Implement blocking output

This allows decoupling `Command::spawn` and `Command::output`. This is
useful for targets which do support launching programs in blocking mode
but do not support multitasking (Eg: UEFI).

This was originally conceived when working on #100316

Signed-off-by: Ayush Singh <ayushsingh1325@gmail.com>

Author: Ayush1325

library/std/src/process.rs

@@ -911,10 +911,8 @@ impl Command {
     /// ```
     #[stable(feature = "process", since = "1.0.0")]
     pub fn output(&mut self) -> io::Result<Output> {
-        self.inner
-            .spawn(imp::Stdio::MakePipe, false)
-            .map(Child::from_inner)
-            .and_then(|p| p.wait_with_output())
+        let (status, stdout, stderr) = self.inner.output()?;
+        Ok(Output { status: ExitStatus(status), stdout, stderr })
     }
 
     /// Executes a command as a child process, waiting for it to finish and

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

@@ -35,6 +35,11 @@ impl Command {
         Ok((Process { handle: Handle::new(process_handle) }, ours))
     }
 
+    pub fn output(&mut self) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+        let (proc, pipes) = self.spawn(Stdio::MakePipe, false)?;
+        crate::sys_common::process::wait_with_output(proc, pipes)
+    }
+
     pub fn exec(&mut self, default: Stdio) -> io::Error {
         if self.saw_nul() {
             return io::const_io_error!(

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

@@ -132,6 +132,11 @@ impl Command {
         }
     }
 
+    pub fn output(&mut self) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+        let (proc, pipes) = self.spawn(Stdio::MakePipe, false)?;
+        crate::sys_common::process::wait_with_output(proc, pipes)
+    }
+
     // Attempts to fork the process. If successful, returns Ok((0, -1))
     // in the child, and Ok((child_pid, -1)) in the parent.
     #[cfg(not(target_os = "linux"))]

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

@@ -20,6 +20,10 @@ impl Command {
         unsupported()
     }
 
+    pub fn output(&mut self) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+        unsupported()
+    }
+
     pub fn exec(&mut self, _default: Stdio) -> io::Error {
         unsupported_err()
     }

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

@@ -108,6 +108,11 @@ impl Command {
         }
     }
 
+    pub fn output(&mut self) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+        let (proc, pipes) = self.spawn(Stdio::MakePipe, false)?;
+        crate::sys_common::process::wait_with_output(proc, pipes)
+    }
+
     pub fn exec(&mut self, default: Stdio) -> io::Error {
         let ret = Command::spawn(self, default, false);
         match ret {

library/std/src/sys/unsupported/pipe.rs

@@ -15,7 +15,7 @@ impl AnonPipe {
         self.0
     }
 
-    pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
+    pub fn read_to_end(&self, _buf: &mut Vec<u8>) -> io::Result<usize> {
         self.0
     }
 

library/std/src/sys/unsupported/process.rs

@@ -75,6 +75,10 @@ impl Command {
     ) -> io::Result<(Process, StdioPipes)> {
         unsupported()
     }
+
+    pub fn output(&mut self) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+        unsupported()
+    }
 }
 
 impl From<AnonPipe> for Stdio {

library/std/src/sys/windows/process.rs

@@ -351,6 +351,11 @@ impl Command {
             ))
         }
     }
+
+    pub fn output(&mut self) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+        let (proc, pipes) = self.spawn(Stdio::MakePipe, false)?;
+        crate::sys_common::process::wait_with_output(proc, pipes)
+    }
 }
 
 impl fmt::Debug for Command {

library/std/src/sys_common/process.rs

@@ -4,7 +4,9 @@
 use crate::collections::BTreeMap;
 use crate::env;
 use crate::ffi::{OsStr, OsString};
-use crate::sys::process::EnvKey;
+use crate::io;
+use crate::sys::pipe::read2;
+use crate::sys::process::{EnvKey, ExitStatus, Process, StdioPipes};
 
 // Stores a set of changes to an environment
 #[derive(Clone, Debug)]
@@ -117,3 +119,30 @@ impl<'a> ExactSizeIterator for CommandEnvs<'a> {
         self.iter.is_empty()
     }
 }
+
+pub fn wait_with_output(
+    mut process: Process,
+    mut pipes: StdioPipes,
+) -> io::Result<(ExitStatus, Vec<u8>, Vec<u8>)> {
+    drop(pipes.stdin.take());
+
+    let (mut stdout, mut stderr) = (Vec::new(), Vec::new());
+    match (pipes.stdout.take(), pipes.stderr.take()) {
+        (None, None) => {}
+        (Some(out), None) => {
+            let res = out.read_to_end(&mut stdout);
+            res.unwrap();
+        }
+        (None, Some(err)) => {
+            let res = err.read_to_end(&mut stderr);
+            res.unwrap();
+        }
+        (Some(out), Some(err)) => {
+            let res = read2(out, &mut stdout, err, &mut stderr);
+            res.unwrap();
+        }
+    }
+
+    let status = process.wait()?;
+    Ok((status, stdout, stderr))
+}