Fix `isatty` in WASI. (#3696)

WASI doesn't have an `isatty` ioctl or syscall, so wasi-libc's `isatty`
implementation uses the file descriptor type and rights to determine if
the file descriptor is likely to be a tty. The real fix here will be to
add an `isatty` call to WASI. But for now, have Wasmtime set the
filetype and rights for file descriptors so that wasi-libc's `isatty`
works as expected.

Cargo.lock

@@ -3129,6 +3129,7 @@ version = "0.33.0"
 dependencies = [
  "anyhow",
  "async-trait",
+ "atty",
  "cap-fs-ext",
  "cap-rand",
  "cap-std",

crates/wasi-common/cap-std-sync/Cargo.toml

@@ -30,6 +30,7 @@ rustix = "0.31.0"
 [target.'cfg(windows)'.dependencies]
 winapi = "0.3"
 lazy_static = "1.4"
+atty = "0.2.14"
 
 [dev-dependencies]
 tempfile = "3.1.0"

crates/wasi-common/cap-std-sync/src/file.rs

@@ -121,6 +121,16 @@ impl WasiFile for File {
     async fn num_ready_bytes(&self) -> Result<u64, Error> {
         Ok(self.0.num_ready_bytes()?)
     }
+    fn isatty(&self) -> bool {
+        #[cfg(unix)]
+        {
+            rustix::io::isatty(&self.0)
+        }
+        #[cfg(windows)]
+        {
+            false
+        }
+    }
     async fn readable(&self) -> Result<(), Error> {
         Err(Error::badf())
     }

crates/wasi-common/cap-std-sync/src/stdio.rs

@@ -35,8 +35,12 @@ impl WasiFile for Stdin {
         Ok(())
     }
     async fn get_filetype(&self) -> Result<FileType, Error> {
+        if self.isatty() {
+            Ok(FileType::CharacterDevice)
+        } else {
             Ok(FileType::Unknown)
         }
+    }
     async fn get_fdflags(&self) -> Result<FdFlags, Error> {
         Ok(FdFlags::empty())
     }
@@ -104,6 +108,16 @@ impl WasiFile for Stdin {
     async fn num_ready_bytes(&self) -> Result<u64, Error> {
         Ok(self.0.num_ready_bytes()?)
     }
+    fn isatty(&self) -> bool {
+        #[cfg(unix)]
+        {
+            rustix::io::isatty(&self.0)
+        }
+        #[cfg(not(unix))]
+        {
+            atty::is(atty::Stream::Stdin)
+        }
+    }
     async fn readable(&self) -> Result<(), Error> {
         Err(Error::badf())
     }
@@ -125,7 +139,7 @@ impl AsFd for Stdin {
 }
 
 macro_rules! wasi_file_write_impl {
-    ($ty:ty) => {
+    ($ty:ty, $ident:ident) => {
         #[async_trait::async_trait]
         impl WasiFile for $ty {
             fn as_any(&self) -> &dyn Any {
@@ -138,8 +152,12 @@ macro_rules! wasi_file_write_impl {
                 Ok(())
             }
             async fn get_filetype(&self) -> Result<FileType, Error> {
+                if self.isatty() {
+                    Ok(FileType::CharacterDevice)
+                } else {
                     Ok(FileType::Unknown)
                 }
+            }
             async fn get_fdflags(&self) -> Result<FdFlags, Error> {
                 Ok(FdFlags::APPEND)
             }
@@ -210,6 +228,16 @@ macro_rules! wasi_file_write_impl {
             async fn num_ready_bytes(&self) -> Result<u64, Error> {
                 Ok(0)
             }
+            fn isatty(&self) -> bool {
+                #[cfg(unix)]
+                {
+                    rustix::io::isatty(&self.0)
+                }
+                #[cfg(not(unix))]
+                {
+                    atty::is(atty::Stream::$ident)
+                }
+            }
             async fn readable(&self) -> Result<(), Error> {
                 Err(Error::badf())
             }
@@ -237,11 +265,11 @@ pub struct Stdout(std::io::Stdout);
 pub fn stdout() -> Stdout {
     Stdout(std::io::stdout())
 }
-wasi_file_write_impl!(Stdout);
+wasi_file_write_impl!(Stdout, Stdout);
 
 pub struct Stderr(std::io::Stderr);
 
 pub fn stderr() -> Stderr {
     Stderr(std::io::stderr())
 }
-wasi_file_write_impl!(Stderr);
+wasi_file_write_impl!(Stderr, Stderr);

crates/wasi-common/src/ctx.rs

@@ -71,15 +71,31 @@ impl WasiCtx {
     }
 
     pub fn set_stdin(&mut self, f: Box<dyn WasiFile>) {
-        self.insert_file(0, f, FileCaps::all());
+        let rights = Self::stdio_rights(&*f);
+        self.insert_file(0, f, rights);
     }
 
     pub fn set_stdout(&mut self, f: Box<dyn WasiFile>) {
-        self.insert_file(1, f, FileCaps::all());
+        let rights = Self::stdio_rights(&*f);
+        self.insert_file(1, f, rights);
     }
 
     pub fn set_stderr(&mut self, f: Box<dyn WasiFile>) {
-        self.insert_file(2, f, FileCaps::all());
+        let rights = Self::stdio_rights(&*f);
+        self.insert_file(2, f, rights);
+    }
+
+    fn stdio_rights(f: &dyn WasiFile) -> FileCaps {
+        let mut rights = FileCaps::all();
+
+        // If `f` is a tty, restrict the `tell` and `seek` capabilities, so
+        // that wasi-libc's `isatty` correctly detects the file descriptor
+        // as a tty.
+        if f.isatty() {
+            rights &= !(FileCaps::TELL | FileCaps::SEEK);
+        }
+
+        rights
     }
 
     pub fn push_preopened_dir(

crates/wasi-common/src/file.rs

@@ -34,6 +34,7 @@ pub trait WasiFile: Send + Sync {
     async fn seek(&self, pos: std::io::SeekFrom) -> Result<u64, Error>; // file op that generates a new stream from a file will supercede this
     async fn peek(&self, buf: &mut [u8]) -> Result<u64, Error>; // read op
     async fn num_ready_bytes(&self) -> Result<u64, Error>; // read op
+    fn isatty(&self) -> bool;
 
     async fn readable(&self) -> Result<(), Error>;
     async fn writable(&self) -> Result<(), Error>;

crates/wasi-common/src/pipe.rs

@@ -180,6 +180,9 @@ impl<R: Read + Any + Send + Sync> WasiFile for ReadPipe<R> {
     async fn num_ready_bytes(&self) -> Result<u64, Error> {
         Ok(0)
     }
+    fn isatty(&self) -> bool {
+        false
+    }
     async fn readable(&self) -> Result<(), Error> {
         Err(Error::badf())
     }
@@ -336,6 +339,9 @@ impl<W: Write + Any + Send + Sync> WasiFile for WritePipe<W> {
     async fn num_ready_bytes(&self) -> Result<u64, Error> {
         Ok(0)
     }
+    fn isatty(&self) -> bool {
+        false
+    }
     async fn readable(&self) -> Result<(), Error> {
         Err(Error::badf())
     }

crates/wasi-common/tokio/src/file.rs

@@ -112,6 +112,9 @@ macro_rules! wasi_file_impl {
             async fn num_ready_bytes(&self) -> Result<u64, Error> {
                 block_on_dummy_executor(|| self.0.num_ready_bytes())
             }
+            fn isatty(&self) -> bool {
+                self.0.isatty()
+            }
 
             #[cfg(not(windows))]
             async fn readable(&self) -> Result<(), Error> {