make Child::try_wait return io::Result<Option<ExitStatus>>

This is much nicer for callers who want to short-circuit real I/O errors
with `?`, because they can write this

    if let Some(status) = foo.try_wait()? {
        ...
    } else {
        ...
    }

instead of this

    match foo.try_wait() {
        Ok(status) => {
            ...
        }
        Err(err) if err.kind() == io::ErrorKind::WouldBlock => {
            ...
        }
        Err(err) => return Err(err),
    }

The original design of `try_wait` was patterned after the `Read` and
`Write` traits, which support both blocking and non-blocking
implementations in a single API. But since `try_wait` is never blocking,
it makes sense to optimize for the non-blocking case.

Tracking issue: https://github.com/rust-lang/rust/issues/38903

4 files changed, 14 insertions, 14 deletions

diff --git a/src/libstd/sys/redox/process.rs b/src/libstd/sys/redox/process.rs
index 50dcd44b42e..60dc03fcf47 100644
--- a/src/libstd/sys/redox/process.rs
+++ b/src/libstd/sys/redox/process.rs
@@ -502,17 +502,17 @@ impl Process {
         Ok(ExitStatus(status as i32))
     }
 
-    pub fn try_wait(&mut self) -> io::Result<ExitStatus> {
+    pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
         if let Some(status) = self.status {
-            return Ok(status)
+            return Ok(Some(status))
         }
         let mut status = 0;
         let pid = cvt(syscall::waitpid(self.pid, &mut status, syscall::WNOHANG))?;
         if pid == 0 {
-            Err(io::Error::from_raw_os_error(syscall::EWOULDBLOCK))
+            Ok(None)
         } else {
             self.status = Some(ExitStatus(status as i32));
-            Ok(ExitStatus(status as i32))
+            Ok(Some(ExitStatus(status as i32)))
         }
     }
 }
diff --git a/src/libstd/sys/unix/process/process_fuchsia.rs b/src/libstd/sys/unix/process/process_fuchsia.rs
index 87acb0ed9b9..0bb2e0c1a83 100644
--- a/src/libstd/sys/unix/process/process_fuchsia.rs
+++ b/src/libstd/sys/unix/process/process_fuchsia.rs
@@ -165,7 +165,7 @@ impl Process {
         Ok(ExitStatus::new(proc_info.rec.return_code))
     }
 
-    pub fn try_wait(&mut self) -> io::Result<ExitStatus> {
+    pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
         use default::Default;
         use sys::process::magenta::*;
 
@@ -179,7 +179,7 @@ impl Process {
             match status {
                 0 => { }, // Success
                 x if x == ERR_TIMED_OUT => {
-                    return Err(io::Error::from(io::ErrorKind::WouldBlock));
+                    return Ok(None);
                 },
                 _ => { panic!("Failed to wait on process handle: {}", status); },
             }
@@ -192,7 +192,7 @@ impl Process {
             return Err(io::Error::new(io::ErrorKind::InvalidData,
                                       "Failed to get exit status of process"));
         }
-        Ok(ExitStatus::new(proc_info.rec.return_code))
+        Ok(Some(ExitStatus::new(proc_info.rec.return_code)))
     }
 }
 
diff --git a/src/libstd/sys/unix/process/process_unix.rs b/src/libstd/sys/unix/process/process_unix.rs
index 0dc1739c1a1..bbc987209e3 100644
--- a/src/libstd/sys/unix/process/process_unix.rs
+++ b/src/libstd/sys/unix/process/process_unix.rs
@@ -249,19 +249,19 @@ impl Process {
         Ok(ExitStatus::new(status))
     }
 
-    pub fn try_wait(&mut self) -> io::Result<ExitStatus> {
+    pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
         if let Some(status) = self.status {
-            return Ok(status)
+            return Ok(Some(status))
         }
         let mut status = 0 as c_int;
         let pid = cvt(unsafe {
             libc::waitpid(self.pid, &mut status, libc::WNOHANG)
         })?;
         if pid == 0 {
-            Err(io::Error::from_raw_os_error(libc::EWOULDBLOCK))
+            Ok(None)
         } else {
             self.status = Some(ExitStatus::new(status));
-            Ok(ExitStatus::new(status))
+            Ok(Some(ExitStatus::new(status)))
         }
     }
 }
diff --git a/src/libstd/sys/windows/process.rs b/src/libstd/sys/windows/process.rs
index d2ad81023e7..1afb3728c9d 100644
--- a/src/libstd/sys/windows/process.rs
+++ b/src/libstd/sys/windows/process.rs
@@ -340,18 +340,18 @@ impl Process {
         }
     }
 
-    pub fn try_wait(&mut self) -> io::Result<ExitStatus> {
+    pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
         unsafe {
             match c::WaitForSingleObject(self.handle.raw(), 0) {
                 c::WAIT_OBJECT_0 => {}
                 c::WAIT_TIMEOUT => {
-                    return Err(io::Error::from_raw_os_error(c::WSAEWOULDBLOCK))
+                    return Ok(None);
                 }
                 _ => return Err(io::Error::last_os_error()),
             }
             let mut status = 0;
             cvt(c::GetExitCodeProcess(self.handle.raw(), &mut status))?;
-            Ok(ExitStatus(status))
+            Ok(Some(ExitStatus(status)))
         }
     }