Rollup merge of rust-lang#121938 - blyxxyz:quadratic-vectored-write, r=Amanieu

Fix quadratic behavior of repeated vectored writes

Some implementations of `Write::write_vectored` in the standard library (`BufWriter`, `LineWriter`, `Stdout`, `Stderr`) check all buffers to calculate the total length. This is O(n) over the number of buffers.

It's common that only a limited number of buffers is written at a time (e.g. 1024 for `writev(2)`). `write_vectored_all` will then call `write_vectored` repeatedly, leading to a runtime of O(n²) over the number of buffers.

This fix is to only calculate as much as needed if it's needed.

Here's a test program:
```rust
#![feature(write_all_vectored)]

use std::fs::File;
use std::io::{BufWriter, IoSlice, Write};
use std::time::Instant;

fn main() {
    let buf = vec![b'\0'; 100_000_000];
    let mut slices: Vec<IoSlice<'_>> = buf.chunks(100).map(IoSlice::new).collect();
    let mut writer = BufWriter::new(File::create("/dev/null").unwrap());

    let start = Instant::now();
    write_smart(&slices, &mut writer);
    println!("write_smart(): {:?}", start.elapsed());

    let start = Instant::now();
    writer.write_all_vectored(&mut slices).unwrap();
    println!("write_all_vectored(): {:?}", start.elapsed());
}

fn write_smart(mut slices: &[IoSlice<'_>], writer: &mut impl Write) {
    while !slices.is_empty() {
        // Only try to write as many slices as can be written
        let res = writer
            .write_vectored(slices.get(..1024).unwrap_or(slices))
            .unwrap();
        slices = &slices[(res / 100)..];
    }
}
```
Before this change:
```
write_smart(): 6.666952ms
write_all_vectored(): 498.437092ms
```
After this change:
```
write_smart(): 6.377158ms
write_all_vectored(): 6.923412ms
```

`LineWriter` (and by extension `Stdout`) isn't fully repaired by this because it looks for newlines. I could open an issue for that after this is merged, I think it's fixable but not trivially.

Author: matthiaskrgr

std/src/io/buffered/bufwriter.rs

@@ -554,35 +554,38 @@ impl<W: ?Sized + Write> Write for BufWriter<W> {
             // same underlying buffer, as otherwise the buffers wouldn't fit in memory). If the
             // computation overflows, then surely the input cannot fit in our buffer, so we forward
             // to the inner writer's `write_vectored` method to let it handle it appropriately.
-            let saturated_total_len =
-                bufs.iter().fold(0usize, |acc, b| acc.saturating_add(b.len()));
+            let mut saturated_total_len: usize = 0;
 
-            if saturated_total_len > self.spare_capacity() {
-                // Flush if the total length of the input exceeds our buffer's spare capacity.
-                // If we would have overflowed, this condition also holds, and we need to flush.
-                self.flush_buf()?;
+            for buf in bufs {
+                saturated_total_len = saturated_total_len.saturating_add(buf.len());
+
+                if saturated_total_len > self.spare_capacity() && !self.buf.is_empty() {
+                    // Flush if the total length of the input exceeds our buffer's spare capacity.
+                    // If we would have overflowed, this condition also holds, and we need to flush.
+                    self.flush_buf()?;
+                }
+
+                if saturated_total_len >= self.buf.capacity() {
+                    // Forward to our inner writer if the total length of the input is greater than or
+                    // equal to our buffer capacity. If we would have overflowed, this condition also
+                    // holds, and we punt to the inner writer.
+                    self.panicked = true;
+                    let r = self.get_mut().write_vectored(bufs);
+                    self.panicked = false;
+                    return r;
+                }
             }
 
-            if saturated_total_len >= self.buf.capacity() {
-                // Forward to our inner writer if the total length of the input is greater than or
-                // equal to our buffer capacity. If we would have overflowed, this condition also
-                // holds, and we punt to the inner writer.
-                self.panicked = true;
-                let r = self.get_mut().write_vectored(bufs);
-                self.panicked = false;
-                r
-            } else {
-                // `saturated_total_len < self.buf.capacity()` implies that we did not saturate.
+            // `saturated_total_len < self.buf.capacity()` implies that we did not saturate.
 
