xkbcomp: where a keysym cannot be resolved, set it to NoSymbol

Where resolve_keysym fails we warn but use the otherwise uninitialized variable
as our keysym. That later ends up in the keymap as random garbage hex value.

Simplest test case, set this in the 'us' keymap:
    key <TLDE>               {      [        xyz ] };

And without this patch we get random garbage:
./build/xkbcli-compile-keymap --layout us | grep TLDE:
    key <TLDE>               {      [      0x018a5cf0 ] };

With this patch, we now get NoSymbol:
./build/xkbcli-compile-keymap --layout us | grep TLDE:
    key <TLDE>               {      [        NoSymbol ] };

src/xkbcomp/parser.y

@@ -726,8 +726,10 @@ KeySyms         :       OBRACE KeySymList CBRACE
 
 KeySym          :       IDENT
                         {
-                            if (!resolve_keysym($1, &$$))
+                            if (!resolve_keysym($1, &$$)) {
                                 parser_warn(param, "unrecognized keysym \"%s\"", $1);
+                                $$ = XKB_KEY_NoSymbol;
+                            }
                             free($1);
                         }
                 |       SECTION { $$ = XKB_KEY_section; }

test/data/symbols/garbage

@@ -0,0 +1,14 @@
+default alphanumeric_keys
+xkb_symbols "garbage" {
+    include "us"
+
+    name[Group1]= "My garbage Layout";
+
+    // The garbage keysym will *not* override the corresponding symbol from the
+    // 'us' layout
+    key <TLDE> { [ keysym_is_garbage, exclam ] };
+
+    // AE13 is unused by 'us', use it to avoid fallback to the 'us' definition.
+    // Define with 2 levels but first level is a garbage symbol.
+    key <AE13> { [ keysym_is_garbage, asciitilde ] };
+};

test/keymap.c

@@ -31,8 +31,51 @@
 
 #include "test.h"
 
-int
-main(void)
+static void
+test_garbage_key(void)
+{
+    struct xkb_context *context = test_get_context(0);
+    struct xkb_keymap *keymap;
+    xkb_keycode_t kc;
+    int nsyms;
+    const xkb_keysym_t *syms;
+    const xkb_layout_index_t first_layout = 0;
+    xkb_level_index_t nlevels;
+
+    assert(context);
+
+    keymap = test_compile_rules(context, NULL, NULL, "garbage", NULL, NULL);
+    assert(keymap);
+
+    /* TLDE uses the 'us' sym on the first level and is thus [grave, exclam] */
+    kc = xkb_keymap_key_by_name(keymap, "TLDE");
+    assert(kc != XKB_KEYCODE_INVALID);
+    nlevels = xkb_keymap_num_levels_for_key(keymap, kc, first_layout);
+    assert(nlevels == 2);
+    nsyms = xkb_keymap_key_get_syms_by_level(keymap, kc, first_layout, 0, &syms);
+    assert(nsyms == 1);
+    assert(*syms == XKB_KEY_grave); /* fallback from 'us' */
+    nsyms = xkb_keymap_key_get_syms_by_level(keymap, kc, first_layout, 1, &syms);
+    assert(nsyms == 1);
+    assert(*syms == XKB_KEY_exclam);
+
+    /* AE13 has no 'us' fallback and ends up as [NoSymbol, asciitilde] */
+    kc = xkb_keymap_key_by_name(keymap, "AE13");
+    assert(kc != XKB_KEYCODE_INVALID);
+    nlevels = xkb_keymap_num_levels_for_key(keymap, kc, first_layout);
+    assert(nlevels == 2);
+    nsyms = xkb_keymap_key_get_syms_by_level(keymap, kc, first_layout, 0, &syms);
+    assert(nsyms == 0);
+    nsyms = xkb_keymap_key_get_syms_by_level(keymap, kc, first_layout, 1, &syms);
+    assert(nsyms == 1);
+    assert(*syms == XKB_KEY_asciitilde);
+
+    xkb_keymap_unref(keymap);
+    xkb_context_unref(context);
+}
+
+static void
+test_keymap(void)
 {
     struct xkb_context *context = test_get_context(0);
     struct xkb_keymap *keymap;
@@ -105,3 +148,12 @@ main(void)
     xkb_keymap_unref(keymap);
     xkb_context_unref(context);
 }
+
+int
+main(void)
+{
+    test_garbage_key();
+    test_keymap();
+
+    return 0;
+}