Auto merge of #138023 - melrief:97227_impl_Seek_for_Take, r=tgross35

Add `std::io::Seek` instance for `std::io::Take`

Library tracking issue [#97227](#97227).
ACP: rust-lang/libs-team#555

1. add a `len` field to `Take` to keep track of the original number of bytes that `Take` could read
2. add a `position()` method to return the current position of the cursor inside `Take`
3. implement `std::io::Seek` for `std::io::Take`

Closes: rust-lang/libs-team#555

Author: bors

library/std/src/io/mod.rs

@@ -1214,7 +1214,7 @@ pub trait Read {
     where
         Self: Sized,
     {
-        Take { inner: self, limit }
+        Take { inner: self, len: limit, limit }
     }
 }
 
@@ -2830,6 +2830,7 @@ impl<T, U> SizeHint for Chain<T, U> {
 #[derive(Debug)]
 pub struct Take<T> {
     inner: T,
+    len: u64,
     limit: u64,
 }
 
@@ -2864,6 +2865,12 @@ impl<T> Take<T> {
         self.limit
     }
 
+    /// Returns the number of bytes read so far.
+    #[unstable(feature = "seek_io_take_position", issue = "97227")]
+    pub fn position(&self) -> u64 {
+        self.len - self.limit
+    }
+
     /// Sets the number of bytes that can be read before this instance will
     /// return EOF. This is the same as constructing a new `Take` instance, so
     /// the amount of bytes read and the previous limit value don't matter when
@@ -2889,6 +2896,7 @@ impl<T> Take<T> {
     /// ```
     #[stable(feature = "take_set_limit", since = "1.27.0")]
     pub fn set_limit(&mut self, limit: u64) {
+        self.len = limit;
         self.limit = limit;
     }
 
@@ -3076,6 +3084,49 @@ impl<T> SizeHint for Take<T> {
     }
 }
 
+#[stable(feature = "seek_io_take", since = "CURRENT_RUSTC_VERSION")]
+impl<T: Seek> Seek for Take<T> {
+    fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
+        let new_position = match pos {
+            SeekFrom::Start(v) => Some(v),
+            SeekFrom::Current(v) => self.position().checked_add_signed(v),
+            SeekFrom::End(v) => self.len.checked_add_signed(v),
+        };
+        let new_position = match new_position {
+            Some(v) if v <= self.len => v,
+            _ => return Err(ErrorKind::InvalidInput.into()),
+        };
+        while new_position != self.position() {
+            if let Some(offset) = new_position.checked_signed_diff(self.position()) {
+                self.inner.seek_relative(offset)?;
+                self.limit = self.limit.wrapping_sub(offset as u64);
+                break;
+            }
+            let offset = if new_position > self.position() { i64::MAX } else { i64::MIN };
+            self.inner.seek_relative(offset)?;
+            self.limit = self.limit.wrapping_sub(offset as u64);
+        }
+        Ok(new_position)
+    }
+
+    fn stream_len(&mut self) -> Result<u64> {
+        Ok(self.len)
+    }
+
+    fn stream_position(&mut self) -> Result<u64> {
+        Ok(self.position())
+    }
+
+    fn seek_relative(&mut self, offset: i64) -> Result<()> {
+        if !self.position().checked_add_signed(offset).is_some_and(|p| p <= self.len) {
+            return Err(ErrorKind::InvalidInput.into());
+        }
+        self.inner.seek_relative(offset)?;
+        self.limit = self.limit.wrapping_sub(offset as u64);
+        Ok(())
+    }
+}
+
 /// An iterator over `u8` values of a reader.
 ///
 /// This struct is generally created by calling [`bytes`] on a reader.

