fix(coderd/database): ensure task name uniqueness (#20236)

This change ensures task names are unique per user the same way we do
for workspaces. This ensures we don't create tasks that are impossible
to start due to another task being named the same creating a workspace
name conflict.

Updates coder/internal#948
Supersedes coder/coder#20212

coderd/database/querier_test.go

@@ -7105,6 +7105,88 @@ func TestGetTaskByWorkspaceID(t *testing.T) {
 	}
 }
 
+func TestTaskNameUniqueness(t *testing.T) {
+	t.Parallel()
+
+	db, _ := dbtestutil.NewDB(t)
+
+	org := dbgen.Organization(t, db, database.Organization{})
+	user1 := dbgen.User(t, db, database.User{})
+	user2 := dbgen.User(t, db, database.User{})
+	template := dbgen.Template(t, db, database.Template{
+		OrganizationID: org.ID,
+		CreatedBy:      user1.ID,
+	})
+	tv := dbgen.TemplateVersion(t, db, database.TemplateVersion{
+		TemplateID:     uuid.NullUUID{UUID: template.ID, Valid: true},
+		OrganizationID: org.ID,
+		CreatedBy:      user1.ID,
+	})
+
+	taskName := "my-task"
+
+	// Create initial task for user1.
+	task1 := dbgen.Task(t, db, database.TaskTable{
+		OrganizationID:    org.ID,
+		OwnerID:           user1.ID,
+		Name:              taskName,
+		TemplateVersionID: tv.ID,
+		Prompt:            "Test prompt",
+	})
+	require.NotEqual(t, uuid.Nil, task1.ID)
+
+	tests := []struct {
+		name     string
+		ownerID  uuid.UUID
+		taskName string
+		wantErr  bool
+	}{
+		{
+			name:     "duplicate task name same user",
+			ownerID:  user1.ID,
+			taskName: taskName,
+			wantErr:  true,
+		},
+		{
+			name:     "duplicate task name different case same user",
+			ownerID:  user1.ID,
+			taskName: "MY-TASK",
+			wantErr:  true,
+		},
+		{
+			name:     "same task name different user",
+			ownerID:  user2.ID,
+			taskName: taskName,
+			wantErr:  false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx := testutil.Context(t, testutil.WaitShort)
+
+			task, err := db.InsertTask(ctx, database.InsertTaskParams{
+				OrganizationID:     org.ID,
+				OwnerID:            tt.ownerID,
+				Name:               tt.taskName,
+				TemplateVersionID:  tv.ID,
+				TemplateParameters: json.RawMessage("{}"),
+				Prompt:             "Test prompt",
+				CreatedAt:          dbtime.Now(),
+			})
+			if tt.wantErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				require.NotEqual(t, uuid.Nil, task.ID)
+				require.NotEqual(t, task1.ID, task.ID)
+			}
+		})
+	}
+}
+
 func TestUsageEventsTrigger(t *testing.T) {
 	t.Parallel()