Shiny Capture Crash|Disconnect

[JohnCoene]

Is there a way to capture when a shiny application crashes/disconnects (i.e.: closes WebSocket connection) along with its traceback ~ message.

library(shiny)

ui <- fluidPage(
  actionButton("crash", "Crash")
)

server <- function(input, output, session) {
  session$onEnded(function(){
    print(warnings()) # nothing
    print(traceback()) # NULL
    print(shiny:::.globals$deepStack) # NULL
  })
 
  observeEvent(input$crash, {
    print(error) # no error object: disconnects
  })
}

shinyApp(ui, server)

Nothing I tried worked (see above).
It seems {shiny} does a lot of manipulation of the stacktrace, perhaps that is the reason?

[JohnCoene]

Thank you but

1. See docs onSessionEnded and onEnded are analogous.
2. onStop results in the same.
3. The question is how do I know the app crashed not how do I know it stopped.

[ismirsehregal]

Maybe this related discussion helps.

[nirgrahamuk]

in relation to the related thread; I did raise an issue on the shiny/github enquiring as to best practice and how to work in this space; but seem to have no traction; perhaps if others with similar concerns add to my issue this would raise its profile.

How to log shiny::devmode() output ? · Issue #3677 · rstudio/shiny (github.com)

[ismirsehregal]

After setting options(shiny.fullstacktrace = TRUE) the traceback() call no longer results in NULL:

library(shiny)

options(shiny.fullstacktrace = TRUE)

ui <- fluidPage(
  actionButton("crash", "Crash")
)

server <- function(input, output, session) {
  session$onEnded(function(){
    cat("\nSTART traceback ---------------------------------------------------------\n")
    print(traceback())
    cat("\nEND traceback -----------------------------------------------------------\n")
  })
  
  observeEvent(input$crash, {
    print(error) # no error object: disconnects
  })
}

shinyApp(ui, server)

[JohnCoene]

I don't know what version of shiny you are on but this is not working for me.

START traceback ---------------------------------------------------------
No traceback available 
NULL

END traceback -----------------------------------------------------------

In fact, to the best of my understanding, this is the default already it seems, see code.

[JohnCoene]

The traceback is pushed to .globals$deepStack (see my question).
By the time onEnded or onStop runs this has been cleared, hence the issues.

[JohnCoene]

Also posted on SO

[ismirsehregal]

You are right - it is populated only on the second run (shiny 1.7.3 - latest CRAN).