library/std/src/io/tests.rs

@@ -416,6 +416,126 @@ fn seek_position() -> io::Result<()> {
     Ok(())
 }
 
+#[test]
+fn take_seek() -> io::Result<()> {
+    let mut buf = Cursor::new(b"0123456789");
+    buf.set_position(2);
+    let mut take = buf.by_ref().take(4);
+    let mut buf1 = [0u8; 1];
+    let mut buf2 = [0u8; 2];
+    assert_eq!(take.position(), 0);
+
+    assert_eq!(take.seek(SeekFrom::Start(0))?, 0);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'2', b'3']);
+    assert_eq!(take.seek(SeekFrom::Start(1))?, 1);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'3', b'4']);
+    assert_eq!(take.seek(SeekFrom::Start(2))?, 2);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'4', b'5']);
+    assert_eq!(take.seek(SeekFrom::Start(3))?, 3);
+    take.read_exact(&mut buf1)?;
+    assert_eq!(buf1, [b'5']);
+    assert_eq!(take.seek(SeekFrom::Start(4))?, 4);
+    assert_eq!(take.read(&mut buf1)?, 0);
+
+    assert_eq!(take.seek(SeekFrom::End(0))?, 4);
+    assert_eq!(take.seek(SeekFrom::End(-1))?, 3);
+    take.read_exact(&mut buf1)?;
+    assert_eq!(buf1, [b'5']);
+    assert_eq!(take.seek(SeekFrom::End(-2))?, 2);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'4', b'5']);
+    assert_eq!(take.seek(SeekFrom::End(-3))?, 1);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'3', b'4']);
+    assert_eq!(take.seek(SeekFrom::End(-4))?, 0);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'2', b'3']);
+
+    assert_eq!(take.seek(SeekFrom::Current(0))?, 2);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'4', b'5']);
+
+    assert_eq!(take.seek(SeekFrom::Current(-3))?, 1);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'3', b'4']);
+
+    assert_eq!(take.seek(SeekFrom::Current(-1))?, 2);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'4', b'5']);
+
+    assert_eq!(take.seek(SeekFrom::Current(-4))?, 0);
+    take.read_exact(&mut buf2)?;
+    assert_eq!(buf2, [b'2', b'3']);
+
+    assert_eq!(take.seek(SeekFrom::Current(2))?, 4);
+    assert_eq!(take.read(&mut buf1)?, 0);
+
+    Ok(())
+}
+
+#[test]
+fn take_seek_error() {
+    let buf = Cursor::new(b"0123456789");
+    let mut take = buf.take(2);
+    assert!(take.seek(SeekFrom::Start(3)).is_err());
+    assert!(take.seek(SeekFrom::End(1)).is_err());
+    assert!(take.seek(SeekFrom::End(-3)).is_err());
+    assert!(take.seek(SeekFrom::Current(-1)).is_err());
+    assert!(take.seek(SeekFrom::Current(3)).is_err());
+}
+
+struct ExampleHugeRangeOfZeroes {
+    position: u64,
+}
+
+impl Read for ExampleHugeRangeOfZeroes {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let max = buf.len().min(usize::MAX);
+        for i in 0..max {
+            if self.position == u64::MAX {
+                return Ok(i);
+            }
+            self.position += 1;
+            buf[i] = 0;
+        }
+        Ok(max)
+    }
+}
+
+impl Seek for ExampleHugeRangeOfZeroes {
+    fn seek(&mut self, pos: io::SeekFrom) -> io::Result<u64> {
+        match pos {
+            io::SeekFrom::Start(i) => self.position = i,
+            io::SeekFrom::End(i) if i >= 0 => self.position = u64::MAX,
+            io::SeekFrom::End(i) => self.position = self.position - i.unsigned_abs(),
+            io::SeekFrom::Current(i) => {
+                self.position = if i >= 0 {
+                    self.position.saturating_add(i.unsigned_abs())
+                } else {
+                    self.position.saturating_sub(i.unsigned_abs())
+                };
+            }
+        }
+        Ok(self.position)
+    }
+}
+
+#[test]
+fn take_seek_big_offsets() -> io::Result<()> {
+    let inner = ExampleHugeRangeOfZeroes { position: 1 };
+    let mut take = inner.take(u64::MAX - 2);
+    assert_eq!(take.seek(io::SeekFrom::Start(u64::MAX - 2))?, u64::MAX - 2);
+    assert_eq!(take.inner.position, u64::MAX - 1);
+    assert_eq!(take.seek(io::SeekFrom::Start(0))?, 0);
+    assert_eq!(take.inner.position, 1);
+    assert_eq!(take.seek(io::SeekFrom::End(-1))?, u64::MAX - 3);
+    assert_eq!(take.inner.position, u64::MAX - 2);
+    Ok(())
+}
+
 // A simple example reader which uses the default implementation of
 // read_to_end.
 struct ExampleSliceReader<'a> {

library/std/src/lib.rs

@@ -325,6 +325,7 @@
 #![feature(try_blocks)]
 #![feature(try_trait_v2)]
 #![feature(type_alias_impl_trait)]
+#![feature(unsigned_signed_diff)]
 // tidy-alphabetical-end
 //
 // Library features (core):