3 files changed, 30 insertions, 12 deletions
diff --git a/library/std/src/sys/unix/kernel_copy.rs b/library/std/src/sys/unix/kernel_copy.rs
index 200dbf06ff8..9687576bb6a 100644
--- a/library/std/src/sys/unix/kernel_copy.rs
+++ b/library/std/src/sys/unix/kernel_copy.rs
@@ -61,6 +61,7 @@ use crate::process::{ChildStderr, ChildStdin, ChildStdout};
 use crate::ptr;
 use crate::sync::atomic::{AtomicBool, AtomicU8, Ordering};
 use crate::sys::cvt;
+use libc::{EBADF, EINVAL, ENOSYS, EOPNOTSUPP, EOVERFLOW, EPERM, EXDEV};
 
 #[cfg(test)]
 mod tests;
@@ -535,7 +536,7 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
                 cvt(copy_file_range(INVALID_FD, ptr::null_mut(), INVALID_FD, ptr::null_mut(), 1, 0))
             };
 
-            if matches!(result.map_err(|e| e.raw_os_error()), Err(Some(libc::EBADF))) {
+            if matches!(result.map_err(|e| e.raw_os_error()), Err(Some(EBADF))) {
                 HAS_COPY_FILE_RANGE.store(AVAILABLE, Ordering::Relaxed);
             } else {
                 HAS_COPY_FILE_RANGE.store(UNAVAILABLE, Ordering::Relaxed);
@@ -573,19 +574,20 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
             Err(err) => {
                 return match err.raw_os_error() {
                     // when file offset + max_length > u64::MAX
-                    Some(libc::EOVERFLOW) => CopyResult::Fallback(written),
-                    Some(
-                        libc::ENOSYS | libc::EXDEV | libc::EINVAL | libc::EPERM | libc::EOPNOTSUPP,
-                    ) => {
+                    Some(EOVERFLOW) => CopyResult::Fallback(written),
+                    Some(ENOSYS | EXDEV | EINVAL | EPERM | EOPNOTSUPP | EBADF) => {
                         // Try fallback io::copy if either:
                         // - Kernel version is < 4.5 (ENOSYS¹)
                         // - Files are mounted on different fs (EXDEV)
                         // - copy_file_range is broken in various ways on RHEL/CentOS 7 (EOPNOTSUPP)
                         // - copy_file_range file is immutable or syscall is blocked by seccomp¹ (EPERM)
                         // - copy_file_range cannot be used with pipes or device nodes (EINVAL)
+                        // - the writer fd was opened with O_APPEND (EBADF²)
                         //
                         // ¹ these cases should be detected by the initial probe but we handle them here
                         //   anyway in case syscall interception changes during runtime
+                        // ² actually invalid file descriptors would cause this too, but in that case
+                        //   the fallback code path is expected to encounter the same error again
                         assert_eq!(written, 0);
                         CopyResult::Fallback(0)
                     }
@@ -649,7 +651,7 @@ fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) ->
             Ok(ret) => written += ret as u64,
             Err(err) => {
                 return match err.raw_os_error() {
-                    Some(libc::ENOSYS | libc::EPERM) => {
+                    Some(ENOSYS | EPERM) => {
                         // syscall not supported (ENOSYS)
                         // syscall is disallowed, e.g. by seccomp (EPERM)
                         match mode {
@@ -659,12 +661,12 @@ fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) ->
                         assert_eq!(written, 0);
                         CopyResult::Fallback(0)
                     }
-                    Some(libc::EINVAL) => {
+                    Some(EINVAL) => {
                         // splice/sendfile do not support this particular file descriptor (EINVAL)
                         assert_eq!(written, 0);
                         CopyResult::Fallback(0)
                     }
-                    Some(os_err) if mode == SpliceMode::Sendfile && os_err == libc::EOVERFLOW => {
+                    Some(os_err) if mode == SpliceMode::Sendfile && os_err == EOVERFLOW => {
                         CopyResult::Fallback(written)
                     }
                     _ => CopyResult::Error(err, written),
diff --git a/library/std/src/sys/unix/kernel_copy/tests.rs b/library/std/src/sys/unix/kernel_copy/tests.rs
index 77369cdd35f..3fe849e23e2 100644
--- a/library/std/src/sys/unix/kernel_copy/tests.rs
+++ b/library/std/src/sys/unix/kernel_copy/tests.rs
@@ -65,6 +65,24 @@ fn copy_specialization() -> Result<()> {
     result.and(rm1).and(rm2)
 }
 
+#[test]
+fn copies_append_mode_sink() -> Result<()> {
+    let tmp_path = tmpdir();
+    let source_path = tmp_path.join("copies_append_mode.source");
+    let sink_path = tmp_path.join("copies_append_mode.sink");
+    let mut source =
+        OpenOptions::new().create(true).truncate(true).write(true).read(true).open(&source_path)?;
+    write!(source, "not empty")?;
+    source.seek(SeekFrom::Start(0))?;
+    let mut sink = OpenOptions::new().create(true).append(true).open(&sink_path)?;
+
+    let copied = crate::io::copy(&mut source, &mut sink)?;
+
+    assert_eq!(copied, 9);
+
+    Ok(())
+}
+
 #[bench]
 fn bench_file_to_file_copy(b: &mut test::Bencher) {
     const BYTES: usize = 128 * 1024;
diff --git a/library/std/src/sys/wasi/fs.rs b/library/std/src/sys/wasi/fs.rs
index 4134ef67671..a119e66d93c 100644
--- a/library/std/src/sys/wasi/fs.rs
+++ b/library/std/src/sys/wasi/fs.rs
@@ -654,13 +654,11 @@ fn open_parent(p: &Path) -> io::Result<(ManuallyDrop<WasiFd>, PathBuf)> {
                 );
                 return Err(io::Error::new(io::ErrorKind::Other, msg));
             }
-            let len = CStr::from_ptr(buf.as_ptr().cast()).to_bytes().len();
-            buf.set_len(len);
-            buf.shrink_to_fit();
+            let relative = CStr::from_ptr(relative_path).to_bytes().to_vec();
 
             return Ok((
                 ManuallyDrop::new(WasiFd::from_raw(fd as u32)),
-                PathBuf::from(OsString::from_vec(buf)),
+                PathBuf::from(OsString::from_vec(relative)),
             ));
         }
     }