add user_error stream (#917)

src/machine/streams.rs

@@ -14,7 +14,7 @@ use std::fmt;
 use std::fs::File;
 use std::hash::{Hash, Hasher};
 use std::io;
-use std::io::{stdout, Cursor, ErrorKind, Read, Seek, SeekFrom, Write};
+use std::io::{stderr, stdout, Cursor, ErrorKind, Read, Seek, SeekFrom, Write};
 use std::mem;
 use std::net::{Shutdown, TcpStream};
 use std::ops::DerefMut;
@@ -114,6 +114,7 @@ enum StreamInstance {
     PausedPrologStream(Vec<u8>, Box<StreamInstance>),
     ReadlineStream(ReadlineStream),
     StaticStr(Cursor<&'static str>),
+    Stderr,
     Stdout,
     TcpStream(ClauseName, TcpStream),
     TlsStream(ClauseName, TlsStream<TcpStream>),
@@ -148,12 +149,13 @@ impl StreamInstance {
             StreamInstance::ReadlineStream(ref mut rl_stream) => rl_stream.read(buf),
             StreamInstance::StaticStr(ref mut src) => src.read(buf),
             StreamInstance::Bytes(ref mut cursor) => cursor.read(buf),
-            StreamInstance::OutputFile(..) | StreamInstance::Stdout | StreamInstance::Null => {
-                Err(std::io::Error::new(
-                    ErrorKind::PermissionDenied,
-                    StreamError::ReadFromOutputStream,
-                ))
-            }
+            StreamInstance::OutputFile(..)
+            | StreamInstance::Stderr
+            | StreamInstance::Stdout
+            | StreamInstance::Null => Err(std::io::Error::new(
+                ErrorKind::PermissionDenied,
+                StreamError::ReadFromOutputStream,
+            )),
         }
     }
 }
@@ -186,6 +188,7 @@ impl fmt::Debug for StreamInstance {
             &StreamInstance::ReadlineStream(ref readline_stream) => {
                 write!(fmt, "ReadlineStream({:?})", readline_stream)
             }
+            &StreamInstance::Stderr => write!(fmt, "Stderr"),
             &StreamInstance::Stdout => write!(fmt, "Stdout"),
             &StreamInstance::TcpStream(_, ref tcp_stream) => {
                 write!(fmt, "TcpStream({:?})", tcp_stream)
@@ -521,7 +524,9 @@ impl Stream {
             | StreamInstance::InputFile(..) => "read",
             StreamInstance::TcpStream(..) | StreamInstance::TlsStream(..) => "read_append",
             StreamInstance::OutputFile(_, _, true) => "append",
-            StreamInstance::Stdout | StreamInstance::OutputFile(_, _, false) => "write",
+            StreamInstance::Stderr
+            | StreamInstance::Stdout
+            | StreamInstance::OutputFile(_, _, false) => "write",
             StreamInstance::Null => "",
         }
     }
@@ -538,6 +543,11 @@ impl Stream {
         Stream::from_inst(StreamInstance::Stdout)
     }
 
+    #[inline]
+    pub fn stderr() -> Self {
+        Stream::from_inst(StreamInstance::Stderr)
+    }
+
     #[inline]
     pub(crate) fn from_tcp_stream(address: ClauseName, tcp_stream: TcpStream) -> Self {
         tcp_stream.set_read_timeout(None).unwrap();
@@ -561,6 +571,14 @@ impl Stream {
         Stream::from_inst(StreamInstance::InputFile(name, file))
     }
 
+    #[inline]
+    pub(crate) fn is_stderr(&self) -> bool {
+        match self.stream_inst.0.borrow().stream_inst {
+            StreamInstance::Stderr => true,
+            _ => false,
+        }
+    }
+
     #[inline]
     pub(crate) fn is_stdout(&self) -> bool {
         match self.stream_inst.0.borrow().stream_inst {
@@ -608,7 +626,8 @@ impl Stream {
     #[inline]
     pub(crate) fn is_output_stream(&self) -> bool {
         match self.stream_inst.0.borrow().stream_inst {
-            StreamInstance::Stdout
+            StreamInstance::Stderr
+            | StreamInstance::Stdout
             | StreamInstance::TcpStream(..)
             | StreamInstance::TlsStream(..)
             | StreamInstance::Bytes(_)
@@ -1110,6 +1129,7 @@ impl Write for Stream {
             StreamInstance::TlsStream(_, ref mut tls_stream) => tls_stream.write(buf),
             StreamInstance::Bytes(ref mut cursor) => cursor.write(buf),
             StreamInstance::Stdout => stdout().write(buf),
+            StreamInstance::Stderr => stderr().write(buf),
             StreamInstance::PausedPrologStream(..)
             | StreamInstance::StaticStr(_)
             | StreamInstance::ReadlineStream(_)
@@ -1127,6 +1147,7 @@ impl Write for Stream {
             StreamInstance::TcpStream(_, ref mut tcp_stream) => tcp_stream.flush(),
             StreamInstance::TlsStream(_, ref mut tls_stream) => tls_stream.flush(),
             StreamInstance::Bytes(ref mut cursor) => cursor.flush(),
+            StreamInstance::Stderr => stderr().flush(),
             StreamInstance::Stdout => stdout().flush(),
             StreamInstance::PausedPrologStream(..)
             | StreamInstance::StaticStr(_)