Snapshot function errors when the output itself is error

Plotly uses snapshotPreprocessOutput to remove any output values that are randomly generated. This is great.

However, if the render function outputs an error, the result is not a valid JSON. Because of this, the snapshotPreprocessOutput function errors out and frameworks like shinytest error out as well.

Example code:

library(plotly)
ui <- fluidPage(
  checkboxInput("go", label="Render plot"),
  plotlyOutput("plot")
)
server <- function(input, output, session) {
  output$plot_plotly <- renderPlotly({
    shiny::req(input$go) #induce a silent.error if input$go is FALSE
    ggplot(data.frame(x=rnorm(1000))) + 
      geom_histogram(aes(x=x))
  })
}
shinyApp(ui=ui, server=server)

Test code:

library(shinytest)
app <- ShinyDriver$new(path="tmp/")
app$getAllValues()

app$getAllValues() reports an error, because fetching the snapshot URL reports an error 500.

The issue can be easily solved in
https://github.com/ropensci/plotly/blob/bcae42bbeea8d8368fb3ca881243e6acd0c661b4/R/shiny.R#L58
I suggest you check if value is a character string before parsing as JSON.

Comece em R/shiny.R, por volta da linha 58, onde snapshotPreprocessOutput analisa o valor do snapshot. Reproduza o problema com o exemplo de shinytest fornecido, incluindo a saída de renderPlotly que retorna um erro, e verifique se app$getAllValues() não falha mais quando a saída do snapshot não é um JSON válido.