Validate R_HOME path early and surface env discovery failures

crates/ark/src/interface.rs

@@ -552,35 +552,41 @@ impl RMain {
             startup::push_ignore_user_r_profile(&mut r_args);
         }
 
-        let r_home = r_home_setup();
+        let r_home = match r_home_setup() {
+            Ok(r_home) => r_home,
+            Err(err) => panic!("Can't set up `R_HOME`: {err}"),
+        };
 
         // `R_HOME` is now defined no matter what and will be used by
         // `r_command()`. Let's discover the other important environment
         // variables set by R's shell script frontend.
         // https://github.com/posit-dev/positron/issues/3637
-        if let Ok(output) =
-            r_command(|command| {
-                // From https://github.com/rstudio/rstudio/blob/74696236/src/cpp/core/r_util/REnvironmentPosix.cpp#L506-L515
-                command.arg("--vanilla").arg("-s").arg("-e").arg(
-                    r#"cat(paste(R.home('share'), R.home('include'), R.home('doc'), sep=';'))"#,
-                );
-            })
-        {
-            if let Ok(vars) = String::from_utf8(output.stdout) {
-                let vars: Vec<&str> = vars.trim().split(';').collect();
-                if vars.len() == 3 {
-                    // Set the R env vars as the R shell script frontend would
-                    unsafe {
-                        std::env::set_var("R_SHARE_DIR", vars[0]);
-                        std::env::set_var("R_INCLUDE_DIR", vars[1]);
-                        std::env::set_var("R_DOC_DIR", vars[2]);
-                    };
+        match r_command(|command| {
+            // From https://github.com/rstudio/rstudio/blob/74696236/src/cpp/core/r_util/REnvironmentPosix.cpp#L506-L515
+            command
+                .arg("--vanilla")
+                .arg("-s")
+                .arg("-e")
+                .arg(r#"cat(paste(R.home('share'), R.home('include'), R.home('doc'), sep=';'))"#);
+        }) {
+            Ok(output) => {
+                if let Ok(vars) = String::from_utf8(output.stdout) {
+                    let vars: Vec<&str> = vars.trim().split(';').collect();
+                    if vars.len() == 3 {
+                        // Set the R env vars as the R shell script frontend would
+                        unsafe {
+                            std::env::set_var("R_SHARE_DIR", vars[0]);
+                            std::env::set_var("R_INCLUDE_DIR", vars[1]);
+                            std::env::set_var("R_DOC_DIR", vars[2]);
+                        };
+                    } else {
+                        log::warn!("Unexpected output for R envvars");
+                    }
                 } else {
-                    log::warn!("Unexpected output for R envvars");
-                }
-            } else {
-                log::warn!("Could not read stdout for R envvars");
-            };
+                    log::warn!("Could not read stdout for R envvars");
+                };
+            },
+            Err(err) => log::error!("Failed to discover R envvars: {err}"),
         };
 
         let libraries = RLibraries::from_r_home_path(&r_home);

crates/ark/src/version.rs

@@ -32,7 +32,7 @@ pub struct RVersion {
 }
 
 pub fn detect_r() -> anyhow::Result<RVersion> {
-    let r_home: String = r_home_setup().to_string_lossy().to_string();
+    let r_home: String = r_home_setup()?.to_string_lossy().to_string();
 
     let output = r_command(|command| {
         command

crates/harp/src/command.rs

@@ -10,6 +10,8 @@ use std::path::PathBuf;
 use std::process::Command;
 use std::process::Output;
 
+use anyhow::anyhow;
+
 use crate::sys::command::COMMAND_R_NAMES;
 
 /// Execute a `Command` for R, trying multiple names for the R executable
@@ -41,28 +43,49 @@ where
 }
 
 /// Use this before calling `r_command()` to ensure that `R_HOME` is set consistently
-pub fn r_home_setup() -> PathBuf {
-    match std::env::var("R_HOME") {
+pub fn r_home_setup() -> anyhow::Result<PathBuf> {
+    // Determine candidate path and the string form to set (we will overwrite
+    // R_HOME in the environment after validation, even if it was already set).
+    let home = match std::env::var("R_HOME") {
         Ok(home) => {
             // Get `R_HOME` from env var, typically set by Positron / CI / kernel specification
-            PathBuf::from(home)
+            home.clone()
         },
+
         Err(_) => {
             // Get `R_HOME` from `PATH`, via `R`
-            let Ok(result) = r_command_from_path(|command| {
+            let result = r_command_from_path(|command| {
                 command.arg("RHOME");
-            }) else {
-                panic!("Can't find R or `R_HOME`");
-            };
+            })
+            .map_err(|err| anyhow!("Can't find R or `R_HOME`: {err}"))?;
 
-            let r_home = String::from_utf8(result.stdout).unwrap();
-            let r_home = r_home.trim();
+            let home = String::from_utf8(result.stdout)
+                .map_err(|err| anyhow!("Invalid UTF-8 from R RHOME output: {err}"))?;
+            home.trim().to_string()
+        },
+    };
+
+    let path = PathBuf::from(home.clone());
+    match path.try_exists() {
+        Ok(true) => {
+            // Ensure `R_HOME` is set in the environment after validation. From
+            // now on, `r_command()` can be used to run exactly the same R as is
+            // running in Ark.
+            unsafe { std::env::set_var("R_HOME", &home) };
+
+            // Check that `R` can be called
+            r_command(|command| {
+                command.arg("RHOME");
+            })
+            .map_err(|err| anyhow!("Can't run R: {err}"))?;
 
-            // Now set `R_HOME`. From now on, `r_command()` can be used to
-            // run exactly the same R as is running in Ark.
-            unsafe { std::env::set_var("R_HOME", r_home) };
-            PathBuf::from(r_home)
+            Ok(path)
         },
+        Ok(false) => Err(anyhow!(
+            "The `R_HOME` path '{}' does not exist.",
+            path.display()
+        )),
+        Err(err) => Err(anyhow!("Can't check if `R_HOME` path exists: {err}")),
     }
 }