rhg: use `Command::exec` instead of `Command::status`

`rhg` is supposed to be a transparent executable, using a subprocess defeats
that purpose. See inline comments for more details.

This also introduces the `which` crate to check if the fallback executable
actually exists to help debugging (plain `execve` doesn't give much
information).

The error code 253 is used to signify that the fallback is not found, but may
mean in the future that it is otherwise invalid if we start being more
specific.

Differential Revision: https://phab.mercurial-scm.org/D12578

rust/Cargo.lock

@@ -505,9 +505,9 b' checksum = "e2abad23fbc42b3700f2f279844d'
 
 [[package]]
 name = "libc"
-version = "0.2.81"
+version = "0.2.124"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1482821306169ec4d07f6aca392a4681f66c75c9918aa49641a2595db64053cb"
+checksum = "21a41fed9d98f27ab1c6d161da622a4fa35e8a54a8adc24bbf3ddd0ef70b0e50"
 
 [[package]]
 name = "libm"
@@ -949,6 +949,7 b' dependencies = ['
  "micro-timer",
  "regex",
  "users",
+ "which",
 ]
 
 [[package]]
@@ -1151,6 +1152,17 b' source = "registry+https://github.com/ru'
 checksum = "1a143597ca7c7793eff794def352d41792a93c481eb1042423ff7ff72ba2c31f"
 
 [[package]]
+name = "which"
+version = "4.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c4fb54e6113b6a8772ee41c3404fb0301ac79604489467e0a9ce1f3e97c24ae"
+dependencies = [
+ "either",
+ "lazy_static",
+ "libc",
+]
+
+[[package]]
 name = "winapi"
 version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"

rust/hg-core/src/exit_codes.rs

@@ -17,3 +17,6 b' pub const UNSUCCESSFUL: ExitCode = 1;'
 
 /// Command or feature not implemented by rhg
 pub const UNIMPLEMENTED: ExitCode = 252;
+
+/// The fallback path is not valid
+pub const INVALID_FALLBACK: ExitCode = 253;

rust/rhg/Cargo.toml

@@ -21,3 +21,4 b' regex = "1.3.9"'
 env_logger = "0.7.1"
 format-bytes = "0.3.0"
 users = "0.11.0"
+which = "4.2.5"

rust/rhg/src/error.rs

@@ -29,6 +29,9 b' pub enum CommandError {'
     /// `rhg` may attempt to silently fall back to Python-based `hg`, which
     /// may or may not support this feature.
     UnsupportedFeature { message: Vec<u8> },
+    /// The fallback executable does not exist (or has some other problem if
+    /// we end up being more precise about broken fallbacks).
+    InvalidFallback { path: Vec<u8>, err: String },
 }
 
 impl CommandError {

rust/rhg/src/main.rs

@@ -13,6 +13,7 b' use hg::utils::files::{get_bytes_from_os'
 use hg::utils::SliceExt;
 use std::collections::HashSet;
 use std::ffi::OsString;
+use std::os::unix::prelude::CommandExt;
 use std::path::PathBuf;
 use std::process::Command;
 
@@ -365,12 +366,14 b' fn exit_code('
             }
         }
         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
         }
+        Err(CommandError::InvalidFallback { .. }) => {
+            exit_codes::INVALID_FALLBACK
+        }
     }
 }
 
@@ -415,6 +418,17 b' fn exit('
         } else {
             log::debug!("falling back (see trace-level log)");
             log::trace!("{}", local_to_utf8(message));
+            if let Err(err) = which::which(executable_path) {
+                exit_no_fallback(
+                    ui,
+                    OnUnsupported::Abort,
+                    Err(CommandError::InvalidFallback {
+                        path: executable.to_owned(),
+                        err: err.to_string(),
+                    }),
+                    use_detailed_exit_code,
+                )
+            }
             // `args` is now `argv[1..]` since we’ve already consumed
             // `argv[0]`
             let mut command = Command::new(executable_path);
@@ -422,19 +436,19 b' fn exit('
             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
-                }
-            }
+            // We don't use subprocess because proper signal handling is harder
+            // and we don't want to keep `rhg` around after a fallback anyway.
+            // For example, if `rhg` is run in the background and falls back to
+            // `hg` which, in turn, waits for a signal, we'll get stuck if
+            // we're doing plain subprocess.
+            //
+            // If `exec` returns, we can only assume our process is very broken
+            // (see its documentation), so only try to forward the error code
+            // when exiting.
+            let err = command.exec();
+            std::process::exit(
+                err.raw_os_error().unwrap_or(exit_codes::ABORT),
+            );
         }
     }
     exit_no_fallback(ui, on_unsupported, result, use_detailed_exit_code)
@@ -471,6 +485,13 b' fn exit_no_fallback('
                 OnUnsupported::Fallback { .. } => unreachable!(),
             }
         }
+        Err(CommandError::InvalidFallback { path, err }) => {
+            let _ = ui.write_stderr(&format_bytes!(
+                b"abort: invalid fallback '{}': {}\n",
+                path,
+                err.as_bytes(),
+            ));
+        }
     }
     std::process::exit(exit_code(&result, use_detailed_exit_code))
 }

tests/test-rhg.t

@@ -179,15 +179,8 b' Fallback to Python'
   [1]
 
   $ rhg cat original --exclude="*.rs" --config rhg.fallback-executable=hg-non-existent
-  tried to fall back to a 'hg-non-existent' sub-process but got error $ENOENT$
-  unsupported feature: error: Found argument '--exclude' which wasn't expected, or isn't valid in this context
-  
-  USAGE:
-      rhg cat [OPTIONS] <FILE>...
-  
-  For more information try --help
-  
-  [252]
+  abort: invalid fallback 'hg-non-existent': cannot find binary path
+  [253]
 
   $ rhg cat original --exclude="*.rs" --config rhg.fallback-executable=rhg
   Blocking recursive fallback. The 'rhg.fallback-executable = rhg' config points to `rhg` itself.