feat: ":write ++p" creates parent dirs #20835

- `:write ++p foo/bar/baz.txt` should create parent directories `foo/bar/` if
   they do not exist
    - Note: `:foo ++…` is usually for options. No existing options have
      a single-char abbreviation (presumably by design), so it's safe to
      special-case `++p` here.
- Same for `writefile(…, 'foo/bar/baz.txt', 'p')`
- `BufWriteCmd` can see the ++p flag via `v:cmdarg`.

closes #19884

3 files changed, 40 insertions, 0 deletions

diff --git a/src/nvim/os/fileio.c b/src/nvim/os/fileio.c
index b1710737d0..280a9c2bee 100644
--- a/src/nvim/os/fileio.c
+++ b/src/nvim/os/fileio.c
@@ -71,6 +71,7 @@ int file_open(FileDescriptor *const ret_fp, const char *const fname, const int f
   FLAG(flags, kFileReadOnly, O_RDONLY, kFalse, wr != kTrue);
 #ifdef O_NOFOLLOW
   FLAG(flags, kFileNoSymlink, O_NOFOLLOW, kNone, true);
+  FLAG(flags, kFileMkDir, O_CREAT|O_WRONLY, kTrue, !(flags & kFileCreateOnly));
 #endif
 #undef FLAG
   // wr is used for kFileReadOnly flag, but on
@@ -78,6 +79,13 @@ int file_open(FileDescriptor *const ret_fp, const char *const fname, const int f
   // `error: variable ‘wr’ set but not used [-Werror=unused-but-set-variable]`
   (void)wr;
 
+  if (flags & kFileMkDir) {
+    int mkdir_ret = os_file_mkdir((char *)fname, 0755);
+    if (mkdir_ret < 0) {
+      return mkdir_ret;
+    }
+  }
+
   const int fd = os_open(fname, os_open_flags, mode);
 
   if (fd < 0) {
diff --git a/src/nvim/os/fileio.h b/src/nvim/os/fileio.h
index da23a54c4e..5e47bbf921 100644
--- a/src/nvim/os/fileio.h
+++ b/src/nvim/os/fileio.h
@@ -35,6 +35,7 @@ typedef enum {
                      ///< be used with kFileCreateOnly.
   kFileNonBlocking = 128,  ///< Do not restart read() or write() syscall if
                            ///< EAGAIN was encountered.
+  kFileMkDir = 256,
 } FileOpenFlags;
 
 static inline bool file_eof(const FileDescriptor *fp)
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 68e96eea6e..3c9578979e 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -946,6 +946,37 @@ int os_mkdir_recurse(const char *const dir, int32_t mode, char **const failed_di
   return 0;
 }
 
+/// Create the parent directory of a file if it does not exist
+///
+/// @param[in] fname Full path of the file name whose parent directories
+///                  we want to create
+/// @param[in] mode  Permissions for the newly-created directory.
+///
+/// @return `0` for success, libuv error code for failure.
+int os_file_mkdir(char *fname, int32_t mode)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  if (!dir_of_file_exists((char_u *)fname)) {
+    char *tail = path_tail_with_sep(fname);
+    char *last_char = tail + strlen(tail) - 1;
+    if (vim_ispathsep(*last_char)) {
+      emsg(_(e_noname));
+      return -1;
+    }
+    char c = *tail;
+    *tail = NUL;
+    int r;
+    char *failed_dir;
+    if ((r = os_mkdir_recurse(fname, mode, &failed_dir) < 0)) {
+      semsg(_(e_mkdir), failed_dir, os_strerror(r));
+      xfree(failed_dir);
+    }
+    *tail = c;
+    return r;
+  }
+  return 0;
+}
+
 /// Create a unique temporary directory.
 ///
 /// @param[in] template Template of the path to the directory with XXXXXX