bug#20153: 24.4.91; destructive add-face-text-property and string deep copying

[Oleh Krehel <ohwoeowho@gmail.com>]

Here's an example of what I'm trying to do:

(setq test-str-1
      #(";; This `is' a test"
        0 3 (fontified nil face font-lock-comment-delimiter-face)
        3 9 (fontified nil face font-lock-comment-face)
        9 11 (fontified nil face (font-lock-constant-face font-lock-comment-face))
        11 19 (fontified nil face font-lock-comment-face)))
(setq test-str-2 (concat test-str-1))
(add-face-text-property 0 (length test-str-2) 'foobar t test-str-2)

I would like to modify `test-str-2' without touching `test-str-1'.
For that, I either need `add-face-text-property' to be non-destructive,
either w.r.t. the string or at least w.r.t. the properties of the
string, or a way to deep-copy a string.

Here's the result of evaling the code above:

test-str-2
;; =>
#(";; This `is' a test" 0 3 (fontified nil face (font-lock-comment-delimiter-face foobar))
  3 9 (fontified nil face (font-lock-comment-face foobar))
  9 11 (fontified nil face (font-lock-constant-face font-lock-comment-face foobar))
  11 19 (fontified nil face (font-lock-comment-face foobar)))
test-str-1
;; =>
#(";; This `is' a test" 0 3 (face font-lock-comment-delimiter-face fontified nil)
  3 9 (face font-lock-comment-face fontified nil)
  9 11 (face (font-lock-constant-face font-lock-comment-face foobar) ; <= foobar is here
        fontified nil)
  11 19 (face font-lock-comment-face fontified nil))

As you see, `test-str-1' was modified.

I'd like to know a straightforward solution to the problem itself, I'm
fine with either way of implementation.

Oleh