input: set input stream to blocking on exit

If stdin is non-blocking, many tools (e.g. cat(1), read(1)) which assume
that stdin is blocking, will break in odd ways:

  read: read error: 0: Resource temporarily unavailable
  cat: -: Resource temporarily unavailable
  rm: error closing file

libuv puts stdin in nonblocking mode, and leaves it that way at exit
(this is apparently by design). So, before this commit, this always
works (because the shell clobbers O_NONBLOCK):

  $ nvim --cmd q
  $ read

...but these forms do _not_ work:

  $ nvim --cmd q && read
  $ echo foo | nvim --cmd q && read
  $ nvim && read

After this commit, all of the above forms work.

Background:

437b4397b9 (diff-41f4d294430cd8c36538999d62681ae2)
https://github.com/fish-shell/fish-shell/issues/176#issuecomment-15800155

- bash (and other shells: zsh, tcsh, fish), upon returning to the
  foreground, always sets fd 0 back to blocking mode. This practice only
  applies to stdin, _not_ stdout or stderr (in practice these fds may be
  affected anyways).
- bash/zsh/tcsh/fish do _not_ restore the non-blocking status of stdin
  when _resuming a job_.
- We do _not_ save/restore the original flags visible to
  fcntl(F_[SG]ETFL), because (counterintuitively) that isn't expected.

Helped-by: oni-link <knil.ino@gmail.com>

Closes #2086
Closes #2377

---

Note: The following implementation of stream_set_blocking() was
discarded, because it resulted in a failed libuv assertion[1]:

  int stream_set_blocking(int fd, bool blocking)
  {
    uv_pipe_t stream;
    uv_pipe_init(uv_default_loop(), &stream, 0);
    uv_pipe_open(&stream, fd);
    int retval = uv_stream_set_blocking((uv_stream_t *)&stream, blocking);
    uv_close((uv_handle_t *)&stream, NULL);
    return retval;
  }

[1] .deps/build/src/libuv/src/unix/core.c:833: uv__io_stop: Assertion `loop->watchers[w->fd] == w' failed.

src/nvim/globals.h

@@ -1217,6 +1217,9 @@ EXTERN char *ignoredp;
 // If a msgpack-rpc channel should be started over stdin/stdout
 EXTERN bool embedded_mode INIT(= false);
 
+/// TTY from which input was gathered.
+EXTERN int global_input_fd INIT(= 0);
+
 /// Used to track the status of external functions.
 /// Currently only used for iconv().
 typedef enum {

src/nvim/if_cscope.c

@@ -820,6 +820,7 @@ err_closing:
     if (execl("/bin/sh", "sh", "-c", cmd, (char *)NULL) == -1)
       PERROR(_("cs_create_connection exec failed"));
 
+    stream_set_blocking(global_input_fd, true);  //normalize stream (#2598)
     exit(127);
   /* NOTREACHED */
   default:      /* parent. */

src/nvim/main.c

@@ -271,17 +271,15 @@ int main(int argc, char **argv)
       || params.output_isatty || params.err_isatty);
 
   if (reading_input) {
-    // Its possible that one of the startup commands(arguments, sourced scripts
-    // or plugins) will prompt the user, so start reading from a tty stream
-    // now. 
-    int fd = fileno(stdin);
+    // One of the startup commands (arguments, sourced scripts or plugins) may
+    // prompt the user, so start reading from a tty now.
+    global_input_fd = fileno(stdin);
     if (!params.input_isatty || params.edit_type == EDIT_STDIN) {
-      // use stderr or stdout since stdin is not a tty and/or could be used to
-      // read the file we'll edit when the "-" argument is given(eg: cat file |
-      // nvim -)
-      fd = params.err_isatty ? fileno(stderr) : fileno(stdout);
+      // Use stderr or stdout since stdin is not a tty and/or could be used to
+      // read the "-" file (eg: cat file | nvim -)
+      global_input_fd = params.err_isatty ? fileno(stderr) : fileno(stdout);
     }
-    input_start_stdin(fd);
+    input_start_stdin(global_input_fd);
   }
 
   // open terminals when opening files that start with term://

src/nvim/misc1.c

@@ -2699,6 +2699,7 @@ void preserve_exit(void)
   }
 
   ml_close_all(false);              // close all memfiles, without deleting
+  stream_set_blocking(global_input_fd, true);  //normalize stream (#2598)
 
   mch_errmsg("Vim: Finished.\n");
 

src/nvim/os/input.c

@@ -8,6 +8,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/os/input.h"
 #include "nvim/os/event.h"
+#include "nvim/os/os.h"
 #include "nvim/os/rstream_defs.h"
 #include "nvim/os/rstream.h"
 #include "nvim/ascii.h"
@@ -61,9 +62,14 @@ void input_start_stdin(int fd)
 void input_stop_stdin(void)
 {
   if (!read_stream) {
+    // In some cases (i.e. "nvim && read") where we do not explicitly play with
+    // std{in,out,err}, some other module/library nevertheless sets the stream
+    // to non-blocking; we still must "fix" the stream (#2598) in those cases.
+    stream_set_blocking(global_input_fd, true);  // normalize stream (#2598)
     return;
   }
 
+  rstream_set_blocking(read_stream, true);  // normalize stream (#2598)
   rstream_stop(read_stream);
   rstream_free(read_stream);
   read_stream = NULL;

src/nvim/os/os.h

@@ -12,6 +12,7 @@
 # include "os/mem.h.generated.h"
 # include "os/env.h.generated.h"
 # include "os/users.h.generated.h"
+# include "os/stream.h.generated.h"
 #endif
 
 #endif  // NVIM_OS_OS_H

src/nvim/os/rstream.c

@@ -297,6 +297,11 @@ void rstream_stop(RStream *rstream)
   }
 }
 
+int rstream_set_blocking(RStream *rstream, bool blocking)
+{
+  return uv_stream_set_blocking(rstream->stream, blocking);
+}
+
 /// Returns the number of bytes ready for consumption in `rstream`
 size_t rstream_pending(RStream *rstream)
 {

src/nvim/os/stream.c

@@ -0,0 +1,26 @@
+// Functions for working with stdio streams (as opposed to RStream/WStream).
+
+#include <stdio.h>
+#include <stdbool.h>
+
+#include <uv.h>
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/stream.c.generated.h"
+#endif
+
+/// Sets the stream associated with `fd` to "blocking" mode.
+///
+/// @return `0` on success, or `-errno` on failure.
+int stream_set_blocking(int fd, bool blocking)
+{
+  int flags = fcntl(fd, F_GETFL, 0);
+  int err = 0;
+  if (!blocking && !(flags & O_NONBLOCK)) {
+    err = fcntl(fd, F_SETFL, flags | O_NONBLOCK);
+  } else if (blocking && (flags & O_NONBLOCK)) {
+    err = fcntl(fd, F_SETFL, flags & ~O_NONBLOCK);
+  }
+  return err == -1 ? -errno : 0;
+}
+