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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/nvim/os/shell.c b/src/nvim/os/shell.c
index e9a3dcbff8..18ee008d66 100644
--- a/src/nvim/os/shell.c
+++ b/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);
 }