mesa: add env var to force enable the KHR_no_error ctx flag

V2: typo know -> known
V3: add security check (Suggested by Nicolai)

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

docs/envvars.html

@@ -46,6 +46,9 @@ sometimes be useful for debugging end-user issues.
 <li>MESA_NO_MMX - if set, disables Intel MMX optimizations
 <li>MESA_NO_3DNOW - if set, disables AMD 3DNow! optimizations
 <li>MESA_NO_SSE - if set, disables Intel SSE optimizations
+<li>MESA_NO_ERROR - if set error checking is disabled as per KHR_no_error.
+   This will result in undefined behaviour for invalid use of the api, but
+   can reduce CPU use for apps that are known to be error free.</li>
 <li>MESA_DEBUG - if set, error messages are printed to stderr.  For example,
    if the application generates a GL_INVALID_ENUM error, a corresponding error
    message indicating where the error occurred, and possibly why, will be

src/mesa/main/context.c

@@ -1208,6 +1208,12 @@ _mesa_initialize_context(struct gl_context *ctx,
    if (!init_attrib_groups( ctx ))
       goto fail;
 
+   /* KHR_no_error is likely to crash, overflow memory, etc if an application
+    * has errors so don't enable it for setuid processes.
+    */
+   if (getenv("MESA_NO_ERROR") && geteuid() == getuid())
+      ctx->Const.ContextFlags |= GL_CONTEXT_FLAG_NO_ERROR_BIT_KHR;
+
    /* setup the API dispatch tables with all nop functions */
    ctx->OutsideBeginEnd = _mesa_alloc_dispatch_table();
    if (!ctx->OutsideBeginEnd)