refactor(os): deduplicate console/fd boilerplate #40509

Problem:
Scattered, redundant boilerplate for Windows console/fd plumbing.

Solution:
- Add os_dup_cloexec()
- Add os_reattach_console_stdio().
- Use os_open_conin_fd().

src/nvim/channel.c

@@ -556,10 +556,8 @@ uint64_t channel_from_stdio(bool rpc, CallbackReader on_output, const char **err
   // Strangely, ConPTY doesn't work if stdin and stdout are pipes. So replace
   // stdin and stdout with CONIN$ and CONOUT$, respectively.
   if (embedded_mode && os_has_conpty_working()) {
-    stdin_dup_fd = os_dup(STDIN_FILENO);
-    os_set_cloexec(stdin_dup_fd);
-    stdout_dup_fd = os_dup(STDOUT_FILENO);
-    os_set_cloexec(stdout_dup_fd);
+    stdin_dup_fd = os_dup_cloexec(STDIN_FILENO);
+    stdout_dup_fd = os_dup_cloexec(STDOUT_FILENO);
     // :restart spawns a replacement server that must not borrow the parent
     // Nvim process console, because that parent process will soon exit.
     const bool restart_alloc_console = os_env_exists(ENV_RESTART_ALLOC_CONSOLE, true);
@@ -577,9 +575,7 @@ uint64_t channel_from_stdio(bool rpc, CallbackReader on_output, const char **err
         ShowWindow(GetConsoleWindow(), SW_HIDE);
       }
     }
-    os_enable_ctrl_c();
-    os_replace_stdin_to_conin();
-    os_replace_stdout_and_stderr_to_conout();
+    os_reattach_console_stdio();
   }
 #else
   if (embedded_mode) {

src/nvim/fileio.c

@@ -84,6 +84,10 @@
 # include <sys/file.h>
 #endif
 
+#ifdef MSWIN
+# include "nvim/os/os_win_console.h"
+#endif
+
 #ifdef OPEN_CHR_FILES
 # include "nvim/charset.h"
 #endif
@@ -1696,11 +1700,8 @@ failed:
       // On Unix, use stderr for stdin, makes shell commands work.
       vim_ignored = dup(2);
 #else
-      // On Windows, use the console input handle for stdin.
-      HANDLE conin = CreateFile("CONIN$", GENERIC_READ | GENERIC_WRITE,
-                                FILE_SHARE_READ, (LPSECURITY_ATTRIBUTES)NULL,
-                                OPEN_EXISTING, 0, (HANDLE)NULL);
-      vim_ignored = _open_osfhandle((intptr_t)conin, _O_RDONLY);
+      // On Windows, use the console input handle (CONIN$) for stdin.
+      vim_ignored = os_open_conin_fd();
 #endif
     }
   }

src/nvim/main.c

@@ -303,10 +303,7 @@ int main(int argc, char **argv)
 #ifdef MSWIN
   int startup_stderr_fd = -1;
   if (embedded_mode) {
-    startup_stderr_fd = os_dup(STDERR_FILENO);
-    if (startup_stderr_fd >= 0) {
-      os_set_cloexec(startup_stderr_fd);
-    }
+    startup_stderr_fd = os_dup_cloexec(STDERR_FILENO);
   }
 #endif
 

src/nvim/os/fs.c

@@ -554,6 +554,20 @@ os_dup_dup:
   return ret;
 }
 
+/// Duplicates file descriptor `fd` and marks the copy close-on-exec (Unix) / non-inheritable
+/// (Windows), so the duped fd is not leaked to spawned child processes.
+///
+/// @return New file descriptor, or libuv error code (< 0) if the dup failed.
+int os_dup_cloexec(const int fd)
+  FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  int newfd = os_dup(fd);
+  if (newfd >= 0) {
+    os_set_cloexec(newfd);
+  }
+  return newfd;
+}
+
 /// Open the file descriptor for stdin.
 int os_open_stdin_fd(void)
 {
@@ -564,7 +578,7 @@ int os_open_stdin_fd(void)
     stdin_dup_fd = os_dup(STDIN_FILENO);
 #ifdef MSWIN
     // Replace the original stdin with the console input handle.
-    os_replace_stdin_to_conin();
+    os_redirect_stdin_to_conin();
 #endif
   }
   return stdin_dup_fd;

src/nvim/os/os_win_console.c

@@ -42,14 +42,14 @@ void os_clear_hwnd(void)
   hWnd = NULL;
 }
 
-void os_replace_stdin_to_conin(void)
+void os_redirect_stdin_to_conin(void)
 {
   close(STDIN_FILENO);
   const int conin_fd = os_open_conin_fd();
   assert(conin_fd == STDIN_FILENO);
 }
 
-void os_replace_stdout_and_stderr_to_conout(void)
+void os_redirect_stdout_stderr_to_conout(void)
 {
   const HANDLE conout_handle =
     CreateFile("CONOUT$",
@@ -66,6 +66,16 @@ void os_replace_stdout_and_stderr_to_conout(void)
   assert(conerr_fd == STDERR_FILENO);
 }
 
+/// Points this process's stdio at the current console: enables Ctrl-C handling and replaces
+/// stdin/stdout/stderr with CONIN$/CONOUT$. The caller must have already acquired a console
+/// (via AttachConsole, AllocConsole, etc.).
+void os_reattach_console_stdio(void)
+{
+  os_enable_ctrl_c();
+  os_redirect_stdin_to_conin();
+  os_redirect_stdout_stderr_to_conout();
+}
+
 /// Detach from the current console and switch stdio to a hidden private one.
 ///
 /// Used when an embedded server must outlive its parent console, while keeping
@@ -75,9 +85,7 @@ void os_swap_to_hidden_console(void)
   FreeConsole();
   AllocConsole();
   ShowWindow(GetConsoleWindow(), SW_HIDE);
-  os_enable_ctrl_c();
-  os_replace_stdin_to_conin();
-  os_replace_stdout_and_stderr_to_conout();
+  os_reattach_console_stdio();
 }
 
 /// Resets Windows console icon if we got an original one on startup.