Auto merge of #77029 - ehuss:command-access, r=dtolnay

Add accessors to Command.

This adds some accessor methods to `Command` to provide a way to access the values set when building the `Command`. An example where this can be useful is to display the command to be executed. This is roughly based on the [`ProcessBuilder`](13b73cdaf7/src/cargo/util/process_builder.rs (L105-L134)) in Cargo.

Possible concerns about the API:
- Values with NULs on Unix will be returned as `"<string-with-nul>"`. I don't think it is practical to avoid this, since otherwise a whole separate copy of all the values would need to be kept in `Command`.
- Does not handle `arg0` on Unix. This can be awkward to support in `get_args` and is rarely used. I figure if someone really wants it, it can be added to `CommandExt` as a separate method.
- Does not offer a way to detect `env_clear`. I'm uncertain if it would be useful for anyone.
- Does not offer a way to get an environment variable by name (`get_env`). I figure this can be added later if anyone really wants it. I think the motivation for this is weak, though. Also, the API could be a little awkward (return a `Option<Option<&OsStr>>`?).
- `get_envs` could skip "cleared" entries and just return `&OsStr` values instead of `Option<&OsStr>`. I'm on the fence here. My use case is to display a shell command, and I only intend it to be roughly equivalent to the actual execution, and I probably won't display `None` entries. I erred on the side of providing extra information, but I suspect many situations will just filter out the `None`s.
- Could implement more iterator stuff (like `DoubleEndedIterator`).

I have not implemented new std items before, so I'm uncertain if the existing issue should be reused, or if a new tracking issue is needed.

cc #44434

library/std/src/sys/unix/process/mod.rs

@@ -1,6 +1,7 @@
-pub use self::process_common::{Command, ExitCode, Stdio, StdioPipes};
+pub use self::process_common::{Command, CommandArgs, ExitCode, Stdio, StdioPipes};
 pub use self::process_inner::{ExitStatus, Process};
 pub use crate::ffi::OsString as EnvKey;
+pub use crate::sys_common::process::CommandEnvs;
 
 mod process_common;
 #[cfg(not(target_os = "fuchsia"))]

library/std/src/sys/unix/process/process_common.rs

@@ -7,11 +7,12 @@ use crate::collections::BTreeMap;
 use crate::ffi::{CStr, CString, OsStr, OsString};
 use crate::fmt;
 use crate::io;
+use crate::path::Path;
 use crate::ptr;
 use crate::sys::fd::FileDesc;
 use crate::sys::fs::File;
 use crate::sys::pipe::{self, AnonPipe};
-use crate::sys_common::process::CommandEnv;
+use crate::sys_common::process::{CommandEnv, CommandEnvs};
 
 #[cfg(not(target_os = "fuchsia"))]
 use crate::sys::fs::OpenOptions;
@@ -184,11 +185,30 @@ impl Command {
     pub fn saw_nul(&self) -> bool {
         self.saw_nul
     }
+
+    pub fn get_program(&self) -> &OsStr {
+        OsStr::from_bytes(self.program.as_bytes())
+    }
+
+    pub fn get_args(&self) -> CommandArgs<'_> {
+        let mut iter = self.args.iter();
+        iter.next();
+        CommandArgs { iter }
+    }
+
+    pub fn get_envs(&self) -> CommandEnvs<'_> {
+        self.env.iter()
+    }
+
+    pub fn get_current_dir(&self) -> Option<&Path> {
+        self.cwd.as_ref().map(|cs| Path::new(OsStr::from_bytes(cs.as_bytes())))
+    }
+
     pub fn get_argv(&self) -> &Vec<*const c_char> {
         &self.argv.0
     }
 
-    pub fn get_program(&self) -> &CStr {
+    pub fn get_program_cstr(&self) -> &CStr {
         &*self.program
     }
 
@@ -402,3 +422,32 @@ impl ExitCode {
         self.0 as i32
     }
 }
+
+pub struct CommandArgs<'a> {
+    iter: crate::slice::Iter<'a, CString>,
+}
+
+impl<'a> Iterator for CommandArgs<'a> {
+    type Item = &'a OsStr;
+    fn next(&mut self) -> Option<&'a OsStr> {
+        self.iter.next().map(|cs| OsStr::from_bytes(cs.as_bytes()))
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+impl<'a> ExactSizeIterator for CommandArgs<'a> {
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
+
+impl<'a> fmt::Debug for CommandArgs<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(self.iter.clone()).finish()
+    }
+}

library/std/src/sys/unix/process/process_fuchsia.rs

@@ -120,7 +120,7 @@ impl Command {
                 | FDIO_SPAWN_CLONE_NAMESPACE
                 | FDIO_SPAWN_CLONE_ENVIRON // this is ignored when envp is non-null
                 | FDIO_SPAWN_CLONE_UTC_CLOCK,
-            self.get_program().as_ptr(),
+            self.get_program_cstr().as_ptr(),
             self.get_argv().as_ptr(),
             envp,
             actions.len() as size_t,

library/std/src/sys/unix/process/process_unix.rs

@@ -245,7 +245,7 @@ impl Command {
             *sys::os::environ() = envp.as_ptr();
         }
 
-        libc::execvp(self.get_program().as_ptr(), self.get_argv().as_ptr());
+        libc::execvp(self.get_program_cstr().as_ptr(), self.get_argv().as_ptr());
         Err(io::Error::last_os_error())
     }
 
@@ -383,7 +383,7 @@ impl Command {
             let envp = envp.map(|c| c.as_ptr()).unwrap_or_else(|| *sys::os::environ() as *const _);
             let ret = libc::posix_spawnp(
                 &mut p.pid,
-                self.get_program().as_ptr(),
+                self.get_program_cstr().as_ptr(),
                 file_actions.0.as_ptr(),
                 attrs.0.as_ptr(),
                 self.get_argv().as_ptr() as *const _,