Fixes from PR

- is_ebadf always returns false
- Allow reading partial characters to buffer
- Allow full UTF-16 in stdin

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>

library/std/src/sys/pal/uefi/stdio.rs

@@ -5,7 +5,36 @@ use crate::os::uefi;
 use crate::ptr::NonNull;
 
 pub struct Stdin {
-    pending: Option<char>,
+    surrogate: Option<u16>,
+    incomplete_utf8: IncompleteUtf8,
+}
+
+struct IncompleteUtf8 {
+    bytes: [u8; 4],
+    len: u8,
+}
+
+impl IncompleteUtf8 {
+    pub const fn new() -> IncompleteUtf8 {
+        IncompleteUtf8 { bytes: [0; 4], len: 0 }
+    }
+
+    // Implemented for use in Stdin::read.
+    fn read(&mut self, buf: &mut [u8]) -> usize {
+        // Write to buffer until the buffer is full or we run out of bytes.
+        let to_write = crate::cmp::min(buf.len(), self.len as usize);
+        buf[..to_write].copy_from_slice(&self.bytes[..to_write]);
+
+        // Rotate the remaining bytes if not enough remaining space in buffer.
+        if usize::from(self.len) > buf.len() {
+            self.bytes.copy_within(to_write.., 0);
+            self.len -= to_write as u8;
+        } else {
+            self.len = 0;
+        }
+
+        to_write
+    }
 }
 
 pub struct Stdout;
@@ -13,46 +42,62 @@ pub struct Stderr;
 
 impl Stdin {
     pub const fn new() -> Stdin {
-        Stdin { pending: None }
+        Stdin { surrogate: None, incomplete_utf8: IncompleteUtf8::new() }
     }
 }
 
 impl io::Read for Stdin {
-    fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
-        let st: NonNull<r_efi::efi::SystemTable> = uefi::env::system_table().cast();
-        let stdin = unsafe { (*st.as_ptr()).con_in };
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        // If there are bytes in the incomplete utf-8, start with those.
+        // (No-op if there is nothing in the buffer.)
+        let mut bytes_copied = self.incomplete_utf8.read(buf);
 
-        // Write any pending character
-        if let Some(ch) = self.pending {
-            if ch.len_utf8() > buf.len() {
-                return Ok(0);
+        let stdin: *mut r_efi::protocols::simple_text_input::Protocol = unsafe {
+            let st: NonNull<r_efi::efi::SystemTable> = uefi::env::system_table().cast();
+            (*st.as_ptr()).con_in
+        };
+
+        if bytes_copied == buf.len() {
+            return Ok(bytes_copied);
+        }
+
+        let ch = simple_text_input_read(stdin)?;
+        // Only 1 character should be returned.
+        let mut ch: Vec<Result<char, crate::char::DecodeUtf16Error>> =
+            if let Some(x) = self.surrogate.take() {
+                char::decode_utf16([x, ch]).collect()
+            } else {
+                char::decode_utf16([ch]).collect()
+            };
+
+        if ch.len() > 1 {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "invalid utf-16 sequence"));
+        }
+
+        match ch.pop().unwrap() {
+            Err(e) => {
+                self.surrogate = Some(e.unpaired_surrogate());
+            }
+            Ok(x) => {
+                // This will always be > 0
+                let buf_free_count = buf.len() - bytes_copied;
+                assert!(buf_free_count > 0);
+
+                if buf_free_count >= x.len_utf8() {
+                    // There is enough space in the buffer for the character.
+                    bytes_copied += x.encode_utf8(&mut buf[bytes_copied..]).len();
+                } else {
+                    // There is not enough space in the buffer for the character.
+                    // Store the character in the incomplete buffer.
+                    self.incomplete_utf8.len =
+                        x.encode_utf8(&mut self.incomplete_utf8.bytes).len() as u8;
+                    // write partial character to buffer.
+                    bytes_copied += self.incomplete_utf8.read(buf);
+                }
             }
-            ch.encode_utf8(buf);
-            buf = &mut buf[ch.len_utf8()..];
-            self.pending = None;
         }
 
-        // Try reading any pending data
-        let inp = read(stdin)?;
-
-        // Check if the key is printiable character
-        if inp == 0x00 {
-            return Err(io::const_io_error!(io::ErrorKind::Interrupted, "Special Key Press"));
-        }
-
-        // The option unwrap is safe since iterator will have 1 element.
-        let ch: char = char::decode_utf16([inp])
-            .next()
-            .unwrap()
-            .map_err(|_| io::const_io_error!(io::ErrorKind::InvalidInput, "Invalid Input"))?;
-        if ch.len_utf8() > buf.len() {
-            self.pending = Some(ch);
-            return Ok(0);
-        }
-
-        ch.encode_utf8(buf);
-
-        Ok(ch.len_utf8())
+        Ok(bytes_copied)
     }
 }
 
@@ -94,11 +139,11 @@ impl io::Write for Stderr {
     }
 }
 
-// UCS-2 character should occupy 3 bytes at most in UTF-8
-pub const STDIN_BUF_SIZE: usize = 3;
+// UTF-16 character should occupy 4 bytes at most in UTF-8
+pub const STDIN_BUF_SIZE: usize = 4;
 
-pub fn is_ebadf(err: &io::Error) -> bool {
-    err.raw_os_error() == Some(r_efi::efi::Status::UNSUPPORTED.as_usize())
+pub fn is_ebadf(_err: &io::Error) -> bool {
+    false
 }
 
 pub fn panic_output() -> Option<impl io::Write> {
@@ -116,6 +161,7 @@ fn write(
     };
 
     let mut utf16: Vec<u16> = utf8.encode_utf16().collect();
+    // NULL terminate the string
     utf16.push(0);
 
     unsafe { simple_text_output(protocol, &mut utf16) }?;
@@ -131,7 +177,9 @@ unsafe fn simple_text_output(
     if res.is_error() { Err(io::Error::from_raw_os_error(res.as_usize())) } else { Ok(()) }
 }
 
-fn read(stdin: *mut r_efi::protocols::simple_text_input::Protocol) -> io::Result<u16> {
+fn simple_text_input_read(
+    stdin: *mut r_efi::protocols::simple_text_input::Protocol,
+) -> io::Result<u16> {
     loop {
         match read_key_stroke(stdin) {
             Ok(x) => return Ok(x.unicode_char),