main: Temporary fix assertion error

This variant uses `fdopen()` which is not standard, but it fixes problem on my 
system. In next commit `scriptin` will use `FileDescriptor*` from os/fileio in 
place of `FILE*`.

src/nvim/main.c

@@ -1057,7 +1057,9 @@ scripterror:
               mch_exit(2);
             }
             if (STRCMP(argv[0], "-") == 0) {
-              scriptin[0] = stdin;
+              const int stdin_dup_fd = os_dup(STDIN_FILENO);
+              FILE *const stdin_dup = fdopen(stdin_dup_fd, "r");
+              scriptin[0] = stdin_dup;
             } else if ((scriptin[0] = mch_fopen(argv[0], READBIN)) == NULL) {
               mch_errmsg(_("Cannot open for reading: \""));
               mch_errmsg(argv[0]);

src/nvim/os/fs.c

@@ -430,6 +430,29 @@ int os_close(const int fd)
   return r;
 }
 
+/// Duplicate file descriptor
+///
+/// @param[in]  fd  File descriptor to duplicate.
+///
+/// @return New file descriptor or libuv error code (< 0).
+int os_dup(const int fd)
+  FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  int ret;
+os_dup_dup:
+  ret = dup(fd);
+  if (ret < 0) {
+    const int error = os_translate_sys_error(errno);
+    errno = 0;
+    if (error == UV_EINTR) {
+      goto os_dup_dup;
+    } else {
+      return error;
+    }
+  }
+  return ret;
+}
+
 /// Read from a file
 ///
 /// Handles EINTR and ENOMEM, but not other errors.

test/functional/core/main_spec.lua

@@ -1,7 +1,9 @@
 local lfs = require('lfs')
 local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
 
 local eq = helpers.eq
+local feed = helpers.feed
 local clear = helpers.clear
 local funcs = helpers.funcs
 local nvim_prog = helpers.nvim_prog
@@ -53,5 +55,51 @@ describe('Command-line option', function()
       local attrs = lfs.attributes(fname)
       eq(#('100500\n'), attrs.size)
     end)
+    it('does not crash after reading from stdin in non-headless mode', function()
+      local screen = Screen.new(40, 8)
+      screen:attach()
+      eq(nil, lfs.attributes(fname))
+      funcs.termopen({
+        nvim_prog_abs(), '-u', 'NONE', '-i', 'NONE',
+         '--cmd', 'set noswapfile shortmess+=IFW fileformats=unix',
+         '-s', '-'
+      })
+      screen:expect([[
+        ^                                        |
+        {1:~                                       }|
+        {1:~                                       }|
+        {1:~                                       }|
+        {1:~                                       }|
+        {2:[No Name]             0,0-1          All}|
+                                                |
+                                                |
+      ]], {
+        [1] = {foreground = 4210943, special = Screen.colors.Grey0},
+        [2] = {special = Screen.colors.Grey0, bold = true, reverse = true}
+      })
+      feed('i:cq<CR><C-\\><C-n>')
+      screen:expect([[
+        ^                                        |
+        [Process exited 1]                      |
+                                                |
+                                                |
+                                                |
+                                                |
+                                                |
+                                                |
+      ]])
+      --[=[ Example of incorrect output:
+      screen:expect([[
+        ^nvim: /var/tmp/portage/dev-libs/libuv-1.|
+        10.2/work/libuv-1.10.2/src/unix/core.c:5|
+        19: uv__close: Assertion `fd > STDERR_FI|
+        LENO' failed.                           |
+                                                |
+        [Process exited 6]                      |
+                                                |
+                                                |
+      ]])
+      ]=]
+    end)
   end)
 end)

test/unit/os/fs_spec.lua

@@ -483,6 +483,22 @@ describe('fs function', function()
       end)
     end)
 
+    describe('os_dup', function()
+      itp('returns new file descriptor', function()
+        local dup0 = fs.os_dup(0)
+        local dup1 = fs.os_dup(1)
+        local dup2 = fs.os_dup(2)
+        local tbl = {[0]=true, [1]=true, [2]=true,
+                     [tonumber(dup0)]=true, [tonumber(dup1)]=true,
+                     [tonumber(dup2)]=true}
+        local i = 0
+        for _, _ in pairs(tbl) do
+          i = i + 1
+        end
+        eq(i, 6)  -- All fds must be unique
+      end)
+    end)
+
     describe('os_open', function()
       local new_file = 'test_new_file'
       local existing_file = 'unit-test-directory/test_existing.file'