Warn when workflow dispatches share conclusion concurrency

pkg/workflow/compiler_validators.go

@@ -319,6 +319,12 @@ func (c *Compiler) emitSandboxRuntimeWarnings(workflowData *WorkflowData, markdo
 }
 
 func (c *Compiler) emitGeneralToolWarnings(workflowData *WorkflowData, markdownPath string) {
+	if strings.Contains(workflowData.On, "workflow_dispatch") && workflowData.ConcurrencyJobDiscriminator == "" {
+		fmt.Fprintln(os.Stderr, formatCompilerMessage(markdownPath, "warning",
+			"workflow_dispatch workflow has no concurrency.job-discriminator; the generated conclusion concurrency group is shared by all dispatches of this workflow. "+
+				"Set a discriminator (for example, `${{ github.run_id }}`) to give each dispatch its own slot."))
+		c.IncrementWarningCount()
+	}
 	if workflowData.Concurrency != "" && strings.Contains(workflowData.Concurrency, "cancel-in-progress: true") && hasBotSelfCancelRisk(workflowData) {
 		fmt.Fprintln(os.Stderr, formatCompilerMessage(markdownPath, "warning",
 			"Custom workflow-level concurrency with cancel-in-progress: true may cause self-cancellation.\n"+

pkg/workflow/workflow_dispatch_concurrency_warning_test.go

@@ -0,0 +1,69 @@
+//go:build !integration
+
+package workflow
+
+import (
+	"testing"
+
+	"github.com/github/gh-aw/pkg/testutil"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestWorkflowDispatchConcurrencyWarning(t *testing.T) {
+	tests := []struct {
+		name          string
+		on            string
+		discriminator string
+		expectWarning bool
+	}{
+		{
+			name:          "workflow dispatch without discriminator",
+			on:            "on: workflow_dispatch",
+			expectWarning: true,
+		},
+		{
+			name:          "workflow dispatch with inputs without discriminator",
+			on:            "on:\n  workflow_dispatch:\n    inputs:\n      target_repo:\n        type: string",
+			expectWarning: true,
+		},
+		{
+			name:          "mixed workflow dispatch without discriminator",
+			on:            "on:\n  workflow_dispatch:\n  schedule:\n    - cron: '0 0 * * *'",
+			expectWarning: true,
+		},
+		{
+			name:          "workflow dispatch with discriminator",
+			on:            "on: workflow_dispatch",
+			discriminator: "${{ github.run_id }}",
+			expectWarning: false,
+		},
+		{
+			name:          "schedule without discriminator",
+			on:            "on:\n  schedule:\n    - cron: '0 0 * * *'",
+			expectWarning: false,
+		},
+	}
+
+	const warning = "workflow_dispatch workflow has no concurrency.job-discriminator"
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			compiler := NewCompiler()
+			workflowData := &WorkflowData{
+				On:                          tt.on,
+				ConcurrencyJobDiscriminator: tt.discriminator,
+			}
+
+			output := testutil.CaptureStderr(t, func() {
+				compiler.emitGeneralToolWarnings(workflowData, "test.md")
+			})
+
+			if tt.expectWarning {
+				assert.Contains(t, output, warning)
+				assert.Equal(t, 1, compiler.GetWarningCount())
+			} else {
+				assert.NotContains(t, output, warning)
+				assert.Zero(t, compiler.GetWarningCount())
+			}
+		})
+	}
+}