mesa: new MESA_EXTENSION_MAX_YEAR env var

If set to year X, only report extensions up to that year.  This is a
work-around for games that try to copy the extensions string to a fixed
size buffer and overflow.  If a game was released in year X, setting
MESA_EXTENSION_MAX_YEAR to that year will likely fix the problem.

docs/envvars.html

@@ -49,6 +49,15 @@ Setting this variable automatically sets the MESA_TEX_PROG variable as well.
 <li>MESA_EXTENSION_OVERRIDE - can be used to enable/disable extensions.
 A value such as "GL_EXT_foo -GL_EXT_bar" will enable the GL_EXT_foo extension
 and disable the GL_EXT_bar extension.
+<li>MESA_EXTENSION_MAX_YEAR - The GL_EXTENSIONS string returned by Mesa is sorted
+by extension year.
+If this variable is set to year X, only extensions defined on or before year
+X will be reported.
+This is to work-around a bug in some games where the extension string is
+copied into a fixed-size buffer without truncating.
+If the extension string is too long, the buffer overrun can cause the game
+to crash.
+This is a work-around for that.
 <li>MESA_GLSL - <a href="shading.html#envvars">shading language compiler options</a>
 </ul>
 

src/mesa/main/extensions.c

@@ -875,12 +875,24 @@ _mesa_make_extension_string(struct gl_context *ctx)
    GLboolean *base = (GLboolean *) &ctx->Extensions;
    const struct extension *i;
    unsigned j;
+   unsigned maxYear = ~0;
 
+   /* Check if the MESA_EXTENSION_MAX_YEAR env var is set */
+   {
+      const char *env = getenv("MESA_EXTENSION_MAX_YEAR");
+      if (env) {
+         maxYear = atoi(env);
+         _mesa_debug(ctx, "Note: limiting GL extensions to %u or earlier\n",
+                     maxYear);
+      }
+   }
 
    /* Compute length of the extension string. */
    count = 0;
    for (i = extension_table; i->name != 0; ++i) {
-      if (base[i->offset] && (i->api_set & (1 << ctx->API))) {
+      if (base[i->offset] &&
+          i->year <= maxYear &&
+          (i->api_set & (1 << ctx->API))) {
 	 length += strlen(i->name) + 1; /* +1 for space */
 	 ++count;
       }
@@ -908,7 +920,9 @@ _mesa_make_extension_string(struct gl_context *ctx)
     */
    j = 0;
    for (i = extension_table; i->name != 0; ++i) {
-      if (base[i->offset] && (i->api_set & (1 << ctx->API))) {
+      if (base[i->offset] &&
+          i->year <= maxYear &&
+          (i->api_set & (1 << ctx->API))) {
          extension_indices[j++] = i - extension_table;
       }
    }