Detect "system editor" at runtime on Linux

On Linux, there is no clearly established "default", but "nano" and "vi"
are obvious choices.

On FreeBSD, "vi" comes with the base system so is the clear default

Author: squell

docs/man/sudoers.5.md

@@ -430,7 +430,7 @@ sudo's behavior can be modified by Default_Entry lines, as explained earlier.  A
 
 * editor
 
-  A colon (‘:’) separated list of editor path names used by **sudoedit** and **visudo**. For **sudoedit**, this list is used to find an editor when none of the SUDO_EDITOR, VISUAL or EDITOR environment variables are set to an editor that exists and is executable.  For **visudo**, it is used as a white list of allowed editors; **visudo** will choose the editor that matches the user's SUDO_EDITOR, VISUAL or EDITOR environment variable if possible, or the  first  editor in  the  list that exists and is executable if not. Unless invoked as **sudoedit**, sudo does not preserve the SUDO_EDITOR, VISUAL or EDITOR environment variables unless they are present in the **env_keep** list. The default on Linux is _/usr/bin/editor_, on FreeBSD _/usr/vim/vi_.
+  A colon (‘:’) separated list of editor path names used by **sudoedit** and **visudo**. For **sudoedit**, this list is used to find an editor when none of the SUDO_EDITOR, VISUAL or EDITOR environment variables are set to an editor that exists and is executable.  For **visudo**, it is used as a white list of allowed editors; **visudo** will choose the editor that matches the user's SUDO_EDITOR, VISUAL or EDITOR environment variable if possible, or the  first  editor in  the  list that exists and is executable if not. Unless invoked as **sudoedit**, sudo does not preserve the SUDO_EDITOR, VISUAL or EDITOR environment variables unless they are present in the **env_keep** list. The default on Linux is _/usr/bin/editor:/usr/bin/nano:/usr/bin/vi_. On FreeBSD the default is _/usr/bin/vi_.
 
 ## Strings that can be used in a boolean context:
 

src/defaults/mod.rs

@@ -21,11 +21,16 @@ use settings_dsl::{
     result_of, storage_of,
 };
 
-pub const SYSTEM_EDITOR: &str = if cfg!(target_os = "linux") {
-    "/usr/bin/editor"
-} else {
+pub fn system_editor() -> &'static str {
+    #[cfg(target_os = "linux")]
+    for editor in ["/usr/bin/editor", "/usr/bin/nano"] {
+        if std::path::Path::new(editor).exists() {
+            return editor;
+        }
+    }
+
     "/usr/bin/vi"
-};
+}
 
 defaults! {
     always_query_group_plugin = false  #ignored
@@ -61,7 +66,7 @@ defaults! {
     passwd_timeout            = (5*60) (!= 0) {fractional_minutes}
     timestamp_timeout         = (15*60) (!= 0) {fractional_minutes}
 
-    editor                    = SYSTEM_EDITOR
+    editor                    = { system_editor() }
     env_editor                = true
 
     env_keep                  = ["COLORS", "DISPLAY", "HOSTNAME", "KRB5CCNAME", "LS_COLORS", "PATH",

src/sudoers/mod.rs

@@ -345,7 +345,7 @@ fn select_editor(settings: &Settings, trusted_env: bool) -> PathBuf {
 
     // fallback on hardcoded path -- always provide something to the caller
 
-    PathBuf::from(defaults::SYSTEM_EDITOR)
+    PathBuf::from(defaults::system_editor())
 }
 
 // a `take_while` variant that does not consume the first non-matching item

src/visudo/mod.rs

@@ -273,7 +273,7 @@ fn edit_sudoers_file(
         sudoers.visudo_editor_path(&host_name, &current_user, &current_user)
     } else {
         // there is no /etc/sudoers config yet, so use a system default
-        PathBuf::from(crate::defaults::SYSTEM_EDITOR)
+        PathBuf::from(crate::defaults::system_editor())
     };
 
     loop {