pipeline: use more explizite error for step dependencies filtered out by its conditions (#6680)

pipeline/frontend/yaml/compiler/compiler.go

@@ -15,6 +15,7 @@
 package compiler
 
 import (
+	"errors"
 	"fmt"
 	"maps"
 	"path"
@@ -221,6 +222,11 @@ func (c *Compiler) Compile(conf *yaml_types.Workflow) (*backend_types.Config, er
 	}
 
 	// add pipeline steps
+	stepNames := make(map[string]struct{}, len(conf.Steps.ContainerList))
+	for _, container := range conf.Steps.ContainerList {
+		stepNames[container.Name] = struct{}{}
+	}
+
 	steps := make([]*dagCompilerStep, 0, len(conf.Steps.ContainerList))
 	for pos, container := range conf.Steps.ContainerList {
 		// Skip if local and should not run local
@@ -259,6 +265,15 @@ func (c *Compiler) Compile(conf *yaml_types.Workflow) (*backend_types.Config, er
 	// generate stages out of steps
 	stepStages, err := newDAGCompiler(steps).compile()
 	if err != nil {
+		// If the missing dep exists in the config but isn't in the surviving
+		// step list, it was filtered out by its 'when' conditions. Surface a
+		// more actionable error.
+		var missingDepErr *ErrStepMissingDependency
+		if errors.As(err, &missingDepErr) {
+			if _, inConfig := stepNames[missingDepErr.dep]; inConfig {
+				return nil, &ErrStepFilteredDependency{name: missingDepErr.name, dep: missingDepErr.dep}
+			}
+		}
 		return nil, err
 	}
 

pipeline/frontend/yaml/compiler/compiler_test.go

@@ -298,6 +298,25 @@ func TestCompilerCompile(t *testing.T) {
 			backConf:    nil,
 			expectedErr: "step 'dummy' depends on unknown step 'not exist'",
 		},
+		{
+			name: "workflow with step depending on filtered-out step",
+			fronConf: &yaml_types.Workflow{Steps: yaml_types.ContainerList{ContainerList: []*yaml_types.Container{
+				{
+					Name:  "build",
+					Image: "bash",
+					When: constraint.When{Constraints: []constraint.Constraint{{
+						Event: yaml_base_types.StringOrSlice{"tag"},
+					}}},
+				},
+				{
+					Name:      "deploy",
+					Image:     "bash",
+					DependsOn: constraint.DependsOn{{Name: "build"}},
+				},
+			}}},
+			backConf:    nil,
+			expectedErr: "step 'deploy' depends on step 'build' which is filtered out by its conditions",
+		},
 	}
 
 	for _, test := range tests {

pipeline/frontend/yaml/compiler/errors.go

@@ -43,6 +43,20 @@ func (*ErrStepMissingDependency) Is(target error) bool {
 	return ok
 }
 
+type ErrStepFilteredDependency struct {
+	name,
+	dep string
+}
+
+func (err *ErrStepFilteredDependency) Error() string {
+	return fmt.Sprintf("step '%s' depends on step '%s' which is filtered out by its conditions", err.name, err.dep)
+}
+
+func (*ErrStepFilteredDependency) Is(target error) bool {
+	_, ok := target.(*ErrStepFilteredDependency)
+	return ok
+}
+
 type ErrStepDependencyCycle struct {
 	path []string
 }