rhg: refactor to pass argv down, instead of caling args_os()

This refactoring makes it easy to patch some command-line preprocessing into rhg.
We use this to support using rhg as a shebang interpreter, for example.

Differential Revision: https://phab.mercurial-scm.org/D12543

rust/rhg/src/blackbox.rs

@@ -1,163 +1,172 b''
 //! Logging for repository events, including commands run in the repository.
 
 use crate::CliInvocation;
 use format_bytes::format_bytes;
 use hg::errors::HgError;
 use hg::repo::Repo;
 use hg::utils::{files::get_bytes_from_os_str, shell_quote};
+use std::ffi::OsString;
 
 const ONE_MEBIBYTE: u64 = 1 << 20;
 
 // TODO: somehow keep defaults in sync with `configitem` in `hgext/blackbox.py`
 const DEFAULT_MAX_SIZE: u64 = ONE_MEBIBYTE;
 const DEFAULT_MAX_FILES: u32 = 7;
 
 // Python does not support %.3f, only %f
 const DEFAULT_DATE_FORMAT: &str = "%Y-%m-%d %H:%M:%S%.3f";
 
 type DateTime = chrono::DateTime<chrono::Local>;
 
 pub struct ProcessStartTime {
     /// For measuring duration
     monotonic_clock: std::time::Instant,
     /// For formatting with year, month, day, etc.
     calendar_based: DateTime,
 }
 
 impl ProcessStartTime {
     pub fn now() -> Self {
         Self {
             monotonic_clock: std::time::Instant::now(),
             calendar_based: chrono::Local::now(),
         }
     }
 }
 
 pub struct Blackbox<'a> {
     process_start_time: &'a ProcessStartTime,
     /// Do nothing if this is `None`
     configured: Option<ConfiguredBlackbox<'a>>,
 }
 
 struct ConfiguredBlackbox<'a> {
     repo: &'a Repo,
     max_size: u64,
     max_files: u32,
     date_format: &'a str,
 }
 
 impl<'a> Blackbox<'a> {
     pub fn new(
         invocation: &'a CliInvocation<'a>,
         process_start_time: &'a ProcessStartTime,
     ) -> Result<Self, HgError> {
         let configured = if let Ok(repo) = invocation.repo {
             if invocation.config.get(b"extensions", b"blackbox").is_none() {
                 // The extension is not enabled
                 None
             } else {
                 Some(ConfiguredBlackbox {
                     repo,
                     max_size: invocation
                         .config
                         .get_byte_size(b"blackbox", b"maxsize")?
                         .unwrap_or(DEFAULT_MAX_SIZE),
                     max_files: invocation
                         .config
                         .get_u32(b"blackbox", b"maxfiles")?
                         .unwrap_or(DEFAULT_MAX_FILES),
                     date_format: invocation
                         .config
                         .get_str(b"blackbox", b"date-format")?
                         .unwrap_or(DEFAULT_DATE_FORMAT),
                 })
             }
         } else {
             // Without a local repository there’s no `.hg/blackbox.log` to
             // write to.
             None
         };
         Ok(Self {
             process_start_time,
             configured,
         })
     }
 
-    pub fn log_command_start(&self) {
+    pub fn log_command_start<'arg>(
+        &self,
+        argv: impl Iterator<Item = &'arg OsString>,
+    ) {
         if let Some(configured) = &self.configured {
-            let message = format_bytes!(b"(rust) {}", format_cli_args());
+            let message = format_bytes!(b"(rust) {}", format_cli_args(argv));
             configured.log(&self.process_start_time.calendar_based, &message);
         }
     }
 
-    pub fn log_command_end(&self, exit_code: i32) {
+    pub fn log_command_end<'arg>(
+        &self,
+        argv: impl Iterator<Item = &'arg OsString>,
+        exit_code: i32,
+    ) {
         if let Some(configured) = &self.configured {
             let now = chrono::Local::now();
             let duration = self
                 .process_start_time
                 .monotonic_clock
                 .elapsed()
                 .as_secs_f64();
             let message = format_bytes!(
                 b"(rust) {} exited {} after {} seconds",
-                format_cli_args(),
+                format_cli_args(argv),
                 exit_code,
                 format_bytes::Utf8(format_args!("{:.03}", duration))
             );
             configured.log(&now, &message);
         }
     }
 }
 
