fall back to Rust for experimental commands

Summary:
# Context

Our last attempt at removing the Python version of `eden redirect` failed (see the SEV attached to D55426809). Our next attempt should be done in a phased manner to avoid similar breakages.

# Solution

We will slowly remove all possible fallbacks to Python so that we can be sure no commands are running the Python version of `eden redirect` prior to deleting the code. New rollout plan is as follows:

1) Remove the Chef recipe that writes `/etc/eden/edenfsctl_rollout.json` (Done)
2) Remove "redirect" from the default list of experimental commands in the Rust code.
3) Add a fallback to Rust if Python parsing fails for experimental commands
4) Add an EdenFS event that's logged every time we fallback to the Python `eden redirect` codepath
5) Monitor Scuba data to ensure the Python codepath isn't being hit
6) Delete Python `eden redirect` altogether

# This diff

Accomplishes #3 by ensuring any experimental command that fails to parse as a valid Python command attempts to run as a Rust command before reporting an error. This would have prevented the SEVs today (since there *was* a valid Rust version of `eden redirect`, we just skipped it altogether).

The next diff will add an EdenFS event that we can add alerting for to ensure this path is never hit. If it is hit, we can investigate why the rollout is failing and adjust accordingly.

Reviewed By: kmancini

Differential Revision: D56282193

fbshipit-source-id: 54c4ff1fc6808f6d44e5d98294302274f34598da

eden/fs/cli/main.py

@@ -107,15 +107,17 @@ def stop_internal_processes(_: str) -> None:
 # For a non-unix system (like Windows), we will define our own error codes.
 try:
     EX_OK: int = os.EX_OK
-    EX_USAGE: int = os.EX_USAGE
     EX_SOFTWARE: int = os.EX_SOFTWARE
     EX_OSFILE: int = os.EX_OSFILE
 except AttributeError:  # On a non-unix system
     EX_OK: int = 0
-    EX_USAGE: int = 64
     EX_SOFTWARE: int = 70
     EX_OSFILE: int = 72
 
+# The Rust CLI depends on this value staying constant. Instead of fetching it
+# from the os library, let's just define it here.
+EX_USAGE: int = 64
+
 
 # We have different mitigations on different platforms due to cmd differences
 def _get_unmount_timeout_suggestions(path: str) -> str:
@@ -2690,7 +2692,14 @@ def main() -> int:
     # Increase how often it's sampled.
     par_telemetry.set_sample_rate(automation=10000)
     parser = create_parser()
-    args = parser.parse_args()
+    try:
+        args = parser.parse_args()
+    except SystemExit:
+        # For some reason argparse calls sys.exit(2) when it encounters a parse
+        # error... This makes it hard for us to distinguish between edenfsctl
+        # failing w/ exit code 2 and a parse error. Let's catch the parse
+        # error and return a more appropriate exit code.
+        return EX_USAGE
 
     # The default event loop on 3.8+ will cause an ugly backtrace when
     # edenfsctl is interrupted. Switching back to the selector event loop.

eden/fs/cli_rs/edenfsctl/src/main.rs

@@ -26,6 +26,9 @@ use edenfs_utils::strip_unc_prefix;
 use fbinit::FacebookInit;
 use tracing_subscriber::filter::EnvFilter;
 
+/// Value used in Python to indicate a command failed to parse
+pub const PYTHON_EDENFSCTL_EX_USAGE: i32 = 64;
+
 fn python_fallback() -> Result<Command> {
     if let Ok(args) = std::env::var("EDENFSCTL_REAL") {
         // We might get a command starting with python.exe here instead of a simple path.
@@ -142,7 +145,14 @@ fn wrapper_main() -> Result<i32> {
                 if cmd.is_enabled() {
                     rust_main(cmd)
                 } else {
-                    fallback(None)
+                    match fallback(None) {
+                        // If the Python version of edenfsctl exited with a
+                        // parse error, we should see if the Rust version
+                        // exists. This helps prevent cases where rollouts
+                        // are not working correctly.
+                        Ok(PYTHON_EDENFSCTL_EX_USAGE) => rust_main(cmd),
+                        res => res,
+                    }
                 }
             }
             // If the command is defined in Rust, then --help will cause

eden/scm/tests/test-eden-commands.t

@@ -7,7 +7,7 @@ test eden commands help
   $ eden --help > /dev/null
   $ eden du --help > /dev/null
   $ eden imnotacommandiswear --help > /dev/null 2>&1
-  [2]
+  [64]
 
 Test a few simple eden commands