context: run commands in the C locale by default

Subprocesses inherit the session environment, so a translated host
locale leaked into builds: perl-based packaging tools
(dpkg-architecture, dpkg-parsechangelog, quilt, ...) warned about
missing locale settings, change their output with the environment,
and dpkg-buildpackage treats some of that output as data.

Default every command to LANG=C and LC_ALL=C; a caller can still
override explicitly through envs().

src/context/api.rs

@@ -243,12 +243,22 @@ impl Context {
     }
 
     /// Make a command inside context
+    ///
+    /// Build tooling must not inherit the session's locale: dpkg-family
+    /// tools and perl-based packaging scripts change their output (and
+    /// dpkg-buildpackage treats some of it as data) with the environment,
+    /// and a translated or mixed locale leaks host state into builds. The
+    /// C locale is the default; a caller can still override it by setting
+    /// LANG/LC_ALL through [`ContextCommand::envs`] afterwards.
     pub fn command<S: AsRef<OsStr>>(&self, program: S) -> ContextCommand<'_> {
         ContextCommand {
             context: self,
             program: program.as_ref().to_string_lossy().to_string(),
             args: Vec::new(),
-            env: Vec::new(),
+            env: vec![
+                ("LANG".to_string(), "C".to_string()),
+                ("LC_ALL".to_string(), "C".to_string()),
+            ],
             cwd: None,
             sink: None,
         }
@@ -530,4 +540,31 @@ mod endpoint_tests {
         let err = ContextConfig::from_endpoint("host:99999").unwrap_err();
         assert_eq!(err, "Invalid port number");
     }
+
+    /// Commands run in the C locale whatever the session environment
+    /// carries: host locale variables must not leak into builds. An
+    /// explicit caller override still wins.
+    #[test]
+    fn commands_default_to_the_c_locale() {
+        let ctx = Context::new(ContextConfig::Local).unwrap();
+
+        let locale = ctx
+            .command("sh")
+            .arg("-c")
+            .arg("printf '%s' \"${LC_ALL:-unset}:${LANG:-unset}\"")
+            .output()
+            .unwrap()
+            .stdout;
+        assert_eq!(String::from_utf8_lossy(&locale), "C:C");
+
+        let locale = ctx
+            .command("sh")
+            .arg("-c")
+            .arg("printf '%s' \"$LC_ALL\"")
+            .env("LC_ALL", "C.UTF-8")
+            .output()
+            .unwrap()
+            .stdout;
+        assert_eq!(String::from_utf8_lossy(&locale), "C.UTF-8");
+    }
 }