 impl ConfiguredBlackbox<'_> {
     fn log(&self, date_time: &DateTime, message: &[u8]) {
         let date = format_bytes::Utf8(date_time.format(self.date_format));
         let user = users::get_current_username().map(get_bytes_from_os_str);
         let user = user.as_deref().unwrap_or(b"???");
         let rev = format_bytes::Utf8(match self.repo.dirstate_parents() {
             Ok(parents) if parents.p2 == hg::revlog::node::NULL_NODE => {
                 format!("{:x}", parents.p1)
             }
             Ok(parents) => format!("{:x}+{:x}", parents.p1, parents.p2),
             Err(_dirstate_corruption_error) => {
                 // TODO: log a non-fatal warning to stderr
                 "???".to_owned()
             }
         });
         let pid = std::process::id();
         let line = format_bytes!(
             b"{} {} @{} ({})> {}\n",
             date,
             user,
             rev,
             pid,
             message
         );
         let result =
             hg::logging::LogFile::new(self.repo.hg_vfs(), "blackbox.log")
                 .max_size(Some(self.max_size))
                 .max_files(self.max_files)
                 .write(&line);
         match result {
             Ok(()) => {}
             Err(_io_error) => {
                 // TODO: log a non-fatal warning to stderr
             }
         }
     }
 }
 
-fn format_cli_args() -> Vec<u8> {
-    let mut args = std::env::args_os();
+fn format_cli_args<'a>(
+    mut args: impl Iterator<Item = &'a OsString>,
+) -> Vec<u8> {
     let _ = args.next(); // Skip the first (or zeroth) arg, the name of the `rhg` executable
     let mut args = args.map(|arg| shell_quote(&get_bytes_from_os_str(arg)));
     let mut formatted = Vec::new();
     if let Some(arg) = args.next() {
         formatted.extend(arg)
     }
     for arg in args {
         formatted.push(b' ');
         formatted.extend(arg)
     }
     formatted
 }

rust/rhg/src/main.rs

@@ -1,716 +1,730 b''
 extern crate log;
 use crate::error::CommandError;
 use crate::ui::{local_to_utf8, Ui};
 use clap::App;
 use clap::AppSettings;
 use clap::Arg;
 use clap::ArgMatches;
 use format_bytes::{format_bytes, join};
 use hg::config::{Config, ConfigSource};
 use hg::exit_codes;
 use hg::repo::{Repo, RepoError};
 use hg::utils::files::{get_bytes_from_os_str, get_path_from_bytes};
 use hg::utils::SliceExt;
 use std::collections::HashSet;
 use std::ffi::OsString;
 use std::path::PathBuf;
 use std::process::Command;
 
 mod blackbox;
 mod color;
 mod error;
 mod ui;
 pub mod utils {
     pub mod path_utils;
 }
 
 fn main_with_result(
+    argv: Vec<OsString>,
     process_start_time: &blackbox::ProcessStartTime,
     ui: &ui::Ui,
     repo: Result<&Repo, &NoRepoInCwdError>,
     config: &Config,
 ) -> Result<(), CommandError> {
     check_unsupported(config, repo)?;
 
     let app = App::new("rhg")
         .global_setting(AppSettings::AllowInvalidUtf8)
         .global_setting(AppSettings::DisableVersion)
         .setting(AppSettings::SubcommandRequired)
         .setting(AppSettings::VersionlessSubcommands)
         .arg(
             Arg::with_name("repository")
                 .help("repository root directory")
                 .short("-R")
                 .long("--repository")
                 .value_name("REPO")
                 .takes_value(true)
                 // Both ok: `hg -R ./foo log` or `hg log -R ./foo`
                 .global(true),
         )
         .arg(
             Arg::with_name("config")
                 .help("set/override config option (use 'section.name=value')")
                 .long("--config")
                 .value_name("CONFIG")
                 .takes_value(true)
                 .global(true)
                 // Ok: `--config section.key1=val --config section.key2=val2`
                 .multiple(true)
                 // Not ok: `--config section.key1=val section.key2=val2`
                 .number_of_values(1),
         )
         .arg(
             Arg::with_name("cwd")
                 .help("change working directory")
                 .long("--cwd")
                 .value_name("DIR")
                 .takes_value(true)
                 .global(true),
         )
         .arg(
             Arg::with_name("color")
                 .help("when to colorize (boolean, always, auto, never, or debug)")
                 .long("--color")
                 .value_name("TYPE")
                 .takes_value(true)
                 .global(true),
         )
         .version("0.0.1");
     let app = add_subcommand_args(app);
 
