Rollup merge of rust-lang#35704 - tbu-:pr_pread_pwrite, r=sfackler

Implement `read_offset` and `write_offset`

These functions allow to read from and write to a file from multiple
threads without changing the per-file cursor, avoiding the race between
the seek and the read.

src/libstd/fs.rs

@@ -348,6 +348,30 @@ impl File {
             inner: self.inner.duplicate()?
         })
     }
+
+    /// Reads a number of bytes starting from a given offset.
+    ///
+    /// The offset is relative to the file start and thus independent from the
+    /// current cursor.
+    ///
+    /// Note that similar to `File::read`, it is not an error to return a short
+    /// read.
+    #[unstable(feature = "file_offset", issue = "35918")]
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.inner.read_at(buf, offset)
+    }
+
+    /// Writes a number of bytes starting from a given offset.
+    ///
+    /// The offset is relative to the file start and thus independent from the
+    /// current cursor.
+    ///
+    /// Note that similar to `File::write`, it is not an error to return a
+    /// short write.
+    #[unstable(feature = "file_offset", issue = "35918")]
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.inner.write_at(buf, offset)
+    }
 }
 
 impl AsInner<fs_imp::File> for File {
@@ -1893,6 +1917,53 @@ mod tests {
         check!(fs::remove_file(filename));
     }
 
+    #[test]
+    fn file_test_io_read_write_at() {
+        let tmpdir = tmpdir();
+        let filename = tmpdir.join("file_rt_io_file_test_read_write_at.txt");
+        let mut buf = [0; 256];
+        let write1 = "asdf";
+        let write2 = "qwer-";
+        let write3 = "-zxcv";
+        let content = "qwer-asdf-zxcv";
+        {
+            let oo = OpenOptions::new().create_new(true).write(true).read(true).clone();
+            let mut rw = check!(oo.open(&filename));
+            assert_eq!(check!(rw.write_at(write1.as_bytes(), 5)), write1.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(rw.read_at(&mut buf, 5)), write1.len());
+            assert_eq!(str::from_utf8(&buf[..write1.len()]), Ok(write1));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(rw.write(write2.as_bytes())), write2.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 5);
+            assert_eq!(check!(rw.read(&mut buf)), write1.len());
+            assert_eq!(str::from_utf8(&buf[..write1.len()]), Ok(write1));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.read_at(&mut buf[..write2.len()], 0)), write2.len());
+            assert_eq!(str::from_utf8(&buf[..write2.len()]), Ok(write2));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.write_at(write3.as_bytes(), 9)), write3.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+        }
+        {
+            let mut read = check!(File::open(&filename));
+            assert_eq!(check!(read.read_at(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(read.seek(SeekFrom::End(-5))), 9);
+            assert_eq!(check!(read.read_at(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(read.read(&mut buf)), write3.len());
+            assert_eq!(str::from_utf8(&buf[..write3.len()]), Ok(write3));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.read_at(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+        }
+        check!(fs::remove_file(&filename));
+    }
+
     #[test]
     fn file_test_stat_is_correct_on_is_file() {
         let tmpdir = tmpdir();

src/libstd/sys/unix/fd.rs

@@ -13,7 +13,7 @@
 use prelude::v1::*;
 
 use io::{self, Read};
-use libc::{self, c_int, size_t, c_void};
+use libc::{self, c_int, off_t, size_t, c_void};
 use mem;
 use sync::atomic::{AtomicBool, Ordering};
 use sys::cvt;
@@ -42,7 +42,7 @@ impl FileDesc {
         let ret = cvt(unsafe {
             libc::read(self.fd,
                        buf.as_mut_ptr() as *mut c_void,
-                       buf.len() as size_t)
+                       buf.len())
         })?;
         Ok(ret as usize)
     }
@@ -52,6 +52,16 @@ impl FileDesc {
         (&mut me).read_to_end(buf)
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        let ret = cvt(unsafe {
+            libc::pread(self.fd,
+                        buf.as_mut_ptr() as *mut c_void,
+                        buf.len(),
+                        offset as off_t)
+        })?;
+        Ok(ret as usize)
+    }
+
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         let ret = cvt(unsafe {
             libc::write(self.fd,
@@ -61,6 +71,16 @@ impl FileDesc {
         Ok(ret as usize)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        let ret = cvt(unsafe {
+            libc::pwrite(self.fd,
+                         buf.as_ptr() as *const c_void,
+                         buf.len(),
+                         offset as off_t)
+        })?;
+        Ok(ret as usize)
+    }
+
     #[cfg(not(any(target_env = "newlib", target_os = "solaris", target_os = "emscripten")))]
     pub fn set_cloexec(&self) -> io::Result<()> {
         unsafe {

src/libstd/sys/unix/fs.rs

@@ -477,10 +477,18 @@ impl File {
         self.0.read_to_end(buf)
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.0.read_at(buf, offset)
+    }
+
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         self.0.write(buf)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.0.write_at(buf, offset)
+    }
+
     pub fn flush(&self) -> io::Result<()> { Ok(()) }
 
     pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {

src/libstd/sys/windows/fs.rs

@@ -312,6 +312,10 @@ impl File {
         self.handle.read(buf)
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.handle.read_at(buf, offset)
+    }
+
     pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
         self.handle.read_to_end(buf)
     }
@@ -320,6 +324,10 @@ impl File {
         self.handle.write(buf)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.handle.write_at(buf, offset)
+    }
+
     pub fn flush(&self) -> io::Result<()> { Ok(()) }
 
     pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {

src/libstd/sys/windows/handle.rs

@@ -106,6 +106,19 @@ impl RawHandle {
         }
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        let mut read = 0;
+        let len = cmp::min(buf.len(), <c::DWORD>::max_value() as usize) as c::DWORD;
+        unsafe {
+            let mut overlapped: c::OVERLAPPED = mem::zeroed();
+            overlapped.Offset = offset as u32;
+            overlapped.OffsetHigh = (offset >> 32) as u32;
+            cvt(c::ReadFile(self.0, buf.as_mut_ptr() as c::LPVOID,
+                            len, &mut read, &mut overlapped))?;
+        }
+        Ok(read as usize)
+    }
+
     pub unsafe fn read_overlapped(&self,
                                   buf: &mut [u8],
                                   overlapped: *mut c::OVERLAPPED)
@@ -176,6 +189,19 @@ impl RawHandle {
         Ok(amt as usize)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        let mut written = 0;
+        let len = cmp::min(buf.len(), <c::DWORD>::max_value() as usize) as c::DWORD;
+        unsafe {
+            let mut overlapped: c::OVERLAPPED = mem::zeroed();
+            overlapped.Offset = offset as u32;
+            overlapped.OffsetHigh = (offset >> 32) as u32;
+            cvt(c::WriteFile(self.0, buf.as_ptr() as c::LPVOID,
+                             len, &mut written, &mut overlapped))?;
+        }
+        Ok(written as usize)
+    }
+
     pub fn duplicate(&self, access: c::DWORD, inherit: bool,
                      options: c::DWORD) -> io::Result<Handle> {
         let mut ret = 0 as c::HANDLE;