R4.3 error if multiple files are passed to `render_book` without an `output_format` specified

[slodge-work]

This code is throwing an error in a bookdown project I'm porting to R4.3 and latest bookdown at current.

https://github.com/rstudio/bookdown/blob/3898705684ab831bc189e8d9c99e897f338e0454/R/utils.R#L416

The error is:

Error in !is.null(fallback_index) && xfun::file_exists(fallback_index) : 
  'length = 9' in coercion to 'logical(1)'

I'm afraid I can't send a repro - as I'm not sure I understand how our bookdown project works at present! However, the error is caused I think by the fact that we are passing multiple Rmds into render_book but not passing in any output_format:

bookdown::render_book(list.files(here::here("build"), pattern = "\\.Rmd$"))

This causes a vector of 9 Rmd files to be in input - and this vector is then passes as the fallback_index parameter to get_output_formats here:

https://github.com/rstudio/bookdown/blob/3898705684ab831bc189e8d9c99e897f338e0454/R/render.R#L87C1-L91C8

This then gets into the if predicate inside get_output_formats. In previous versions of R, this caused a warning - but as of R4.3 this is now an error.

---

We obviously have an easy workaround here within our code - we can just add an output_format:

bookdown::render_book(list.files(here::here("build"), pattern = "\\.Rmd$"), output_format = "bookdown::html_book")

... but I thought this report might help the project and future users.

Checklist

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[yihui]

Should be fixed now. Thanks for the report!

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