-    let matches = app.clone().get_matches_safe()?;
+    let matches = app.clone().get_matches_from_safe(argv.iter())?;
 
     let (subcommand_name, subcommand_matches) = matches.subcommand();
 
     // Mercurial allows users to define "defaults" for commands, fallback
     // if a default is detected for the current command
     let defaults = config.get_str(b"defaults", subcommand_name.as_bytes());
     if defaults?.is_some() {
         let msg = "`defaults` config set";
         return Err(CommandError::unsupported(msg));
     }
 
     for prefix in ["pre", "post", "fail"].iter() {
         // Mercurial allows users to define generic hooks for commands,
         // fallback if any are detected
         let item = format!("{}-{}", prefix, subcommand_name);
         let hook_for_command = config.get_str(b"hooks", item.as_bytes())?;
         if hook_for_command.is_some() {
             let msg = format!("{}-{} hook defined", prefix, subcommand_name);
             return Err(CommandError::unsupported(msg));
         }
     }
     let run = subcommand_run_fn(subcommand_name)
         .expect("unknown subcommand name from clap despite AppSettings::SubcommandRequired");
     let subcommand_args = subcommand_matches
         .expect("no subcommand arguments from clap despite AppSettings::SubcommandRequired");
 
     let invocation = CliInvocation {
         ui,
         subcommand_args,
         config,
         repo,
     };
 
     if let Ok(repo) = repo {
         // We don't support subrepos, fallback if the subrepos file is present
         if repo.working_directory_vfs().join(".hgsub").exists() {
             let msg = "subrepos (.hgsub is present)";
             return Err(CommandError::unsupported(msg));
         }
     }
 
     if config.is_extension_enabled(b"blackbox") {
         let blackbox =
             blackbox::Blackbox::new(&invocation, process_start_time)?;
-        blackbox.log_command_start();
+        blackbox.log_command_start(argv.iter());
         let result = run(&invocation);
-        blackbox.log_command_end(exit_code(
+        blackbox.log_command_end(argv.iter(), exit_code(
             &result,
             // TODO: show a warning or combine with original error if
             // `get_bool` returns an error
             config
                 .get_bool(b"ui", b"detailed-exit-code")
                 .unwrap_or(false),
         ));
         result
     } else {
         run(&invocation)
     }
 }
 
-fn main() {
+fn rhg_main(argv: Vec<OsString>) -> ! {
     // Run this first, before we find out if the blackbox extension is even
     // enabled, in order to include everything in-between in the duration
     // measurements. Reading config files can be slow if they’re on NFS.
     let process_start_time = blackbox::ProcessStartTime::now();
 
     env_logger::init();
 
-    let early_args = EarlyArgs::parse(std::env::args_os());
+    let early_args = EarlyArgs::parse(&argv);
 
     let initial_current_dir = early_args.cwd.map(|cwd| {
         let cwd = get_path_from_bytes(&cwd);
         std::env::current_dir()
             .and_then(|initial| {
                 std::env::set_current_dir(cwd)?;
                 Ok(initial)
             })
             .unwrap_or_else(|error| {
                 exit(
+                    &argv,
                     &None,
                     &Ui::new_infallible(&Config::empty()),
                     OnUnsupported::Abort,
                     Err(CommandError::abort(format!(
                         "abort: {}: '{}'",
                         error,
                         cwd.display()
                     ))),
                     false,
                 )
             })
     });
 
     let mut non_repo_config =
         Config::load_non_repo().unwrap_or_else(|error| {
             // Normally this is decided based on config, but we don’t have that
             // available. As of this writing config loading never returns an
             // "unsupported" error but that is not enforced by the type system.
             let on_unsupported = OnUnsupported::Abort;
 
             exit(
+                &argv,
                 &initial_current_dir,
                 &Ui::new_infallible(&Config::empty()),
                 on_unsupported,
                 Err(error.into()),
                 false,
             )
         });
 
