proposal: os: add Root.ReadFile & Root.WriteFile

[neild]

I propose adding ReadFile and WriteFile methods to *os.Root:

package os

// ReadFile reads the named file in the root and returns the contents.
// See [ReadFile] for more details.
func (r *Root) ReadFile(name string) ([]byte, error)

// WriteFile writes data to the named file in the root, creating it if necessary.
// See [WriteFile] for more details.
func (r *Root) WriteFile(name string, data []byte, perm FileMode) error

These are purely convenience functions (as are os.ReadFile and os.WriteFile). I propose adding them because:

1. os.ReadFile and os.WriteFile are very convenient convenience functions, and it's a shame not to have an equivalent that works with os.Root.
2. The io/fs package not only includes a ReadFile convenience function, it includes a ReadFileFS variant of FS containing a ReadFile method. I think that points at this being something we consider core filesystem functionality.

[gabyhelp]

Related Issues

• proposal: os: add FS() (filesystem simply using os functions) #47803 (closed)
• proposal: os: extend Root FS with interface io/fs.SubFS #72054

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[irsl]

Could we have an os level shortcut methods as well (similar to os.OpenInRoot)? By having that, one wouldn't need to bother with closing the root for one-time operations.

Without that, the following oneliner:

data, err:= os.ReadFile(filepath.Join(dir, file))

Would be around 6:

r, err:= os.OpenRoot(dir)
if err != nil {
   log.Fatalf(...)
}
defer r.Close()
data, err:= r.ReadFile(file)