std: Clamp max read/write sizes on Unix

Turns out that even though all these functions take a `size_t` they don't
actually work that well with anything larger than the maximum value of
`ssize_t`, the return value. Furthermore it looks like OSX rejects any
read/write requests larger than `INT_MAX - 1`. Handle all these cases by just
clamping the maximum size of a read/write on Unix to a platform-specific value.

Closes #38590

1 files changed, 2 insertions, 7 deletions

diff --git a/src/libstd/sys/windows/handle.rs b/src/libstd/sys/windows/handle.rs
index 10b86ba44bc..fdb9483fe1c 100644
--- a/src/libstd/sys/windows/handle.rs
+++ b/src/libstd/sys/windows/handle.rs
@@ -19,7 +19,6 @@ use ptr;
 use sys::c;
 use sys::cvt;
 use sys_common::io::read_to_end_uninitialized;
-use u32;
 
 /// An owned container for `HANDLE` object, closing them on Drop.
 ///
@@ -83,9 +82,7 @@ impl RawHandle {
 
     pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
         let mut read = 0;
-        // ReadFile takes a DWORD (u32) for the length so it only supports
-        // reading u32::MAX bytes at a time.
-        let len = cmp::min(buf.len(), u32::MAX as usize) as c::DWORD;
+        let len = cmp::min(buf.len(), <c::DWORD>::max_value() as usize) as c::DWORD;
         let res = cvt(unsafe {
             c::ReadFile(self.0, buf.as_mut_ptr() as c::LPVOID,
                         len, &mut read, ptr::null_mut())
@@ -181,9 +178,7 @@ impl RawHandle {
 
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         let mut amt = 0;
-        // WriteFile takes a DWORD (u32) for the length so it only supports
-        // writing u32::MAX bytes at a time.
-        let len = cmp::min(buf.len(), u32::MAX as usize) as c::DWORD;
+        let len = cmp::min(buf.len(), <c::DWORD>::max_value() as usize) as c::DWORD;
         cvt(unsafe {
             c::WriteFile(self.0, buf.as_ptr() as c::LPVOID,
                          len, &mut amt, ptr::null_mut())