     non_repo_config
         .load_cli_args(early_args.config, early_args.color)
         .unwrap_or_else(|error| {
             exit(
+                &argv,
                 &initial_current_dir,
                 &Ui::new_infallible(&non_repo_config),
                 OnUnsupported::from_config(&non_repo_config),
                 Err(error.into()),
                 non_repo_config
                     .get_bool(b"ui", b"detailed-exit-code")
                     .unwrap_or(false),
             )
         });
 
     if let Some(repo_path_bytes) = &early_args.repo {
         lazy_static::lazy_static! {
             static ref SCHEME_RE: regex::bytes::Regex =
                 // Same as `_matchscheme` in `mercurial/util.py`
                 regex::bytes::Regex::new("^[a-zA-Z0-9+.\\-]+:").unwrap();
         }
         if SCHEME_RE.is_match(&repo_path_bytes) {
             exit(
+                &argv,
                 &initial_current_dir,
                 &Ui::new_infallible(&non_repo_config),
                 OnUnsupported::from_config(&non_repo_config),
                 Err(CommandError::UnsupportedFeature {
                     message: format_bytes!(
                         b"URL-like --repository {}",
                         repo_path_bytes
                     ),
                 }),
                 // TODO: show a warning or combine with original error if
                 // `get_bool` returns an error
                 non_repo_config
                     .get_bool(b"ui", b"detailed-exit-code")
                     .unwrap_or(false),
             )
         }
     }
     let repo_arg = early_args.repo.unwrap_or(Vec::new());
     let repo_path: Option<PathBuf> = {
         if repo_arg.is_empty() {
             None
         } else {
             let local_config = {
                 if std::env::var_os("HGRCSKIPREPO").is_none() {
                     // TODO: handle errors from find_repo_root
                     if let Ok(current_dir_path) = Repo::find_repo_root() {
                         let config_files = vec![
                             ConfigSource::AbsPath(
                                 current_dir_path.join(".hg/hgrc"),
                             ),
                             ConfigSource::AbsPath(
                                 current_dir_path.join(".hg/hgrc-not-shared"),
                             ),
                         ];
                         // TODO: handle errors from
                         // `load_from_explicit_sources`
                         Config::load_from_explicit_sources(config_files).ok()
                     } else {
                         None
                     }
                 } else {
                     None
                 }
             };
 
             let non_repo_config_val = {
                 let non_repo_val = non_repo_config.get(b"paths", &repo_arg);
                 match &non_repo_val {
                     Some(val) if val.len() > 0 => home::home_dir()
                         .unwrap_or_else(|| PathBuf::from("~"))
                         .join(get_path_from_bytes(val))
                         .canonicalize()
                         // TODO: handle error and make it similar to python
                         // implementation maybe?
                         .ok(),
                     _ => None,
                 }
             };
 
             let config_val = match &local_config {
                 None => non_repo_config_val,
                 Some(val) => {
                     let local_config_val = val.get(b"paths", &repo_arg);
                     match &local_config_val {
                         Some(val) if val.len() > 0 => {
                             // presence of a local_config assures that
                             // current_dir
                             // wont result in an Error
                             let canpath = hg::utils::current_dir()
                                 .unwrap()
                                 .join(get_path_from_bytes(val))
                                 .canonicalize();
                             canpath.ok().or(non_repo_config_val)
                         }
                         _ => non_repo_config_val,
                     }
                 }
             };
             config_val.or(Some(get_path_from_bytes(&repo_arg).to_path_buf()))
         }
     };
 
     let repo_result = match Repo::find(&non_repo_config, repo_path.to_owned())
     {
         Ok(repo) => Ok(repo),
         Err(RepoError::NotFound { at }) if repo_path.is_none() => {
             // Not finding a repo is not fatal yet, if `-R` was not given
             Err(NoRepoInCwdError { cwd: at })
         }
         Err(error) => exit(
+            &argv,
             &initial_current_dir,
             &Ui::new_infallible(&non_repo_config),
             OnUnsupported::from_config(&non_repo_config),
             Err(error.into()),
             // TODO: show a warning or combine with original error if
             // `get_bool` returns an error
             non_repo_config
                 .get_bool(b"ui", b"detailed-exit-code")
                 .unwrap_or(false),
         ),
     };
 