-                // SAFETY: We checked whether or not the spare capacity was large enough above. If
-                // it was, then we're safe already. If it wasn't, we flushed, making sufficient
-                // room for any input <= the buffer size, which includes this input.
-                unsafe {
-                    bufs.iter().for_each(|b| self.write_to_buffer_unchecked(b));
-                };
+            // SAFETY: We checked whether or not the spare capacity was large enough above. If
+            // it was, then we're safe already. If it wasn't, we flushed, making sufficient
+            // room for any input <= the buffer size, which includes this input.
+            unsafe {
+                bufs.iter().for_each(|b| self.write_to_buffer_unchecked(b));
+            };
 
-                Ok(saturated_total_len)
-            }
+            Ok(saturated_total_len)
         } else {
             let mut iter = bufs.iter();
             let mut total_written = if let Some(buf) = iter.by_ref().find(|&buf| !buf.is_empty()) {

std/src/io/buffered/linewritershim.rs

@@ -175,6 +175,10 @@ impl<'a, W: ?Sized + Write> Write for LineWriterShim<'a, W> {
         }
 
         // Find the buffer containing the last newline
+        // FIXME: This is overly slow if there are very many bufs and none contain
+        // newlines. e.g. writev() on Linux only writes up to 1024 slices, so
+        // scanning the rest is wasted effort. This makes write_all_vectored()
+        // quadratic.
         let last_newline_buf_idx = bufs
             .iter()
             .enumerate()
@@ -215,9 +219,14 @@ impl<'a, W: ?Sized + Write> Write for LineWriterShim<'a, W> {
 
         // Don't try to reconstruct the exact amount written; just bail
         // in the event of a partial write
-        let lines_len = lines.iter().map(|buf| buf.len()).sum();
-        if flushed < lines_len {
-            return Ok(flushed);
+        let mut lines_len: usize = 0;
+        for buf in lines {
+            // With overlapping/duplicate slices the total length may in theory
+            // exceed usize::MAX
+            lines_len = lines_len.saturating_add(buf.len());
+            if flushed < lines_len {
+                return Ok(flushed);
+            }
         }
 
         // Now that the write has succeeded, buffer the rest (or as much of the

std/src/io/stdio.rs

@@ -128,8 +128,8 @@ impl Write for StdoutRaw {
     }
 
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
-        let total = bufs.iter().map(|b| b.len()).sum();
-        handle_ebadf(self.0.write_vectored(bufs), total)
+        let total = || bufs.iter().map(|b| b.len()).sum();
+        handle_ebadf_lazy(self.0.write_vectored(bufs), total)
     }
 
     #[inline]
@@ -160,8 +160,8 @@ impl Write for StderrRaw {
     }
 
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
-        let total = bufs.iter().map(|b| b.len()).sum();
-        handle_ebadf(self.0.write_vectored(bufs), total)
+        let total = || bufs.iter().map(|b| b.len()).sum();
+        handle_ebadf_lazy(self.0.write_vectored(bufs), total)
     }
 
     #[inline]
@@ -193,6 +193,13 @@ fn handle_ebadf<T>(r: io::Result<T>, default: T) -> io::Result<T> {
     }
 }
 
+fn handle_ebadf_lazy<T>(r: io::Result<T>, default: impl FnOnce() -> T) -> io::Result<T> {
+    match r {
+        Err(ref e) if stdio::is_ebadf(e) => Ok(default()),
+        r => r,
+    }
+}
+
 /// A handle to the standard input stream of a process.
 ///
 /// Each handle is a shared reference to a global buffer of input data to this