Clean up some stale comments and restore libc usage in flog_safe

flog_safe should be explicitly async-signal-safe functions; let's avoid
nix in that module for this reason.

src/common.rs

@@ -1468,7 +1468,7 @@ fn can_be_encoded(wc: char) -> bool {
 }
 
 /// Call read, blocking and repeating on EINTR. Exits on EAGAIN.
-/// \return the number of bytes read, or 0 on EOF. On EAGAIN, returns -1 if nothing was read.
+/// \return the number of bytes read, or 0 on EOF, or an error.
 pub fn read_blocked(fd: RawFd, buf: &mut [u8]) -> nix::Result<usize> {
     loop {
         let res = nix::unistd::read(fd, buf);

src/fork_exec/flog_safe.rs

@@ -84,7 +84,9 @@ pub fn flog_impl_async_safe(fd: i32, s: impl FloggableDisplayAsyncSafe) {
     let bytes: &[u8] = s.to_flog_str_async_safe(&mut storage);
     // Note we deliberately do not retry on signals, etc.
     // This is used to report error messages after fork() in the child process.
-    let _ = nix::unistd::write(fd, bytes);
+    unsafe {
+        let _ = libc::write(fd, bytes.as_ptr() as *const libc::c_void, bytes.len());
+    }
 }
 
 /// Variant of FLOG which is async-safe to use after fork().

src/history/file.rs

@@ -228,7 +228,6 @@ fn should_mmap() -> bool {
 }
 
 /// Read from `fd` to fill `dest`, zeroing any unused space.
-// Return true on success, false on failure.
 fn read_from_fd(fd: RawFd, mut dest: &mut [u8]) -> nix::Result<()> {
     while !dest.is_empty() {
         match nix::unistd::read(fd, dest) {

src/reader.rs

@@ -684,7 +684,7 @@ fn read_ni(parser: &Parser, fd: RawFd, io: &IoChain) -> i32 {
         return 1;
     }
 
-    // Read all data into a std::string.
+    // Read all data into a vec.
     let mut fd_contents = Vec::with_capacity(usize::try_from(buf.st_size).unwrap());
     loop {
         let mut buff = [0_u8; 4096];