     let config = if let Ok(repo) = &repo_result {
         repo.config()
     } else {
         &non_repo_config
     };
     let ui = Ui::new(&config).unwrap_or_else(|error| {
         exit(
+            &argv,
             &initial_current_dir,
             &Ui::new_infallible(&config),
             OnUnsupported::from_config(&config),
             Err(error.into()),
             config
                 .get_bool(b"ui", b"detailed-exit-code")
                 .unwrap_or(false),
         )
     });
     let on_unsupported = OnUnsupported::from_config(config);
 
     let result = main_with_result(
+        argv.iter().map(|s| s.to_owned()).collect(),
         &process_start_time,
         &ui,
         repo_result.as_ref(),
         config,
     );
     exit(
+        &argv,
         &initial_current_dir,
         &ui,
         on_unsupported,
         result,
         // TODO: show a warning or combine with original error if `get_bool`
         // returns an error
         config
             .get_bool(b"ui", b"detailed-exit-code")
             .unwrap_or(false),
     )
 }
 
+fn main() -> ! {
+    rhg_main(std::env::args_os().collect())
+}
+
 fn exit_code(
     result: &Result<(), CommandError>,
     use_detailed_exit_code: bool,
 ) -> i32 {
     match result {
         Ok(()) => exit_codes::OK,
         Err(CommandError::Abort {
             message: _,
             detailed_exit_code,
         }) => {
             if use_detailed_exit_code {
                 *detailed_exit_code
             } else {
                 exit_codes::ABORT
             }
         }
         Err(CommandError::Unsuccessful) => exit_codes::UNSUCCESSFUL,
 
         // Exit with a specific code and no error message to let a potential
         // wrapper script fallback to Python-based Mercurial.
         Err(CommandError::UnsupportedFeature { .. }) => {
             exit_codes::UNIMPLEMENTED
         }
     }
 }
 
-fn exit(
+fn exit<'a>(
+    original_args: &'a [OsString],
     initial_current_dir: &Option<PathBuf>,
     ui: &Ui,
     mut on_unsupported: OnUnsupported,
     result: Result<(), CommandError>,
     use_detailed_exit_code: bool,
 ) -> ! {
     if let (
         OnUnsupported::Fallback { executable },
         Err(CommandError::UnsupportedFeature { message }),
     ) = (&on_unsupported, &result)
     {
-        let mut args = std::env::args_os();
+        let mut args = original_args.iter();
         let executable = match executable {
             None => {
                 exit_no_fallback(
                     ui,
                     OnUnsupported::Abort,
                     Err(CommandError::abort(
                         "abort: 'rhg.on-unsupported=fallback' without \
                                 'rhg.fallback-executable' set.",
                     )),
                     false,
                 );
             }
             Some(executable) => executable,
         };
         let executable_path = get_path_from_bytes(&executable);
         let this_executable = args.next().expect("exepcted argv[0] to exist");
         if executable_path == &PathBuf::from(this_executable) {
             // Avoid spawning infinitely many processes until resource
             // exhaustion.
             let _ = ui.write_stderr(&format_bytes!(
                 b"Blocking recursive fallback. The 'rhg.fallback-executable = {}' config \
                 points to `rhg` itself.\n",
                 executable
             ));
             on_unsupported = OnUnsupported::Abort
         } else {
             log::debug!("falling back (see trace-level log)");
             log::trace!("{}", local_to_utf8(message));
             // `args` is now `argv[1..]` since we’ve already consumed
             // `argv[0]`
             let mut command = Command::new(executable_path);
             command.args(args);
             if let Some(initial) = initial_current_dir {
                 command.current_dir(initial);
             }
             let result = command.status();
             match result {
                 Ok(status) => std::process::exit(
                     status.code().unwrap_or(exit_codes::ABORT),
                 ),
                 Err(error) => {
                     let _ = ui.write_stderr(&format_bytes!(
                         b"tried to fall back to a '{}' sub-process but got error {}\n",
                         executable, format_bytes::Utf8(error)
                     ));
                     on_unsupported = OnUnsupported::Abort
                 }
             }
         }
     }
     exit_no_fallback(ui, on_unsupported, result, use_detailed_exit_code)
 }
 
