fix(shell): ceci n'est pas une pipe

On linux /dev/stdin is defined as a symlink to /proc/self/fd/0
This in turn is defined as a "magic" symlink which is allowed to point
to internal kernel objects which really does not have a file
name. As a glaring inconsistency, fopen("/proc/self/fd/0", "r")
works if fd was originally opened using pipe() but not using
socketpair(). As it happens UV_CREATE_PIPE does not create pipes
but creates socket pairs. These two unfortunate conditions
means that using /dev/stdin and similar does not work in
shell commands in nvim on linux. as a work around, override
libuv's descicion and create an actual pipe pair.

This change is not needed on BSD:s but done unconditionally for simplicity,
except for on windows where it is not done for stdout because of windows

fixes #35984

runtime/doc/news.txt

@@ -426,6 +426,11 @@ These existing features changed their behavior.
 - Windows: Paths like "\Windows" and "/Windows" are now considered to be
   absolute paths (to the current drive) and no longer relative.
 
+• When 'shelltemp' is off, shell commands now use `pipe()` and not `socketpair()`
+  for input and output. This matters mostly for Linux where some command lines
+  using "/dev/stdin" and similiar would break as these special files can be
+  reopened when backed by pipes but not when backed by socket pairs.
+
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*
 

src/nvim/event/libuv_proc.c

@@ -60,27 +60,58 @@ int libuv_proc_spawn(LibuvProc *uvproc)
     uvproc->uvopts.env = NULL;
   }
 
+  int to_close[3] = { -1, -1, -1 };
+
   if (!proc->in.closed) {
-    uvproc->uvstdio[0].flags = UV_CREATE_PIPE | UV_READABLE_PIPE;
+    uv_file pipe_pair[2];
+    int client_flags = 0;
 #ifdef MSWIN
-    uvproc->uvstdio[0].flags |= proc->overlapped ? UV_OVERLAPPED_PIPE : 0;
+    client_flags |= proc->overlapped ? UV_NONBLOCK_PIPE : 0;
 #endif
-    uvproc->uvstdio[0].data.stream = (uv_stream_t *)(&proc->in.uv.pipe);
+
+    // As of libuv 1.51, UV_CREATE_PIPE can only create pipes
+    // using socketpair(), not pipe(). We want the latter on linux
+    // as socket pairs behave different in some confusing ways, like
+    // breaking /proc/0/fd/0 which is disowned by the linux socket maintainer.
+    uv_pipe(pipe_pair, client_flags, UV_NONBLOCK_PIPE);
+
+    uvproc->uvstdio[0].flags = UV_INHERIT_FD;
+    uvproc->uvstdio[0].data.fd = pipe_pair[0];
+    to_close[0] = pipe_pair[0];
+
+    uv_pipe_open(&proc->in.uv.pipe, pipe_pair[1]);
   }
 
   if (!proc->out.s.closed) {
-    uvproc->uvstdio[1].flags = UV_CREATE_PIPE | UV_WRITABLE_PIPE;
 #ifdef MSWIN
+    // TODO(bfredl): in theory it would have been nice if the uv_pipe() branch
+    // also worked for windows but IOCP happens because of reasons.
+    uvproc->uvstdio[1].flags = UV_CREATE_PIPE | UV_WRITABLE_PIPE;
     // pipe must be readable for IOCP to work on Windows.
     uvproc->uvstdio[1].flags |= proc->overlapped
                                 ? (UV_READABLE_PIPE | UV_OVERLAPPED_PIPE) : 0;
-#endif
     uvproc->uvstdio[1].data.stream = (uv_stream_t *)(&proc->out.s.uv.pipe);
+#else
+    uv_file pipe_pair[2];
+    uv_pipe(pipe_pair, UV_NONBLOCK_PIPE, 0);
+
+    uvproc->uvstdio[1].flags = UV_INHERIT_FD;
+    uvproc->uvstdio[1].data.fd = pipe_pair[1];
+    to_close[1] = pipe_pair[1];
+
+    uv_pipe_open(&proc->out.s.uv.pipe, pipe_pair[0]);
+#endif
   }
 
   if (!proc->err.s.closed) {
-    uvproc->uvstdio[2].flags = UV_CREATE_PIPE | UV_WRITABLE_PIPE;
-    uvproc->uvstdio[2].data.stream = (uv_stream_t *)(&proc->err.s.uv.pipe);
+    uv_file pipe_pair[2];
+    uv_pipe(pipe_pair, UV_NONBLOCK_PIPE, 0);
+
+    uvproc->uvstdio[2].flags = UV_INHERIT_FD;
+    uvproc->uvstdio[2].data.fd = pipe_pair[1];
+    to_close[2] = pipe_pair[1];
+
+    uv_pipe_open(&proc->err.s.uv.pipe, pipe_pair[0]);
   } else if (proc->fwd_err) {
     uvproc->uvstdio[2].flags = UV_INHERIT_FD;
     uvproc->uvstdio[2].data.fd = STDERR_FILENO;
@@ -92,10 +123,16 @@ int libuv_proc_spawn(LibuvProc *uvproc)
     if (uvproc->uvopts.env) {
       os_free_fullenv(uvproc->uvopts.env);
     }
-    return status;
+    goto exit;
   }
 
