Add option to use Windows default DLL search strategy (for Conda) (#972)

* add option to use windows default dll search strategy (for conda)

* add more windows guards

* Contain `USE_STANDARD_DLL_SEARCH_PATH` in `windows/` folder (#973)

* Update crates/ark/src/main.rs

Co-authored-by: Davis Vaughan <[email protected]>

* reflow & propagate new name

* rustfmt

---------

Co-authored-by: Davis Vaughan <[email protected]>

Author: jmcphers

crates/ark/src/main.rs

@@ -1,7 +1,7 @@
 //
 // main.rs
 //
-// Copyright (C) 2022-2024 Posit Software, PBC. All rights reserved.
+// Copyright (C) 2022-2025 Posit Software, PBC. All rights reserved.
 //
 //
 
@@ -30,31 +30,46 @@ thread_local! {
 
 fn print_usage() {
     println!("Ark {}, an R Kernel.", env!("CARGO_PKG_VERSION"));
-    println!(
+    print!(
         r#"
 Usage: ark [OPTIONS]
 
 Available options:
 
---connection_file FILE   Start the kernel with the given JSON connection file
-                         (see the Jupyter kernel documentation for details)
--- arg1 arg2 ...         Set the argument list to pass to R; defaults to
-                         --interactive
---startup-file FILE      An R file to run on session startup
---session-mode MODE      The mode in which the session is running (console, notebook, background)
---no-capture-streams     Do not capture stdout/stderr from R
---default-repos          Set the default repositories to use, by name:
-                         "rstudio" ('cran.rstudio.com', the default), or
-                         "posit-ppm" ('packagemanager.posit.co', subject to availability), or
-                         "none" (do not alter the 'repos' option in any way)
---repos-conf             Set the default repositories to use from a configuration file
-                         containing a list of named repositories (`name = url`)
---default-ppm-repo       Set the default repositories to a custom Posit Package Manager URL.
---version                Print the version of Ark
---log FILE               Log to the given file (if not specified, stdout/stderr
-                         will be used)
---install                Install the kernel spec for Ark
---help                   Print this help message
+--connection_file FILE       Start the kernel with the given JSON connection file
+                             (see the Jupyter kernel documentation for details)
+-- arg1 arg2 ...             Set the argument list to pass to R; defaults to
+                             --interactive
+--startup-file FILE          An R file to run on session startup
+--session-mode MODE          The mode in which the session is running (console, notebook, background)
+--no-capture-streams         Do not capture stdout/stderr from R
+--default-repos              Set the default repositories to use, by name:
+                             "rstudio" ('cran.rstudio.com', the default), or
+                             "posit-ppm" ('packagemanager.posit.co', subject to availability), or
+                             "none" (do not alter the 'repos' option in any way)
+--repos-conf                 Set the default repositories to use from a configuration file
+                             containing a list of named repositories (`name = url`)
+--default-ppm-repo           Set the default repositories to a custom Posit Package Manager URL.
+--version                    Print the version of Ark
+--log FILE                   Log to the given file (if not specified, stdout/stderr
+                             will be used)
+--install                    Install the kernel spec for Ark"#
+    );
+
+    // Windows-specific options
+    #[cfg(target_os = "windows")]
+    print!(
+        r#"
+--standard-dll-search-order  Use the standard Windows DLL search order (including the PATH environment
+                             variable) when loading R. Useful for R installations that depend on DLLs
+                             other than those in system folders and R install folder, such as Conda.
+                             The R shared library folder must be on the PATH for this to work."#
+    );
+
+    print!(
+        r#"
+--help                       Print this help message
+
 "#
     );
 }
@@ -82,6 +97,8 @@ fn main() -> anyhow::Result<()> {
     let mut has_action = false;
     let mut capture_streams = true;
     let mut default_repos = DefaultRepos::Auto;
+    #[cfg(target_os = "windows")]
+    let mut use_windows_dll_search_path = false;
 
     while let Some(arg) = argv.next() {
         match arg.as_str() {
@@ -136,6 +153,8 @@ fn main() -> anyhow::Result<()> {
                 return Ok(());
             },
             "--no-capture-streams" => capture_streams = false,
+            #[cfg(target_os = "windows")]
+            "--standard-dll-search-order" => use_windows_dll_search_path = true,
             "--default-repos" => {
                 if let Some(repos) = argv.next() {
                     if default_repos != DefaultRepos::Auto {
@@ -408,6 +427,13 @@ fn main() -> anyhow::Result<()> {
     // Parse the connection file
     let (connection_file, registration_file) = kernel::read_connection(connection_file.as_str());
 
+    // Set Windows DLL search path option before starting the kernel
+    // (this must be done before R libraries are loaded)
+    #[cfg(target_os = "windows")]
+    if use_windows_dll_search_path {
+        harp::sys::library::set_use_standard_dll_search_path(true);
+    }
+
     // Connect the Jupyter kernel and start R.
     // Does not return!
     start_kernel(

crates/harp/src/sys/windows/library.rs

@@ -1,11 +1,13 @@
 /*
  * library.rs
  *
- * Copyright (C) 2024 Posit Software, PBC. All rights reserved.
+ * Copyright (C) 2024-2025 Posit Software, PBC. All rights reserved.
  *
  */
 
 use std::path::PathBuf;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering;
 
 use libloading::os::windows::Library;
 use libloading::os::windows::LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR;
@@ -74,6 +76,18 @@ impl RLibraries {
 }
 
 pub fn open_r_shared_library(path: &PathBuf) -> Result<libloading::Library, libloading::Error> {
+    // Check if we should use the standard DLL search path. Note that Windows' standard DLL
+    // search path includes the PATH environment variable, so this is useful for R installations
+    // that depend on DLLs other than those in system folders and R install folder, such as Conda.
+    //
+    // However, this does _not_ work for typical R installations because they rely on DLLs in R's own
+    // shared library folder, which is not part of Windows' standard DLL search heuristic.
+    if use_standard_dll_search_path() {
+        // Use the standard DLL search order (no special flags)
+        let library = unsafe { Library::new(path) };
+        return library.map(|library| library.into());
+    }
+
     // Each R shared library may have its own set of DLL dependencies. For example,
     // `R.dll` depends on `Rblas.dll` and some DLLs in system32. For each of the R DLLs we load,
     // the combination of R's DLL folder (i.e. `bin/x64`) and the system32 folder are enough to
@@ -100,3 +114,20 @@ pub fn find_r_shared_library_folder(path: &PathBuf) -> PathBuf {
         path.join("bin").join("x64")
     }
 }
+
+/// When true, use the standard Windows DLL search path instead of the restricted
+/// search path (DLL load dir + system32). This is a Windows-only setting.
+///
+/// See https://github.com/posit-dev/ark/pull/972 for details.
+///
+/// This must be set by [set_use_standard_dll_search_path()] before
+/// [RLibraries::from_r_home_path()].
+static USE_STANDARD_DLL_SEARCH_PATH: AtomicBool = AtomicBool::new(false);
+
+pub fn set_use_standard_dll_search_path(value: bool) {
+    USE_STANDARD_DLL_SEARCH_PATH.store(value, Ordering::SeqCst);
+}
+
+fn use_standard_dll_search_path() -> bool {
+    USE_STANDARD_DLL_SEARCH_PATH.load(Ordering::SeqCst)
+}