Add edenfsctl --checkout-dir flag

Summary:
Adds a flag to edenfsctl to let us specify a checkout directory outside our current working directory. This is necessary if we're going to use `buck2 run edenfsctl` as an `eden-dev` replacement, since buck requires us to run from the source directory, which may be different from the checkout we want edenfsctl to operate on.

With this change, we can approximate `eden-dev` like this:

```
% buck2 run edenfsctl -- --config-dir=$HOME/local/eden-dev-state --checkout-dir=$HOME/fbsource-dev trace fs
```

This is implemented by altering both the Rust and Python entry points to accept the new flag and change to the given directory.

Reviewed By: xavierd

Differential Revision: D38961247

fbshipit-source-id: 814306871ebf01f1cabab87c98e85adce68b4ac8

eden/fs/cli/main.py

@@ -2180,6 +2180,7 @@ def create_parser() -> argparse.ArgumentParser:
     parser.add_argument(
         "--home-dir", help="Path to directory where .edenrc config file is stored."
     )
+    parser.add_argument("--checkout-dir", help=argparse.SUPPRESS)
     parser.add_argument(
         "--version", "-v", action="store_true", help="Print EdenFS version."
     )
@@ -2227,6 +2228,17 @@ def normalize_path_arg(path_arg: str, may_need_tilde_expansion: bool = False) ->
     return path_arg
 
 
+def set_working_directory(args: argparse.Namespace) -> Optional[int]:
+    if args.checkout_dir is None:
+        return
+
+    try:
+        os.chdir(args.checkout_dir)
+    except OSError as e:
+        print(f"Unable to change to checkout directory: {e}", file=sys.stderr)
+        return EX_OSFILE
+
+
 def is_working_directory_stale() -> bool:
     try:
         os.getcwd()
@@ -2277,6 +2289,10 @@ Please run "cd / && cd -" to update your shell's working directory."""
 
 
 async def async_main(parser: argparse.ArgumentParser, args: argparse.Namespace) -> int:
+    set_return_code = set_working_directory(args)
+    if set_return_code is not None:
+        return set_return_code
+
     # Before doing anything else check that the current working directory is valid.
     # This helps catch the case where a user is trying to run the EdenFS CLI inside
     # a stale eden mount point.

eden/fs/cli_rs/edenfs-commands/src/lib.rs

@@ -61,6 +61,10 @@ pub struct MainCommand {
     #[clap(long, parse(from_str = expand_path))]
     home_dir: Option<PathBuf>,
 
+    /// Path to directory within a checkout.
+    #[clap(long, parse(from_str = expand_path), hide = true)]
+    checkout_dir: Option<PathBuf>,
+
     #[clap(long)]
     pub debug: bool,
 
@@ -151,7 +155,21 @@ impl MainCommand {
         )
     }
 
+    fn set_working_directory(&self) -> Result<()> {
+        if let Some(checkout_dir) = &self.checkout_dir {
+            std::env::set_current_dir(checkout_dir).with_context(|| {
+                format!(
+                    "Unable to change to checkout directory: {}",
+                    checkout_dir.display()
+                )
+            })?;
+        }
+        Ok(())
+    }
+
     pub fn run(self) -> Result<ExitCode> {
+        self.set_working_directory()?;
+
         // For command line program, we don't really need concurrency. Schedule everything in
         // current thread should be sufficient.
         let runtime = tokio::runtime::Builder::new_current_thread()