Add a built-in for setting the PPM repo in a running session.

This commit adds a `.ps.set_ppm_repo()` built-in that works the same way
as the existing CLI option, but allows the repo to be set at runtime.

It is intended to support making the
`positron.r.packageManagerRepository` setting apply without needing to
restart/recreate existing R sessions.

Part of posit-dev/positron#10965.

Signed-off-by: Aaron Jacobs <aaron.jacobs@posit.co>

Author: atheriel

crates/ark/src/modules/positron/repos.R

@@ -19,10 +19,13 @@ apply_repo_defaults <- function(
         repos <- defaults
     } else {
         if ("CRAN" %in% names(repos) && "CRAN" %in% names(defaults)) {
-            # If a CRAN repository is set to @CRAN@, and a default provides a
-            # specific URL, override it. This is the only instance in which we
-            # replace an already-set repository option with a default.
-            if (identical(repos[["CRAN"]], "@CRAN@")) {
+            # If a CRAN repository is set to @CRAN@ or is marked as having been
+            # updated by "the IDE" *and* a default provides a specific URL,
+            # override it.
+            #
+            # This is the only instance in which we replace an already-set
+            # repository option with a default.
+            if (identical(repos[["CRAN"]], "@CRAN@") || isTRUE(attr(repos, "IDE"))) {
                 repos[["CRAN"]] <- defaults[["CRAN"]]
 
                 # Flag this CRAN repository as being set by the IDE. This flag is
@@ -39,3 +42,25 @@ apply_repo_defaults <- function(
     }
     options(repos = repos)
 }
+
+#' Set the Posit Package Manager repository
+#'
+#' Sets the Posit Package Manager repository URL for the current session. The
+#' URL will be processed to point to a Linux distribution-specific binary URL if
+#' applicable.
+#'
+#' This function only overrides the CRAN repository when Ark has previously set
+#' it or when it uses placeholder `"@CRAN@"`.
+#'
+#' @param url A PPM repository URL. Must be in the form
+#'   `"https://host/repo/snapshot"`, e.g.,
+#'   `"https://packagemanager.posit.co/cran/latest"`.
+#'
+#' @return `NULL`, invisibly.
+#'
+#' @export
+.ps.set_ppm_repo <- function(url) {
+    # Use Ark's built-in PPM binary URL detection.
+    url <- .ps.Call("ps_get_ppm_binary_url", url)
+    apply_repo_defaults(c(CRAN = url))
+}

crates/ark/src/repos.rs

@@ -13,9 +13,11 @@ use std::io::BufRead;
 use std::io::BufReader;
 use std::path::PathBuf;
 
+use anyhow::Context;
 use harp::exec::RFunction;
 use harp::exec::RFunctionExt;
 use harp::RObject;
+use libr::SEXP;
 
 use crate::modules::ARK_ENVS;
 
@@ -346,6 +348,28 @@ fn get_ppm_binary_package_repo(repo_url: Option<url::Url>) -> String {
     }
 }
 
+#[harp::register]
+pub extern "C-unwind" fn ps_get_ppm_binary_url(url: SEXP) -> anyhow::Result<SEXP> {
+    let url_string = unsafe { RObject::view(url).to::<String>().context("`url` must be a string")? };
+    if url_string.is_empty() {
+        return Err(anyhow::anyhow!("Empty Package Manager URL provided"));
+    }
+
+    // Validate the PPM URL structure.
+    let parsed = url::Url::parse(&url_string).context("Invalid URL format")?;
+    let segments = parsed
+        .path_segments()
+        .ok_or_else(|| anyhow::anyhow!("Package Manager URL must have a path"))?;
+    if segments.count() != 2 {
+        return Err(anyhow::anyhow!(
+            "Package Manager URL must have exactly 2 path segments (e.g., /cran/latest)"
+        ));
+    }
+
+    let final_url = get_ppm_binary_package_repo(Some(parsed));
+    Ok(RObject::from(final_url).sexp)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;