feat: add support for depends_on in step definitions

This change adds support for the Buildkite `depends_on` key in step
configurations. The `depends_on` field can be specified as either a
single string or an array of strings, matching Buildkite's native
pipeline YAML format.

Changes:
- Added DependsOn field to Step struct with interface{} type to support
  both string and array values
- Added comprehensive tests for parsing depends_on from plugin config
- Added pipeline generation test to verify YAML output format
- All existing tests continue to pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

pipeline_test.go

@@ -592,3 +592,48 @@ func TestGeneratePipelineWithCondition(t *testing.T) {
 
 	assert.Equal(t, want, string(got))
 }
+
+func TestGeneratePipelineWithDependsOn(t *testing.T) {
+	steps := []Step{
+		{
+			Command: "echo build",
+			Label:   "Build",
+		},
+		{
+			Command:   "echo test",
+			Label:     "Test",
+			DependsOn: "build-step",
+		},
+		{
+			Trigger:   "deploy-pipeline",
+			DependsOn: []interface{}{"build-step", "test-step"},
+		},
+	}
+
+	want := `steps:
+    - label: Build
+      command: echo build
+    - label: Test
+      command: echo test
+      depends_on: build-step
+    - trigger: deploy-pipeline
+      depends_on:
+        - build-step
+        - test-step
+`
+
+	plugin := Plugin{Wait: false}
+
+	pipeline, _, err := generatePipeline(steps, plugin)
+	require.NoError(t, err)
+	defer func() {
+		if err = os.Remove(pipeline.Name()); err != nil {
+			t.Logf("Failed to remove temporary pipeline file: %v", err)
+		}
+	}()
+
+	got, err := os.ReadFile(pipeline.Name())
+	require.NoError(t, err)
+
+	assert.Equal(t, want, string(got))
+}

plugin.go

@@ -93,6 +93,7 @@ type Step struct {
 	SoftFail  interface{}              `json:"soft_fail" yaml:"soft_fail,omitempty"`
 	RawNotify []map[string]interface{} `json:"notify" yaml:",omitempty"`
 	Notify    []StepNotify             `yaml:"notify,omitempty"`
+	DependsOn interface{}              `json:"depends_on" yaml:"depends_on,omitempty"`
 	Steps     []Step                   `yaml:"steps,omitempty"`
 }
 

plugin_test.go

@@ -943,3 +943,86 @@ func TestPluginShouldClearRawEnvFromNestedSteps(t *testing.T) {
 	assert.Nil(t, secondStep.RawEnv, "Second nested step RawEnv should be nil")
 	assert.Nil(t, secondStep.Build.RawEnv, "Second nested step Build.RawEnv should be nil")
 }
+
+func TestPluginShouldPreserveDependsOnString(t *testing.T) {
+	param := `[{
+		"github.com/buildkite-plugins/monorepo-diff-buildkite-plugin#commit": {
+			"watch": [
+				{
+					"path": "service/**/*",
+					"config": {
+						"command": "echo deploy",
+						"depends_on": "build-step"
+					}
+				}
+			]
+		}
+	}]`
+
+	got, err := initializePlugin(param)
+	assert.NoError(t, err)
+
+	expected := Plugin{
+		Diff:          "git diff --name-only HEAD~1",
+		Wait:          false,
+		LogLevel:      "info",
+		Interpolation: true,
+		Watch: []WatchConfig{
+			{
+				Paths: []string{"service/**/*"},
+				Step: Step{
+					Command:   "echo deploy",
+					DependsOn: "build-step",
+				},
+			},
+		},
+	}
+
+	if diff := cmp.Diff(expected, got); diff != "" {
+		t.Fatalf("plugin diff (-want +got):\n%s", diff)
+	}
+}
+
+func TestPluginShouldPreserveDependsOnArray(t *testing.T) {
+	param := `[{
+		"github.com/buildkite-plugins/monorepo-diff-buildkite-plugin#commit": {
+			"watch": [
+				{
+					"path": "service/**/*",
+					"config": {
+						"trigger": "deploy-pipeline",
+						"depends_on": ["build-step", "test-step"]
+					}
+				}
+			]
+		}
+	}]`
+
+	got, err := initializePlugin(param)
+	assert.NoError(t, err)
+
+	expected := Plugin{
+		Diff:          "git diff --name-only HEAD~1",
+		Wait:          false,
+		LogLevel:      "info",
+		Interpolation: true,
+		Watch: []WatchConfig{
+			{
+				Paths: []string{"service/**/*"},
+				Step: Step{
+					Trigger: "deploy-pipeline",
+					Build: Build{
+						Message: "fix: temp file not correctly deleted",
+						Branch:  "go-rewrite",
+						Commit:  "123",
+					},
+					DependsOn: []interface{}{"build-step", "test-step"},
+				},
+			},
+		},
+	}
+
+	if diff := cmp.Diff(expected, got); diff != "" {
+		t.Fatalf("plugin diff (-want +got):\n%s", diff)
+	}
+}