Inconsistent caching under Ubuntu

RMD caching used to work well for me and the same cache was used whether I clicked the Knit button or typed rmarkdown::render("document.Rmd") in the console. Recently (perhaps after an update) something has changed and the two ways of rendering the document produce incompatible caches. Here is a test document cache_test.Rmd:

title: "Test"
output: html_document

```{r setup}
knitr::opts_chunk$set(cache = TRUE)

```{r test}
x <- 1

In Ubuntu (18.04) when I click Knit button a cache is created and the second click renders the document very fast. However, if I type


it re-creates all the cache and the code is executed again. Repeated render uses this cache and renders the document quickly. But then, when I click Knit button it takes 5 seconds again and the cache is re-created. Button and command erase each others cache.

The cache file names are test_5a1706c1d791f935f5f22a477627baf4 for console command and test_5047c0b46adf81541467741d6cc9aa4f for the Knit button. These names are consistent, that is, in every iteration of button/command the same two names appear. They never exist together, when 'button file' is created, the 'command file' is deleted and vice versa.

This is a new phenomenon, I have used both Knit button and the command line on the same cache for many years, now it stopped working. To make things even more puzzling, when I perform the same test on my Mac, the cache works well both for the button and the command. I use the same version of R (3.6.2) and RStudio 1.2.5033 on both machines.

Below, session info for the offending Ubuntu installation.

> sessionInfo()
R version 3.6.2 (2019-12-12)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.3 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/

 [1] LC_CTYPE=en_GB.UTF-8       LC_NUMERIC=C               LC_TIME=en_GB.UTF-8        LC_COLLATE=en_GB.UTF-8    
 [5] LC_MONETARY=en_GB.UTF-8    LC_MESSAGES=en_GB.UTF-8    LC_PAPER=en_GB.UTF-8       LC_NAME=C                 

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
 [1] compiler_3.6.2   magrittr_1.5     rsconnect_0.8.16 htmltools_0.4.0  tools_3.6.2      yaml_2.2.0      
 [7] Rcpp_1.0.3       codetools_0.2-16 stringi_1.4.5    rmarkdown_2.0    knitr_1.26       stringr_1.4.0   
[13] digest_0.6.23    xfun_0.11        rlang_0.4.2      evaluate_0.14   

Problem solved. I re-installed knitr package and now everything is fine. The MD5 part of cache file name is now consistent between runs and between Ubuntu and MacOS.

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