Add CALLR_NO_TEMP_DLLS env var

Closes #273.

R/load-client.R

@@ -27,7 +27,7 @@ load_client_lib <- function(sofile = NULL, pxdir = NULL) {
   # install path since that library might be shared (e.g. in tests)
   need_cleanup <- TRUE
 
-  if (is.null(sofile)) {
+  if (is.null(sofile) || Sys.getenv("CALLR_NO_TEMP_DLLS", "false") == "true") {
     sofile <- sofile_in_processx()
     lib <- dyn.load(sofile)
     need_cleanup <- FALSE

README.Rmd

@@ -227,6 +227,19 @@ callr::rcmd("config", "CC")
 
 This returns a list with three components: the standard output, the standard error, and the exit (status) code of the `R CMD` command.
 
+## Configuration
+
+### Environment variables
+
+* `CALLR_NO_TEMP_DLLS`: If `true`, then callr does not use a temporary
+  directory to copy the client DLL files from, in the subprocess. By
+  default callr copies the DLL file that drives the callr subprocess into
+  a temporary directory and loads it from there. This is mainly to avoid
+  locking a DLL file in the package library, on Windows. If this default
+  causes issues for you, set it to `true`, and then callr will use the DLL
+  file from the installed processx package. See also
+  https://github.com/r-lib/callr/issues/273.
+
 ## Code of Conduct
 
 Please note that the callr project is released with a

README.md

@@ -346,6 +346,19 @@ callr::rcmd("config", "CC")
 This returns a list with three components: the standard output, the
 standard error, and the exit (status) code of the `R CMD` command.
 
+## Configuration
+
+### Environment variables
+
+* `CALLR_NO_TEMP_DLLS`: If `true`, then callr does not use a temporary
+  directory to copy the client DLL files from, in the subprocess. By
+  default callr copies the DLL file that drives the callr subprocess into
+  a temporary directory and loads it from there. This is mainly to avoid
+  locking a DLL file in the package library, on Windows. If this default
+  causes issues for you, set it to `true`, and then callr will use the DLL
+  file from the installed processx package. See also
+  https://github.com/r-lib/callr/issues/273.
+
 ## Code of Conduct
 
 Please note that the callr project is released with a [Contributor Code

tests/testthat/test-load-client.R

@@ -89,7 +89,7 @@ test_that("set_stdout_file, set_setderr_file", {
     c(readLines(f1), readLines(f2))
   }
 
-  ret <- callr::r(do)  
+  ret <- callr::r(do)
   expect_equal(ret, c("this is output", "this is error"))
 })
 
@@ -109,3 +109,24 @@ test_that("init function of client lib is run", {
   # init function
   expect_false(unclass(pxlib$.lib)$dynamicLookup)
 })
+
+test_that("CALLR_NO_TEMP_DLLS", {
+  skip_on_cran()
+  if (.Platform$OS.type != "windows") skip("Windows only")
+
+  # If not set, then it should come from the temporary location
+  withr::local_envvar(CALLR_NO_TEMP_DLLS = NA_character_)
+  dlls <- callr::r(function() ps::ps_shared_libs())$path
+  px <- grep("processx.*client.dll", dlls)
+  cr <- grep("callr.*client.dll", dlls)
+  expect_true(length(px) == 0)
+  expect_true(length(cr) >= 1)
+
+  # If set, then it should come from processx
+  withr::local_envvar(CALLR_NO_TEMP_DLLS = "true")
+  dlls <- callr::r(function() ps::ps_shared_libs())$path
+  px <- grep("processx.*client.dll", dlls)
+  cr <- grep("callr.*client.dll", dlls)
+  expect_true(length(px) >= 1)
+  expect_true(length(cr) == 0)
+})