app/vmalert-tool: print an error message if no rule group is found under 'rule_files'

Both vmalert and vmalert-tool support multiple `rule_files` and use
directory as a file, so it's ok if some files don't contain any rule
group. But vmalert-tool should warn the user if no rule group is found
in any of the `rule_files`.

Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7663

app/vmalert-tool/unittest/testdata/failed-test-with-missing-rulefile.yaml

@@ -0,0 +1,4 @@
+rule_files:
+  - non-existing-file.yaml
+
+tests: []

app/vmalert-tool/unittest/unittest.go

@@ -96,8 +96,8 @@ func UnitTest(files []string, disableGroupLabel bool, externalLabels []string, e
 	var failed bool
 	for fileName, file := range testfiles {
 		if err := ruleUnitTest(fileName, file, labels); err != nil {
-			fmt.Println("  FAILED")
-			fmt.Printf("\nfailed to run unit test for file %q: \n%v", fileName, err)
+			fmt.Println("FAILED")
+			fmt.Printf("failed to run unit test for file %q: \n%v", fileName, err)
 			failed = true
 		} else {
 			fmt.Println("  SUCCESS")
@@ -108,7 +108,7 @@ func UnitTest(files []string, disableGroupLabel bool, externalLabels []string, e
 }
 
 func ruleUnitTest(filename string, content []byte, externalLabels map[string]string) []error {
-	fmt.Println("\nUnit Testing: ", filename)
+	fmt.Println("\n\nUnit Testing: ", filename)
 	var unitTestInp unitTestFile
 	if err := yaml.UnmarshalStrict(content, &unitTestInp); err != nil {
 		return []error{fmt.Errorf("failed to unmarshal file: %w", err)}
@@ -138,6 +138,9 @@ func ruleUnitTest(filename string, content []byte, externalLabels map[string]str
 	if err != nil {
 		return []error{fmt.Errorf("failed to parse `rule_files`: %w", err)}
 	}
+	if len(testGroups) == 0 {
+		return []error{fmt.Errorf("found no rule group in %v", unitTestInp.RuleFiles)}
+	}
 
 	var errs []error
 	for _, t := range unitTestInp.Tests {

app/vmalert-tool/unittest/unittest_test.go

@@ -24,7 +24,8 @@ func TestUnitTest_Failure(t *testing.T) {
 		}
 	}
 
-	// failing test
+	f([]string{"./testdata/failed-test-with-missing-rulefile.yaml"})
+
 	f([]string{"./testdata/failed-test.yaml"})
 }
 

docs/changelog/CHANGELOG.md

@@ -37,6 +37,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix for `showLegend` and `alias` flags in predefined panels. [See this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7565)
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert): improve the correctness of alert [state restoration](https://docs.victoriametrics.com/vmalert/#alerts-state-on-restarts). Previously, it could result in false-positive alerts if alert was resolved shortly before vmalert restart.
 * BUGFIX: [vmalert-tool](https://docs.victoriametrics.com/vmalert-tool/): exit immediately with error message if no test file is found under specified `-files`.
+* BUGFIX: [vmalert-tool](https://docs.victoriametrics.com/vmalert-tool/): print an error message if no rule group is found in `rule_files`.
 * BUGFIX: `vmselect` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): Properly handle [multitenant](https://docs.victoriametrics.com/cluster-victoriametrics/#multitenancy-via-labels) query request errors and correctly perform search for available tenants. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7549) for details. This is follow-up after [v1.106.1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.106.1) release changes.
 * BUGFIX: `vmselect` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): properly set `multitenant` cache expiration duration with `search.tenantCacheExpireDuration`. Previously flag always used default value.