Better Python detection (fix #187)

src/main.rs

@@ -78,7 +78,7 @@ fn run() -> Result<(), Error> {
     let mut builder = formatted_timed_builder();
 
     if config.verbose() {
-        builder.filter(Some("topgrade"), LevelFilter::Debug);
+        builder.filter(Some("topgrade"), LevelFilter::Trace);
     }
 
     builder.init();

src/steps/os/linux.rs

@@ -4,8 +4,11 @@ use crate::terminal::{print_separator, print_warning};
 use crate::utils::{require, require_option, which};
 use failure::ResultExt;
 use ini::Ini;
+use log::{debug, error};
 use serde::Deserialize;
-use std::path::PathBuf;
+use std::io;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Output};
 use walkdir::WalkDir;
 
 static OS_RELEASE_PATH: &str = "/etc/os-release";
@@ -29,6 +32,16 @@ pub enum Distribution {
     Solus,
 }
 
+/// Returns whether the given Python path is the system Python in Arch Linux
+fn arch_system_python(path: &Path) -> Result<bool, io::Error> {
+    debug!("Python is in {}", path.display());
+    Command::new("pacman").arg("-Qqo").arg(&path).output().map(|output| {
+        debug!("Pacman output: {:?}", output);
+        let Output { status, stdout, .. } = output;
+        status.success() && String::from_utf8(stdout).unwrap() == "python\n"
+    })
+}
+
 impl Distribution {
     fn parse_os_release(os_release: &ini::Ini) -> Result<Self, Error> {
         let section = os_release.general_section();
@@ -111,7 +124,12 @@ fn upgrade_arch_linux(sudo: &Option<PathBuf>, cleanup: bool, run_type: RunType)
 
     if let Some(yay) = which("yay") {
         if let Some(python) = which("python") {
-            if python != PathBuf::from("/usr/bin/python") {
+            let is_system_python = arch_system_python(&python).unwrap_or_else(|e| {
+                error!("Error detecting system Python: {}", e);
+                false
+            });
+
+            if !is_system_python {
                 print_warning(format!(
                     "Python detected at {:?}, which is probably not the system Python.
 It's dangerous to run yay since Python based AUR packages will be installed in the wrong location",