   proc->pid = uvproc->uv.pid;
+exit:
+  for (int i = 0; i < 3; i++) {
+    if (to_close[i] > -1) {
+      close(to_close[i]);
+    }
+  }
   return status;
 }
 

test/client/uv_stream.lua

@@ -150,9 +150,10 @@ ProcStream.__index = ProcStream
 --- @param env string[]?
 --- @param io_extra uv.uv_pipe_t?
 --- @param on_exit fun(closed: integer?)?  Called after the child process exits.
+--- @param forward_stderr forward stderr from the nvim process. otherwise collect it.
 --- `closed` is the timestamp (uv.now()) when close() was called, or nil if it wasn't.
 --- @return test.ProcStream
-function ProcStream.spawn(argv, env, io_extra, on_exit)
+function ProcStream.spawn(argv, env, io_extra, on_exit, forward_stderr)
   local self = setmetatable({
     collect_text = false,
     output = function(self)
@@ -176,9 +177,10 @@ function ProcStream.spawn(argv, env, io_extra, on_exit)
   for i = 2, #argv do
     args[#args + 1] = argv[i]
   end
+  local stderr = forward_stderr and 1 or self._child_stderr
   --- @diagnostic disable-next-line:missing-fields
   self._proc, self._pid = uv.spawn(prog, {
-    stdio = { self._child_stdin, self._child_stdout, self._child_stderr, io_extra },
+    stdio = { self._child_stdin, self._child_stdout, stderr, io_extra },
     args = args,
     --- @diagnostic disable-next-line:assign-type-mismatch
     env = env,

test/functional/core/job_spec.lua

@@ -1339,6 +1339,30 @@ describe('jobs', function()
       ]])
     end
   end)
+
+  it('uses real pipes for stdin/stdout #35984', function()
+    if skip(is_os('win'), 'Not applicable for Windows') then
+      return
+    end
+    -- this fails on linux if we used socketpair() for stdin and stdout,
+    -- which libuv does if you ask to create stdio streams for you
+    local val = exec_lua(function()
+      local output
+      local job = vim.fn.jobstart('wc /dev/stdin > /dev/stdout', {
+        stdout_buffered = true,
+        on_stdout = function(_, data, _)
+          output = data
+        end,
+      })
+      vim.fn.chansend(job, 'foo\nbar baz\n')
+      vim.fn.chanclose(job, 'stdin')
+      vim.fn.jobwait({ job })
+      return output
+    end)
+    eq(2, #val, val)
+    eq({ '2', '3', '12', '/dev/stdin' }, vim.split(val[1], '%s+', { trimempty = true }))
+    eq('', val[2])
+  end)
 end)
 
 describe('pty process teardown', function()

test/functional/testnvim.lua

@@ -515,7 +515,7 @@ function M.new_session(keep, ...)
       )
       io.stdout:flush()
     end
-  end)
+  end, true)
   n_processes = n_processes + 1
 
   local new_session = Session.new(proc)

test/functional/vimscript/system_spec.lua

@@ -196,10 +196,10 @@ describe('system()', function()
       n.set_shell_powershell()
       eq('ああ\n', eval([[system('Write-Output "ああ"')]]))
       -- Sanity test w/ default encoding
-      -- * on Windows, expected to default to Western European enc
+      -- * on Windows, UTF-8 still works.
       -- * on Linux, expected to default to UTF8
       command([[let &shellcmdflag = '-NoLogo -NoProfile -ExecutionPolicy RemoteSigned -Command ']])
-      eq(is_os('win') and '??\n' or 'ああ\n', eval([[system('Write-Output "ああ"')]]))
+      eq('ああ\n', eval([[system('Write-Output "ああ"')]]))
     end)
 
     it('`echo` and waits for its return', function()
@@ -548,10 +548,10 @@ describe('systemlist()', function()
     n.set_shell_powershell()
     eq({ is_os('win') and 'あ\r' or 'あ' }, eval([[systemlist('Write-Output あ')]]))
     -- Sanity test w/ default encoding
-    -- * on Windows, expected to default to Western European enc
+    -- * on Windows, UTF-8 still works.
     -- * on Linux, expected to default to UTF8
     command([[let &shellcmdflag = '-NoLogo -NoProfile -ExecutionPolicy RemoteSigned -Command ']])
-    eq({ is_os('win') and '?\r' or 'あ' }, eval([[systemlist('Write-Output あ')]]))
+    eq({ is_os('win') and 'あ\r' or 'あ' }, eval([[systemlist('Write-Output あ')]]))
   end)
 end)