feat(api): support handling stdin stream in remote ui

src/nvim/api/vim.c

@@ -2661,3 +2661,35 @@ end:
   xfree(cmdline);
   return result;
 }
+
+/// Invokes the nvim server to read from stdin when it is not a tty
+///
+/// It enables functionalities like:
+/// - echo "1f u c4n r34d th1s u r34lly n33d t0 g37 r357"| nvim -
+/// - cat path/to/a/file | nvim -
+/// It has to be called before |nvim_ui_attach()| is called in order
+/// to ensure proper functioning.
+///
+/// @param channel_id: The channel id of the GUI-client
+/// @param filedesc: The file descriptor of the GUI-client process' stdin
+/// @param implicit: Tells if read_stdin call is implicit.
+///                  i.e for cases like `echo xxx | nvim`
+/// @param[out] err Error details, if any
+void nvim_read_stdin(uint64_t channel_id, Integer filedesc, Error *err)
+FUNC_API_SINCE(6) FUNC_API_REMOTE_ONLY
+{
+  if (starting != NO_SCREEN) {
+    api_set_error(err, kErrorTypeValidation,
+                  "nvim_read_stdin must be called before nvim_ui_attach");
+    return;
+  }
+  if (filedesc < 0) {
+    api_set_error(err, kErrorTypeValidation,
+                  "file descriptor must be non-negative");
+    return;
+  }
+
+  stdin_filedesc = (int)filedesc;
+  implicit_readstdin = implicit;
+  return;
+}

src/nvim/globals.h

@@ -704,6 +704,7 @@ EXTERN int RedrawingDisabled INIT(= 0);
 
 EXTERN int readonlymode INIT(= false);      // Set to true for "view"
 EXTERN int recoverymode INIT(= false);      // Set to true for "-r" option
+EXTERN int stdin_filedesc INIT(= -1);       // stdin filedesc set by embedder
 
 // typeahead buffer
 EXTERN typebuf_T typebuf INIT(= { NULL, NULL, 0, 0, 0, 0, 0, 0, 0 });
@@ -847,6 +848,9 @@ EXTERN linenr_T printer_page_num;
 EXTERN bool typebuf_was_filled INIT(= false);     // received text from client
                                                   // or from feedkeys()
 
+EXTERN bool implicit_readstdin INIT(= false);     // Used in embed job created
+                                                  // by TUI process only in
+                                                  // builtin tui
 
 #ifdef BACKSLASH_IN_FILENAME
 EXTERN char psepc INIT(= '\\');            // normal path separator character

src/nvim/main.c

@@ -454,7 +454,7 @@ int main(int argc, char **argv)
   // writing end of the pipe doesn't like, e.g., in case stdin and stderr
   // are the same terminal: "cat | vim -".
   // Using autocommands here may cause trouble...
-  if (params.edit_type == EDIT_STDIN && !recoverymode) {
+  if ((params.edit_type == EDIT_STDIN || implicit_readstdin) && !recoverymode) {
     read_stdin();
   }