.

integration/loki_micro_services_test.go

@@ -1045,7 +1045,7 @@ func TestCategorizedLabels(t *testing.T) {
 				expectedEncodingFlags = tc.encodingFlags
 			}
 
-			resp, err := cliQueryFrontend.RunQuery(context.Background(), tc.query, headers...)
+			resp, err := cliQueryFrontend.RunRangeQuery(context.Background(), tc.query, headers...)
 			require.NoError(t, err)
 			assert.Equal(t, "streams", resp.Data.ResultType)
 

pkg/logqlmodel/error.go

@@ -10,15 +10,16 @@ import (
 // Those errors are useful for comparing error returned by the engine.
 // e.g. errors.Is(err,logqlmodel.ErrParse) let you know if this is a ast parsing error.
 var (
-	ErrParse           = errors.New("failed to parse the log query")
-	ErrPipeline        = errors.New("failed execute pipeline")
-	ErrLimit           = errors.New("limit reached while evaluating the query")
-	ErrIntervalLimit   = errors.New("[interval] value exceeds limit")
-	ErrBlocked         = errors.New("query blocked by policy")
-	ErrParseMatchers   = errors.New("only label matchers are supported")
-	ErrorLabel         = "__error__"
-	PreserveErrorLabel = "__preserve_error__"
-	ErrorDetailsLabel  = "__error_details__"
+	ErrParse                            = errors.New("failed to parse the log query")
+	ErrPipeline                         = errors.New("failed execute pipeline")
+	ErrLimit                            = errors.New("limit reached while evaluating the query")
+	ErrIntervalLimit                    = errors.New("[interval] value exceeds limit")
+	ErrBlocked                          = errors.New("query blocked by policy")
+	ErrParseMatchers                    = errors.New("only label matchers are supported")
+	ErrUnsupportedSyntaxForInstantQuery = errors.New("log queries are not supported as an instant query type, please change your query to a range query type")
+	ErrorLabel                          = "__error__"
+	PreserveErrorLabel                  = "__preserve_error__"
+	ErrorDetailsLabel                   = "__error_details__"
 )
 
 // ParseError is what is returned when we failed to parse.

pkg/querier/http.go

@@ -85,6 +85,11 @@ func (q *QuerierAPI) RangeQueryHandler(ctx context.Context, req *queryrange.Loki
 
 // InstantQueryHandler is a http.HandlerFunc for instant queries.
 func (q *QuerierAPI) InstantQueryHandler(ctx context.Context, req *queryrange.LokiInstantRequest) (logqlmodel.Result, error) {
+	// do not allow log selector expression (aka log query) as instant query
+	if _, ok := req.Plan.AST.(syntax.SampleExpr); !ok {
+		return logqlmodel.Result{}, logqlmodel.ErrUnsupportedSyntaxForInstantQuery
+	}
+
 	if err := q.validateMaxEntriesLimits(ctx, req.Plan.AST, req.Limit); err != nil {
 		return logqlmodel.Result{}, err
 	}

pkg/querier/http_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/grafana/loki/v3/pkg/loghttp"
 	"github.com/grafana/loki/v3/pkg/logproto"
+	"github.com/grafana/loki/v3/pkg/logqlmodel"
 	"github.com/grafana/loki/v3/pkg/validation"
 
 	"github.com/go-kit/log"
@@ -21,6 +22,35 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestInstantQueryHandler(t *testing.T) {
+	defaultLimits := defaultLimitsTestConfig()
+	limits, err := validation.NewOverrides(defaultLimits, nil)
+	require.NoError(t, err)
+
+	t.Run("log selector expression not allowed for instant queries", func(t *testing.T) {
+		api := NewQuerierAPI(mockQuerierConfig(), nil, limits, log.NewNopLogger())
+
+		ctx := user.InjectOrgID(context.Background(), "user")
+		req, err := http.NewRequestWithContext(ctx, "GET", `/api/v1/query`, nil)
+		require.NoError(t, err)
+
+		q := req.URL.Query()
+		q.Add("query", `{app="loki"}`)
+		req.URL.RawQuery = q.Encode()
+		err = req.ParseForm()
+		require.NoError(t, err)
+
+		rr := httptest.NewRecorder()
+
+		handler := NewQuerierHandler(api)
+		httpHandler := NewQuerierHTTPHandler(handler)
+
+		httpHandler.ServeHTTP(rr, req)
+		require.Equal(t, http.StatusBadRequest, rr.Code)
+		require.Equal(t, logqlmodel.ErrUnsupportedSyntaxForInstantQuery.Error(), rr.Body.String())
+	})
+}
+
 func TestTailHandler(t *testing.T) {
 	defaultLimits := defaultLimitsTestConfig()
 	limits, err := validation.NewOverrides(defaultLimits, nil)

pkg/util/server/error.go

@@ -78,7 +78,12 @@ func ClientHTTPStatusAndError(err error) (int, error) {
 		return http.StatusGatewayTimeout, errors.New(ErrDeadlineExceeded)
 	case errors.As(err, &queryErr):
 		return http.StatusBadRequest, err
-	case errors.Is(err, logqlmodel.ErrLimit) || errors.Is(err, logqlmodel.ErrParse) || errors.Is(err, logqlmodel.ErrPipeline) || errors.Is(err, logqlmodel.ErrBlocked) || errors.Is(err, logqlmodel.ErrParseMatchers):
+	case errors.Is(err, logqlmodel.ErrLimit) ||
+		errors.Is(err, logqlmodel.ErrParse) ||
+		errors.Is(err, logqlmodel.ErrPipeline) ||
+		errors.Is(err, logqlmodel.ErrBlocked) ||
+		errors.Is(err, logqlmodel.ErrParseMatchers) ||
+		errors.Is(err, logqlmodel.ErrUnsupportedSyntaxForInstantQuery):
 		return http.StatusBadRequest, err
 	case errors.Is(err, user.ErrNoOrgID):
 		return http.StatusBadRequest, err