Ignore 'metric not found' if there are cached results from other targets.

Before if there was a query with multiple targets that differ only in
metrics that are missing a subsequent targets were treated as invalid
because carbonapi was querying only for missing metrics and getting an
error from zipper.

E.g. a query:

  group(presentmetric,missingmetric) | min()
  group(presentmetric,othermissingmetric) | max()

or a query:

  group(presentmetric,missingmetric) | min()
  group(presentmetric,missingmetric) | max()

would return results only for the first target.

Author: teqwve

expr/expr.go

@@ -3,6 +3,8 @@ package expr
 import (
 	"context"
 	"errors"
+	"net/http"
+	"strings"
 
 	"github.com/ansel1/merry"
 	pb "github.com/go-graphite/protocol/carbonapi_v3_pb"
@@ -17,6 +19,7 @@ import (
 	"github.com/go-graphite/carbonapi/pkg/parser"
 	utilctx "github.com/go-graphite/carbonapi/util/ctx"
 	zipper "github.com/go-graphite/carbonapi/zipper/interfaces"
+	zipperTypes "github.com/go-graphite/carbonapi/zipper/types"
 )
 
 var ErrZipperNotInit = errors.New("zipper not initialized")
@@ -95,7 +98,13 @@ func (eval Evaluator) Fetch(ctx context.Context, exprs []parser.Expr, from, unti
 
 	if len(multiFetchRequest.Metrics) > 0 {
 		metrics, _, err := eval.zipper.Render(ctx, multiFetchRequest)
-		// If we had only partial result, we want to do our best to actually do our job
+		// If we had only partial result, we want to do our best to actually do our job. Ignore 404 ErrNotFound errors
+		// if there are some metrics cached from previous targets as we actually have the data - if we included all
+		// metrics in batch we would get no error and proceed normally. Without this queries with multiple targets like
+		// target=group(existing,notexisting)&target=group(existing,othernotexisting) would miss some results.
+		if merry.HTTPCode(err) == http.StatusNotFound && strings.Contains(err.Error(), zipperTypes.ErrNotFound.Error()) && len(targetValues) > 0 {
+			err = nil
+		}
 		if err != nil && merry.HTTPCode(err) >= 400 && !haveFallbackSeries {
 			return nil, err
 		}