debugcurrentexe: command to show the path to the main executable

Summary:
This command prints the path to the main executable. It's similar to
`sys.executable` in `debugshell`, but without using Python.

The main motivation is to better support the node IPC channel on Windows.
The problem is that by default, libc (CRT, C runtime) fds (not confused with
Windows OS handles) are *not* inherited. To inherit them you'll need to set
the `lpReserved2` in `STARTUPINFOA` struct when creating a process. The field
is [documented](https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/ns-processthreadsapi-startupinfoa) as:

  cbReserved2
  Reserved for use by the C Run-time; must be zero.

  lpReserved2
  Reserved for use by the C Run-time; must be NULL.

Rust stdlib does not provide a way to set them in the C runtime compatible way
to pass C fds. nodejs/libuv sets them directly, ignoring the "must be NULL"
suggestions.

On Windows, there are multiple layers of wrappers today.

For the telemetry wrapper we might teach it to translate the fd to Windows HANDLE
and teach the main binary to also accept a HANDLE.

Reviewed By: sggutier

Differential Revision: D50020026

fbshipit-source-id: 1798a30848b2b76add63f5637594784e6e7e096c

eden/scm/lib/hgcommands/src/commands/debug.rs

@@ -14,6 +14,7 @@ pub use super::IO;
 
 commands! {
     mod args;
+    mod currentexe;
     mod dumpinternalconfig;
     mod dumpindexedlog;
     mod dumptrace;

eden/scm/lib/hgcommands/src/commands/debug/currentexe.rs

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+use clidispatch::ReqCtx;
+
+use super::ConfigSet;
+use super::NoOpts;
+use super::Result;
+
+pub fn run(ctx: ReqCtx<NoOpts>, _config: &mut ConfigSet) -> Result<u8> {
+    let path = std::env::current_exe()?;
+    let out = format!("{}\n", path.display());
+    ctx.io().write(out)?;
+    Ok(0)
+}
+
+pub fn aliases() -> &'static str {
+    "debugcurrentexe"
+}
+
+pub fn doc() -> &'static str {
+    "show the path to the main executable"
+}
+
+pub fn synopsis() -> Option<&'static str> {
+    None
+}

eden/scm/tests/test-completion.t

@@ -110,6 +110,7 @@ Show debug commands if there are no other candidates
   debugconfig
   debugcopytrace
   debugcreatestreamclonebundle
+  debugcurrentexe
   debugdag
   debugdata
   debugdatapack
@@ -437,6 +438,7 @@ Show all commands + options
   debugcomplete: options
   debugcopytrace: source, dest
   debugcreatestreamclonebundle: 
+  debugcurrentexe: 
   debugdag: bookmarks, branches, dots, spaces
   debugdata: changelog, manifest, dir
   debugdatapack: long, node, node-delta

eden/scm/tests/test-help.t

@@ -937,6 +937,8 @@ Test list of internal help commands
                  trace the copy of the given files from source to dest commit
    debugcreatestreamclonebundle
                  create a stream clone bundle file
+   debugcurrentexe
+                 show the path to the main executable
    debugdag      format the changelog or an index DAG as a concise textual
                  description
    debugdata     dump the contents of a data file revision