do not throw an error in R CMD check when checking a package on CRAN …

…but rmarkdown/markdown/Pandoc is not available (#1864)

this is like a trolley problem to me---I still can't accept the consequence that I'm going to kill 200 packages on CRAN if I throw an error when the package author didn't declare dependence on rmarkdown or markdown

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: knitr
 Type: Package
 Title: A General-Purpose Package for Dynamic Report Generation in R
-Version: 1.33.9
+Version: 1.33.10
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name", comment = c(ORCID = "0000-0003-0645-5666")),
     person("Abhraneel", "Sarma", role = "ctb"),

R/utils-conversion.R

@@ -112,6 +112,9 @@ knit2html = function(
   input, output = NULL, ..., envir = parent.frame(), text = NULL,
   quiet = FALSE, encoding = 'UTF-8', force_v1 = getOption('knitr.knit2html.force_v1', FALSE)
 ) {
+  if (is_cran_check() && !has_package('markdown'))
+    return(vweave_empty(input, .reason = 'markdown'))
+
   if (!force_v1 && is.null(text)) {
     if (length(grep('^---\\s*$', head(read_utf8(input), 1)))) warning2(
       'It seems you should call rmarkdown::render() instead of knitr::knit2html() ',

R/utils-vignettes.R

@@ -85,9 +85,9 @@ vtangle_empty = function(file, ...) {
 }
 
 # when neither Pandoc nor markdown is available, just silently skip the vignette
-vweave_empty = function(file, ...) {
+vweave_empty = function(file, ..., .reason = 'Pandoc') {
   out = with_ext(file, 'html')
-  writeLines('The vignette could not be built because Pandoc is not available.', out)
+  writeLines(sprintf('The vignette could not be built because %s is not available.', .reason), out)
   out
 }
 
@@ -97,6 +97,8 @@ register_vignette_engines = function(pkg) {
   vig_engine('docco_linear', vweave_docco_linear, '[.][Rr](md|markdown)$')
   vig_engine('docco_classic', vweave_docco_classic, '[.][Rr]mk?d$')
   vig_engine('rmarkdown', function(...) {
+    if (is_cran_check() && !has_package('rmarkdown'))
+      return(vweave_empty(..., .reason = 'rmarkdown'))
     if (pandoc_available()) {
       vweave_rmarkdown(...)
     } else {

R/utils.R

@@ -167,6 +167,15 @@ is_lyx = function() {
   grepl('[.]Rnw$', args[1]) && !is.na(Sys.getenv('LyXDir', NA))
 }
 
+# detect if running on CRAN (assuming that CRAN does not set CI or NOT_CRAN=true)
+is_cran = function() {
+  !any(tolower(Sys.getenv(c('CI', 'NOT_CRAN'))) == 'true')
+}
+
+is_cran_check = function() {
+  is_cran() && is_R_CMD_check()
+}
+
 # round a number to getOption('digits') decimal places by default, and format()
 # it using significant digits if the option knitr.digits.signif = TRUE
 round_digits = function(x) {