Optimize Seek::stream_len impl for File

It uses the file metadata on Unix with a fallback for files incorrectly
reported as zero-sized. It uses `GetFileSizeEx` on Windows.

This reduces the number of syscalls needed for determining the file size
of an open file from 3 to 1.

Author: tbu-

library/std/src/fs.rs

@@ -823,9 +823,38 @@ impl Write for &File {
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Seek for &File {
+    /// Seek to an offset, in bytes in a file.
+    ///
+    /// See [`Seek::seek`] docs for more info.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// This function currently corresponds to the `lseek64` function on Unix
+    /// and the `SetFilePointerEx` function on Windows. Note that this [may
+    /// change in the future][changes].
+    ///
+    /// [changes]: io#platform-specific-behavior
     fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
         self.inner.seek(pos)
     }
+
+    /// Returns the length of this file (in bytes).
+    ///
+    /// See [`Seek::stream_len`] docs for more info.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// This function currently corresponds to the `statx` function on Linux
+    /// (with fallbacks) and the `GetFileSizeEx` function on Windows. Note that
+    /// this [may change in the future][changes].
+    ///
+    /// [changes]: io#platform-specific-behavior
+    fn stream_len(&mut self) -> io::Result<u64> {
+        if let Some(result) = self.inner.size() {
+            return result;
+        }
+        io::stream_len_default(self)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -872,6 +901,9 @@ impl Seek for File {
     fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
         (&*self).seek(pos)
     }
+    fn stream_len(&mut self) -> io::Result<u64> {
+        (&*self).stream_len()
+    }
 }
 
 #[stable(feature = "io_traits_arc", since = "1.73.0")]

library/std/src/io/mod.rs

@@ -1995,16 +1995,7 @@ pub trait Seek {
     /// ```
     #[unstable(feature = "seek_stream_len", issue = "59359")]
     fn stream_len(&mut self) -> Result<u64> {
-        let old_pos = self.stream_position()?;
-        let len = self.seek(SeekFrom::End(0))?;
-
-        // Avoid seeking a third time when we were already at the end of the
-        // stream. The branch is usually way cheaper than a seek operation.
-        if old_pos != len {
-            self.seek(SeekFrom::Start(old_pos))?;
-        }
-
-        Ok(len)
+        stream_len_default(self)
     }
 
     /// Returns the current seek position from the start of the stream.
@@ -2066,6 +2057,21 @@ pub trait Seek {
     }
 }
 
+#[doc(hidden)]
+#[unstable(feature = "internal_stream_len_default", issue = "none")]
+pub fn stream_len_default<T: Seek + ?Sized>(self_: &mut T) -> Result<u64> {
+    let old_pos = self_.stream_position()?;
+    let len = self_.seek(SeekFrom::End(0))?;
+
+    // Avoid seeking a third time when we were already at the end of the
+    // stream. The branch is usually way cheaper than a seek operation.
+    if old_pos != len {
+        self_.seek(SeekFrom::Start(old_pos))?;
+    }
+
+    Ok(len)
+}
+
 /// Enumeration of possible methods to seek within an I/O object.
 ///
 /// It is used by the [`Seek`] trait.

library/std/src/sys/pal/hermit/fs.rs

@@ -426,6 +426,10 @@ impl File {
         Err(Error::from_raw_os_error(22))
     }
 
+    pub fn size(&self) -> Option<io::Result<u64>> {
+        None
+    }
+
     pub fn duplicate(&self) -> io::Result<File> {
         Err(Error::from_raw_os_error(22))
     }

library/std/src/sys/pal/solid/fs.rs

@@ -455,6 +455,10 @@ impl File {
         }
     }
 
+    pub fn size(&self) -> Option<io::Result<u64>> {
+        None
+    }
+
     pub fn duplicate(&self) -> io::Result<File> {
         unsupported()
     }

library/std/src/sys/pal/unix/fs.rs

@@ -1307,6 +1307,15 @@ impl File {
         Ok(n as u64)
     }
 
+    pub fn size(&self) -> Option<io::Result<u64>> {
+        match self.file_attr().map(|attr| attr.size()) {
+            // Fall back to default implementation if the returned size is 0,
+            // we might be in a proc mount.
+            Ok(0) => None,
+            result => Some(result),
+        }
+    }
+
     pub fn duplicate(&self) -> io::Result<File> {
         self.0.duplicate().map(File)
     }

library/std/src/sys/pal/unsupported/fs.rs

@@ -238,6 +238,10 @@ impl File {
         self.0
     }
 
+    pub fn size(&self) -> io::Result<u64> {
+        self.0
+    }
+
     pub fn duplicate(&self) -> io::Result<File> {
         self.0
     }

library/std/src/sys/pal/wasi/fs.rs

@@ -465,6 +465,10 @@ impl File {
         self.fd.seek(pos)
     }
 
+    pub fn size(&self) -> Option<io::Result<u64>> {
+        None
+    }
+
     pub fn duplicate(&self) -> io::Result<File> {
         // https://github.com/CraneStation/wasmtime/blob/master/docs/WASI-rationale.md#why-no-dup
         unsupported()

library/std/src/sys/pal/windows/c/bindings.txt

@@ -2336,6 +2336,7 @@ Windows.Win32.Storage.FileSystem.FlushFileBuffers
 Windows.Win32.Storage.FileSystem.GetFileAttributesW
 Windows.Win32.Storage.FileSystem.GetFileInformationByHandle
 Windows.Win32.Storage.FileSystem.GetFileInformationByHandleEx
+Windows.Win32.Storage.FileSystem.GetFileSizeEx
 Windows.Win32.Storage.FileSystem.GetFileType
 Windows.Win32.Storage.FileSystem.GETFINALPATHNAMEBYHANDLE_FLAGS
 Windows.Win32.Storage.FileSystem.GetFinalPathNameByHandleW

library/std/src/sys/pal/windows/c/windows_sys.rs

@@ -273,6 +273,10 @@ extern "system" {
     ) -> BOOL;
 }
 #[link(name = "kernel32")]
+extern "system" {
+    pub fn GetFileSizeEx(hfile: HANDLE, lpfilesize: *mut i64) -> BOOL;
+}
+#[link(name = "kernel32")]
 extern "system" {
     pub fn GetFileType(hfile: HANDLE) -> FILE_TYPE;
 }

library/std/src/sys/pal/windows/fs.rs

@@ -501,6 +501,12 @@ impl File {
         Ok(newpos as u64)
     }
 
+    pub fn size(&self) -> Option<io::Result<u64>> {
+        let mut result = 0;
+        Some(cvt(unsafe { c::GetFileSizeEx(self.handle.as_raw_handle(), &mut result) })
+            .map(|_| result))
+    }
+
     pub fn duplicate(&self) -> io::Result<File> {
         Ok(Self { handle: self.handle.try_clone()? })
     }