system('foo &', 'bar'): Show error, don't crash.

Closes #3529
Closes #5241

In Vim,
    :echo system('cat - &', 'foo')
works because for both system() and :! Vim writes input to a temp file and uses
shell syntax to redirect the file to the backgrounded `cat` (get_cmd_output()
.. make_filter_cmd()).

In Nvim,
    :echo system('cat - &', 'foo')
fails because we write the input directly via pipes (shell.c:do_os_system()),
but (per POSIX[1]) backgrounded process input stream is redirected from
/dev/null (unless overridden by shell redirection; supported only by some shells
[2]), so our writes are ignored, the process exits quickly, and if we are
writing data larger than the buffer size we'll see EPIPE.

This still works:
    :%w !tee > foo1358.txt &
but this does not:
    :%w !tee foo1358.txt &
though it *should* (why doesn't it?) because we still do the temp file dance
in do_bang() .. do_filter().

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_03_02
[2] http://unix.stackexchange.com/a/71218

runtime/doc/eval.txt

@@ -6838,23 +6838,30 @@ system({cmd} [, {input}])				*system()* *E677*
 		Get the output of the shell command {cmd} as a |string|.  {cmd}
 		will be run the same as in |jobstart()|.  See |systemlist()|
 		to get the output as a |List|.
+		Not to be used for interactive commands.
 
-		When {input} is given and is a string this string is written 
-		to a file and passed as stdin to the command.  The string is 
-		written as-is, you need to take care of using the correct line 
-		separators yourself.
-		If {input} is given and is a |List| it is written to the file
-		in a way |writefile()| does with {binary} set to "b" (i.e.
-		with a newline between each list item with newlines inside
-		list items converted to NULs).  
-		Pipes are not used.
+		If {input} is a string it is written to a pipe and passed as
+		stdin to the command.  The string is written as-is, line
+		separators are not changed.
+		If {input} is a |List| it is written to the pipe as
+		|writefile()| does with {binary} set to "b" (i.e. with
+		a newline between each list item, and newlines inside list
+		items converted to NULs).
+								*E5677*
+		Note: system() cannot write to or read from backgrounded ("&")
+		shell commands, e.g.: >
+		    :echo system("cat - &", "foo"))
+<		which is equivalent to: >
+		    $ echo foo | bash -c 'cat - &'
+<		The pipes are disconnected (unless overridden by shell
+		redirection syntax) before input can reach it. Use
+		|jobstart()| instead.
 
 		Note: Use |shellescape()| or |::S| with |expand()| or 
 		|fnamemodify()| to escape special characters in a command 
 		argument.  Newlines in {cmd} may cause the command to fail.  
 		The characters in 'shellquote' and 'shellxquote' may also 
 		cause trouble.
-		This is not to be used for interactive commands.
 
 		The result is a String.  Example: >
 		    :let files = system("ls " .  shellescape(expand('%:h')))
@@ -6869,9 +6876,6 @@ system({cmd} [, {input}])				*system()* *E677*
 		The command executed is constructed using several options when
 		{cmd} is a string: 'shell' 'shellcmdflag' {cmd}
 
-		The command will be executed in "cooked" mode, so that a
-		CTRL-C will interrupt the command (on Unix at least).
-
 		The resulting error code can be found in |v:shell_error|.
 		This function will fail in |restricted-mode|.
 

runtime/doc/vim_diff.txt

@@ -141,10 +141,10 @@ are always available and may be used simultaneously in separate plugins.  The
 `neovim` pip package must be installed to use Python plugins in Nvim (see
 |provider-python|).
 
-|:!| and |system()| do not support "interactive" commands; use |:terminal| for
-that instead. Terminal Vim supports interactive |:!| and |system()|, but gui
-Vim does not. See ":help gui-pty" in Vim:
-   http://vimdoc.sourceforge.net/htmldoc/gui_x11.html#gui-pty
+|:!| does not support "interactive" commands. Use |:terminal| instead.
+(GUI Vim has a similar limitation, see ":help gui-pty" in Vim.)
+
+|system()| does not support writing/reading "backgrounded" commands. |E5677|
 
 |mkdir()| behaviour changed:
 1. Assuming /tmp/foo does not exist and /tmp can be written to

src/nvim/event/rstream.c

@@ -112,8 +112,8 @@ static void read_cb(uv_stream_t *uvstream, ssize_t cnt, const uv_buf_t *buf)
         // to `alloc_cb` will return the same unused pointer(`rbuffer_produced`
         // won't be called)
         && cnt != 0) {
-      DLOG("Closing Stream(%p) because of %s(%zd)", stream,
-           uv_strerror((int)cnt), cnt);
+      DLOG("Closing Stream (%p): %s (%s)", stream,
+           uv_err_name((int)cnt), os_strerror((int)cnt));
       // Read error or EOF, either way stop the stream and invoke the callback
       // with eof == true
       uv_read_stop(uvstream);

src/nvim/os/shell.c

@@ -545,6 +545,16 @@ static size_t write_output(char *output, size_t remaining, bool to_buffer,
 
 static void shell_write_cb(Stream *stream, void *data, int status)
 {
+  if (status) {
+    // Can happen if system() tries to send input to a shell command that was
+    // backgrounded (:call system("cat - &", "foo")). #3529 #5241
+    EMSG2(_("E5677: Error writing input to shell-command: %s"),
+          uv_err_name(status));
+  }
+  if (stream->closed) {  // Process may have exited before this write.
+    ELOG("stream was already closed");
+    return;
+  }
   stream_close(stream, NULL, NULL);
 }