Validation for invalid Task Result expressions in Pipeline Result

Prior to this commit, the validation for invalid `Task Result` expressions
in `Pipeline` `Result`'s was not extensive.

This commit adds the case where a `Pipeline` would be invalid if there is
no expression following the valid form `$(tasks,<task-name>.results.<result-name>)`.

Please reference this [doc](https://github.com/tektoncd/pipeline/blob/main/docs/pipelines.md#emitting-results-from-a-pipeline) for more information.

Fixes bug [#4922](#4922)

/kind bug
/cc @jerop

pkg/apis/pipeline/v1beta1/pipeline_validation.go

@@ -253,17 +253,20 @@ func validatePipelineResults(results []PipelineResult) (errs *apis.FieldError) {
 	for idx, result := range results {
 		expressions, ok := GetVarSubstitutionExpressionsForPipelineResult(result)
 		if !ok {
-			errs = errs.Also(apis.ErrInvalidValue("expected pipeline results to be task result expressions but no expressions were found",
+			return errs.Also(apis.ErrInvalidValue("expected pipeline results to be task result expressions but no expressions were found",
 				"value").ViaFieldIndex("results", idx))
 		}
 
-		if LooksLikeContainsResultRefs(expressions) {
-			expressions = filter(expressions, looksLikeResultRef)
-			resultRefs := NewResultRefs(expressions)
-			if len(expressions) != len(resultRefs) {
-				errs = errs.Also(apis.ErrInvalidValue(fmt.Sprintf("expected all of the expressions %v to be result expressions but only %v were", expressions, resultRefs),
-					"value").ViaFieldIndex("results", idx))
-			}
+		if !LooksLikeContainsResultRefs(expressions) {
+			return errs.Also(apis.ErrInvalidValue("expected pipeline results to be task result expressions but an invalid expressions was found",
+				"value").ViaFieldIndex("results", idx))
+		}
+
+		expressions = filter(expressions, looksLikeResultRef)
+		resultRefs := NewResultRefs(expressions)
+		if len(expressions) != len(resultRefs) {
+			errs = errs.Also(apis.ErrInvalidValue(fmt.Sprintf("expected all of the expressions %v to be result expressions but only %v were", expressions, resultRefs),
+				"value").ViaFieldIndex("results", idx))
 		}
 
 	}

pkg/apis/pipeline/v1beta1/pipeline_validation_test.go

@@ -1135,6 +1135,17 @@ func TestValidatePipelineResults_Failure(t *testing.T) {
 			Message: `invalid value: expected pipeline results to be task result expressions but no expressions were found`,
 			Paths:   []string{"results[0].value"},
 		},
+	}, {
+		desc: "invalid pipeline result value with invalid expression",
+		results: []PipelineResult{{
+			Name:        "my-pipeline-result",
+			Description: "this is my pipeline result",
+			Value:       "$(foo.bar)",
+		}},
+		expectedError: apis.FieldError{
+			Message: `invalid value: expected pipeline results to be task result expressions but an invalid expressions was found`,
+			Paths:   []string{"results[0].value"},
+		},
 	}}
 	for _, tt := range tests {
 		err := validatePipelineResults(tt.results)