fix: Add test to assure echo "$@" >&2 works as expected (#1284)

Previously it would print the arguments twice, now it only prints
it once as we avoid adding the 'trick' to the script if it's already
present.

This helps with custom git credential helpers who endorse to
to call them like `!credential-helper $@`, in which case the
added arguments are already part of the script.

See https://github.com/languitar/pass-git-helper for detailed
documentation on how that should usually work.

Git seems to manage to not automatically add '$@' when calling
credential helpers, but it's something that is done by `gix-command`
automatically when a command should be invoked that receives arguments
*and* has to be evaluated by a shell.

The current implementation is very naive, but should also work
for 99.9% of the cases out there.

Author: Byron

gix-command/src/lib.rs

@@ -120,7 +120,7 @@ mod prepare {
             self
         }
 
-        /// Use a shell, but try to split arguments by hand if this be safely done without a shell.
+        /// Use a shell, but try to split arguments by hand if this can be safely done without a shell.
         ///
         /// If that's not the case, use a shell instead.
         pub fn with_shell_allow_argument_splitting(mut self) -> Self {
@@ -202,7 +202,14 @@ mod prepare {
                         let mut cmd = Command::new(if cfg!(windows) { "sh" } else { "/bin/sh" });
                         cmd.arg("-c");
                         if !prep.args.is_empty() {
-                            prep.command.push(" \"$@\"")
+                            if prep.command.to_str().map_or(true, |cmd| !cmd.contains("$@")) {
+                                prep.command.push(" \"$@\"");
+                            } else {
+                                gix_trace::debug!(
+                                    "Will not add '$@' to '{:?}' as it seems to contain it already",
+                                    prep.command
+                                );
+                            }
                         }
                         cmd.arg(prep.command);
                         cmd.arg("--");

gix-command/tests/command.rs

@@ -324,10 +324,20 @@ mod prepare {
             "this always needs a shell as we need tilde expansion"
         );
     }
+
+    #[test]
+    fn script_with_dollar_at() {
+        let cmd = std::process::Command::from(gix_command::prepare("echo \"$@\" >&2").with_shell().arg("store"));
+        assert_eq!(
+            format!("{cmd:?}"),
+            format!(r#""{SH}" "-c" "echo \"$@\" >&2" "--" "store""#),
+            "this is how credential helpers have to work as for some reason they don't get '$@' added in Git.\
+            We deal with it by not doubling the '$@' argument, which seems more flexible."
+        );
+    }
 }
 
 mod spawn {
-    #[cfg(unix)]
     use bstr::ByteSlice;
 
     #[test]
@@ -360,6 +370,19 @@ mod spawn {
         Ok(())
     }
 
+    #[test]
+    fn script_with_dollar_at() -> crate::Result {
+        let out = std::process::Command::from(gix_command::prepare("echo \"$@\"").with_shell().arg("arg"))
+            .spawn()?
+            .wait_with_output()?;
+        assert_eq!(
+            out.stdout.to_str_lossy().trim(),
+            "arg",
+            "the argument is just mentioned once"
+        );
+        Ok(())
+    }
+
     #[test]
     fn direct_command_execution_searches_in_path() -> crate::Result {
         assert!(gix_command::prepare(if cfg!(unix) { "ls" } else { "dir.exe" })