[gdk-pixbuf: 3/7] gif: Fix multiple LZW clear codes breaking decoding

[Emmanuele Bassi <ebassi src gnome org>]

commit b9b12bfd5c6f8394cc9a4a548bd6dc0f5fb2f70b
Author: Robert Ancell <robert ancell canonical com>
Date:   Tue Dec 4 16:51:36 2018 +1300

    gif: Fix multiple LZW clear codes breaking decoding
    
    The following was occurring:
    1. A clear code is detected in the LZW stream
    2. The first code is detected as a clear.
    3. The following code is returned as a color index, this breaks if it is a
       clear.
    
    There were two codepaths in use, one for handling the first clear in the LZW
    sequence and another for handling clears within the sequence. The former
    handled sequential clears correctly, the latter did not.
    
    The solution is to the correct codepath and remove the other one. This
    simplification should not affect other decoding (as confirmed by the test
    suite).

 gdk-pixbuf/io-gif.c                    | 40 +---------------------------------
 tests/test-images/gif-test-suite/TESTS |  4 ++--
 2 files changed, 3 insertions(+), 41 deletions(-)
---
diff --git a/gdk-pixbuf/io-gif.c b/gdk-pixbuf/io-gif.c
index 3986635bc..244d8788f 100644
--- a/gdk-pixbuf/io-gif.c
+++ b/gdk-pixbuf/io-gif.c
@@ -86,7 +86,6 @@ enum {
        GIF_GET_COLORMAP2,
        GIF_PREPARE_LZW,
        GIF_LZW_FILL_BUFFER,
-       GIF_LZW_CLEAR_CODE,
        GIF_GET_LZW,
        GIF_DONE
 };
@@ -162,7 +161,6 @@ struct _GifContext
        int code_lastbit;
        int code_done;
        int code_last_byte;
-       int lzw_code_pending;
 
        /* lzw context */
        gint lzw_fresh;
@@ -554,29 +552,6 @@ get_code (GifContext *context,
        return ret;
 }
 
-
-static void
-set_gif_lzw_clear_code (GifContext *context)
-{
-       context->state = GIF_LZW_CLEAR_CODE;
-       context->lzw_code_pending = -1;
-}
-
-static int
-gif_lzw_clear_code (GifContext *context)
-{
-       gint code;
-
-       code = get_code (context, context->lzw_code_size);
-       if (code == -3)
-               return -0;
-
-       context->lzw_firstcode = context->lzw_oldcode = code;
-       context->lzw_code_pending = code;
-       context->state = GIF_GET_LZW;
-       return 0;
-}
-
 #define CHECK_LZW_SP() G_STMT_START {                                           \
         if ((guchar *)context->lzw_sp >=                                        \
             (guchar *)context->lzw_stack + sizeof (context->lzw_stack)) {       \
@@ -596,12 +571,6 @@ lzw_read_byte (GifContext *context)
        gint my_retval;
        register int i;
 
-       if (context->lzw_code_pending != -1) {
-               retval = context->lzw_code_pending;
-               context->lzw_code_pending = -1;
-               return retval;
-       }
-
        if (context->lzw_fresh) {
                context->lzw_fresh = FALSE;
                do {
@@ -632,8 +601,7 @@ lzw_read_byte (GifContext *context)
                        context->lzw_max_code_size = 2 * context->lzw_clear_code;
                        context->lzw_max_code = context->lzw_clear_code + 2;
                        context->lzw_sp = context->lzw_stack;
-
-                       set_gif_lzw_clear_code (context);
+                       context->lzw_fresh = TRUE;
                        return -3;
                } else if (code == context->lzw_end_code) {
                        int count;
@@ -1136,7 +1104,6 @@ static void
 gif_set_prepare_lzw (GifContext *context)
 {
        context->state = GIF_PREPARE_LZW;
-       context->lzw_code_pending = -1;
 }
 static int
 gif_prepare_lzw (GifContext *context)
@@ -1444,11 +1411,6 @@ gif_main_loop (GifContext *context)
                        retval = gif_lzw_fill_buffer (context);
                        break;
 
-               case GIF_LZW_CLEAR_CODE:
-                        LOG("clear_code\n");
-                       retval = gif_lzw_clear_code (context);
-                       break;
-
                case GIF_GET_LZW:
                         LOG("get_lzw\n");
                        retval = gif_get_lzw (context);
diff --git a/tests/test-images/gif-test-suite/TESTS b/tests/test-images/gif-test-suite/TESTS
index cc986661e..1d4a3f13f 100644
--- a/tests/test-images/gif-test-suite/TESTS
+++ b/tests/test-images/gif-test-suite/TESTS
@@ -33,8 +33,8 @@ extra-data
 no-clear
 no-eoi
 no-clear-and-eoi
-#many-clears
-#double-clears
+many-clears
+double-clears
 invalid-colors
 max-width
 max-height