 fn exit_no_fallback(
     ui: &Ui,
     on_unsupported: OnUnsupported,
     result: Result<(), CommandError>,
     use_detailed_exit_code: bool,
 ) -> ! {
     match &result {
         Ok(_) => {}
         Err(CommandError::Unsuccessful) => {}
         Err(CommandError::Abort {
             message,
             detailed_exit_code: _,
         }) => {
             if !message.is_empty() {
                 // Ignore errors when writing to stderr, we’re already exiting
                 // with failure code so there’s not much more we can do.
                 let _ = ui.write_stderr(&format_bytes!(b"{}\n", message));
             }
         }
         Err(CommandError::UnsupportedFeature { message }) => {
             match on_unsupported {
                 OnUnsupported::Abort => {
                     let _ = ui.write_stderr(&format_bytes!(
                         b"unsupported feature: {}\n",
                         message
                     ));
                 }
                 OnUnsupported::AbortSilent => {}
                 OnUnsupported::Fallback { .. } => unreachable!(),
             }
         }
     }
     std::process::exit(exit_code(&result, use_detailed_exit_code))
 }
 
 macro_rules! subcommands {
     ($( $command: ident )+) => {
         mod commands {
             $(
                 pub mod $command;
             )+
         }
 
         fn add_subcommand_args<'a, 'b>(app: App<'a, 'b>) -> App<'a, 'b> {
             app
             $(
                 .subcommand(commands::$command::args())
             )+
         }
 
         pub type RunFn = fn(&CliInvocation) -> Result<(), CommandError>;
 
         fn subcommand_run_fn(name: &str) -> Option<RunFn> {
             match name {
                 $(
                     stringify!($command) => Some(commands::$command::run),
                 )+
                 _ => None,
             }
         }
     };
 }
 
 subcommands! {
     cat
     debugdata
     debugrequirements
     debugignorerhg
     files
     root
     config
     status
 }
 
 pub struct CliInvocation<'a> {
     ui: &'a Ui,
     subcommand_args: &'a ArgMatches<'a>,
     config: &'a Config,
     /// References inside `Result` is a bit peculiar but allow
     /// `invocation.repo?` to work out with `&CliInvocation` since this
     /// `Result` type is `Copy`.
     repo: Result<&'a Repo, &'a NoRepoInCwdError>,
 }
 
 struct NoRepoInCwdError {
     cwd: PathBuf,
 }
 
 /// CLI arguments to be parsed "early" in order to be able to read
 /// configuration before using Clap. Ideally we would also use Clap for this,
 /// see <https://github.com/clap-rs/clap/discussions/2366>.
 ///
 /// These arguments are still declared when we do use Clap later, so that Clap
 /// does not return an error for their presence.
 struct EarlyArgs {
     /// Values of all `--config` arguments. (Possibly none)
     config: Vec<Vec<u8>>,
     /// Value of all the `--color` argument, if any.
     color: Option<Vec<u8>>,
     /// Value of the `-R` or `--repository` argument, if any.
     repo: Option<Vec<u8>>,
     /// Value of the `--cwd` argument, if any.
     cwd: Option<Vec<u8>>,
 }
 
 impl EarlyArgs {
-    fn parse(args: impl IntoIterator<Item = OsString>) -> Self {
+    fn parse<'a>(args: impl IntoIterator<Item = &'a OsString>) -> Self {
         let mut args = args.into_iter().map(get_bytes_from_os_str);
         let mut config = Vec::new();
         let mut color = None;
         let mut repo = None;
         let mut cwd = None;
         // Use `while let` instead of `for` so that we can also call
         // `args.next()` inside the loop.
         while let Some(arg) = args.next() {
             if arg == b"--config" {
                 if let Some(value) = args.next() {
                     config.push(value)
                 }
             } else if let Some(value) = arg.drop_prefix(b"--config=") {
                 config.push(value.to_owned())
             }
 
             if arg == b"--color" {
                 if let Some(value) = args.next() {
                     color = Some(value)
                 }
             } else if let Some(value) = arg.drop_prefix(b"--color=") {
                 color = Some(value.to_owned())
             }
 
             if arg == b"--cwd" {
                 if let Some(value) = args.next() {
                     cwd = Some(value)
                 }
             } else if let Some(value) = arg.drop_prefix(b"--cwd=") {
                 cwd = Some(value.to_owned())
             }
 
             if arg == b"--repository" || arg == b"-R" {
                 if let Some(value) = args.next() {
                     repo = Some(value)
                 }
             } else if let Some(value) = arg.drop_prefix(b"--repository=") {
                 repo = Some(value.to_owned())
             } else if let Some(value) = arg.drop_prefix(b"-R") {
                 repo = Some(value.to_owned())
             }
         }
         Self {
             config,
             color,
             repo,
             cwd,
         }
     }
 }
 
