main/extensions: Add early extension override structures

During the early one_time_init phase of context creation, we
initialize two global gl_extensions structures.

We read the MESA_EXTENSION_OVERRIDE environment variable, and store
positive and negative overrides in two structures:
* struct gl_extensions _mesa_extension_override_enables
* struct gl_extensions _mesa_extension_override_disables

These are filled before the driver initializes extensions and
constants, therefore the driver can make adjustments based on the
desired overrides.

This can be useful during development of a new extension where the
extension is only partially ready. The driver can't actually advertise
support for the extension, but if it sees that the override is set for
the extension, then it can expose more supported parts of the
extension, such as upgrading context constants.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/main/context.c

@@ -396,6 +396,8 @@ one_time_init( struct gl_context *ctx )
       assert( sizeof(GLint) == 4 );
       assert( sizeof(GLuint) == 4 );
 
+      _mesa_one_time_init_extension_overrides();
+
       _mesa_get_cpu_features();
 
       for (i = 0; i < 256; i++) {

src/mesa/main/extensions.c

@@ -37,6 +37,9 @@
 #include "macros.h"
 #include "mtypes.h"
 
+struct gl_extensions _mesa_extension_override_enables;
+struct gl_extensions _mesa_extension_override_disables;
+
 enum {
    DISABLE = 0,
    GLL = 1 << API_OPENGL_COMPAT,       /* GL Legacy / Compatibility */
@@ -587,6 +590,54 @@ get_extension_override( struct gl_context *ctx )
 }
 
 
+/**
+ * \brief Initialize extension override tables.
+ *
+ * This should be called one time early during first context initialization.
+ */
+void
+_mesa_one_time_init_extension_overrides(void)
+{
+   const char *env_const = _mesa_getenv("MESA_EXTENSION_OVERRIDE");
+   char *env;
+   char *ext;
+   size_t offset;
+
+   memset(&_mesa_extension_override_enables, 0, sizeof(struct gl_extensions));
+   memset(&_mesa_extension_override_disables, 0, sizeof(struct gl_extensions));
+
+   if (env_const == NULL) {
+      return;
+   }
+
+   /* Copy env_const because strtok() is destructive. */
+   env = strdup(env_const);
+   for (ext = strtok(env, " "); ext != NULL; ext = strtok(NULL, " ")) {
+      int enable;
+      switch (ext[0]) {
+      case '+':
+         enable = 1;
+         ++ext;
+         break;
+      case '-':
+         enable = 0;
+         ++ext;
+         break;
+      default:
+         enable = 1;
+         break;
+      }
+
+      offset = set_extension(&_mesa_extension_override_enables, ext, enable);
+      if (offset != 0 && (offset != o(dummy_true) || enable != GL_FALSE)) {
+         ((GLboolean *) &_mesa_extension_override_disables)[offset] = !enable;
+      }
+   }
+
+   free(env);
+}
+
+
 /**
  * \brief Initialize extension tables and enable default extensions.
  *

src/mesa/main/extensions.h

@@ -39,9 +39,12 @@
 #include "glheader.h"
 
 struct gl_context;
+struct gl_extensions;
 
 extern void _mesa_enable_sw_extensions(struct gl_context *ctx);
 
+extern void _mesa_one_time_init_extension_overrides(void);
+
 extern void _mesa_init_extensions(struct gl_context *ctx);
 
 extern GLubyte *_mesa_make_extension_string(struct gl_context *ctx);
@@ -52,4 +55,7 @@ _mesa_get_extension_count(struct gl_context *ctx);
 extern const GLubyte *
 _mesa_get_enabled_extension(struct gl_context *ctx, GLuint index);
 
+extern struct gl_extensions _mesa_extension_override_enables;
+extern struct gl_extensions _mesa_extension_override_disables;
+
 #endif