 /// What to do when encountering some unsupported feature.
 ///
 /// See `HgError::UnsupportedFeature` and `CommandError::UnsupportedFeature`.
 enum OnUnsupported {
     /// Print an error message describing what feature is not supported,
     /// and exit with code 252.
     Abort,
     /// Silently exit with code 252.
     AbortSilent,
     /// Try running a Python implementation
     Fallback { executable: Option<Vec<u8>> },
 }
 
 impl OnUnsupported {
     const DEFAULT: Self = OnUnsupported::Abort;
 
     fn from_config(config: &Config) -> Self {
         match config
             .get(b"rhg", b"on-unsupported")
             .map(|value| value.to_ascii_lowercase())
             .as_deref()
         {
             Some(b"abort") => OnUnsupported::Abort,
             Some(b"abort-silent") => OnUnsupported::AbortSilent,
             Some(b"fallback") => OnUnsupported::Fallback {
                 executable: config
                     .get(b"rhg", b"fallback-executable")
                     .map(|x| x.to_owned()),
             },
             None => Self::DEFAULT,
             Some(_) => {
                 // TODO: warn about unknown config value
                 Self::DEFAULT
             }
         }
     }
 }
 
 /// The `*` extension is an edge-case for config sub-options that apply to all
 /// extensions. For now, only `:required` exists, but that may change in the
 /// future.
 const SUPPORTED_EXTENSIONS: &[&[u8]] =
     &[b"blackbox", b"share", b"sparse", b"narrow", b"*"];
 
 fn check_extensions(config: &Config) -> Result<(), CommandError> {
     if let Some(b"*") = config.get(b"rhg", b"ignored-extensions") {
         // All extensions are to be ignored, nothing to do here
         return Ok(());
     }
 
     let enabled: HashSet<&[u8]> = config
         .get_section_keys(b"extensions")
         .into_iter()
         .map(|extension| {
             // Ignore extension suboptions. Only `required` exists for now.
             // `rhg` either supports an extension or doesn't, so it doesn't
             // make sense to consider the loading of an extension.
             extension.split_2(b':').unwrap_or((extension, b"")).0
         })
         .collect();
 
     let mut unsupported = enabled;
     for supported in SUPPORTED_EXTENSIONS {
         unsupported.remove(supported);
     }
 
     if let Some(ignored_list) = config.get_list(b"rhg", b"ignored-extensions")
     {
         for ignored in ignored_list {
             unsupported.remove(ignored.as_slice());
         }
     }
 
     if unsupported.is_empty() {
         Ok(())
     } else {
         let mut unsupported: Vec<_> = unsupported.into_iter().collect();
         // Sort the extensions to get a stable output
         unsupported.sort();
         Err(CommandError::UnsupportedFeature {
             message: format_bytes!(
                 b"extensions: {} (consider adding them to 'rhg.ignored-extensions' config)",
                 join(unsupported, b", ")
             ),
         })
     }
 }
 
 fn check_unsupported(
     config: &Config,
     repo: Result<&Repo, &NoRepoInCwdError>,
 ) -> Result<(), CommandError> {
     check_extensions(config)?;
 
     if std::env::var_os("HG_PENDING").is_some() {
         // TODO: only if the value is `== repo.working_directory`?
         // What about relative v.s. absolute paths?
         Err(CommandError::unsupported("$HG_PENDING"))?
     }
 
     if let Ok(repo) = repo {
         if repo.has_subrepos()? {
             Err(CommandError::unsupported("sub-repositories"))?
         }
     }
 
     if config.has_non_empty_section(b"encode") {
         Err(CommandError::unsupported("[encode] config"))?
     }
 
     if config.has_non_empty_section(b"decode") {
         Err(CommandError::unsupported("[decode] config"))?
     }
 
     Ok(())
 }