[Mesa-dev] [PATCH] Revert "mesa: remove unnecessary 'sort by year' for the GL extensions"

[Emil Velikov]

This reverts commit 3d81e11b49366b5636b8524ba0f8c7076e3fdf34.

As reported by Federico, some games require the 'sort by year' since
they truncate the extensions which do not fit the fixed size string
array.

Seemingly I did not consider that as the documentation (both Mesa and
Nvidia) mention about program crashes ... which are worked around by
setting the env. variable.

This commit reinstates the workaround and enhances the documentation.

Cc: Federico Dossena <info at fdossena.com>
Cc: Timothy Arceri <tarceri at itsqueeze.com>
Cc: Marek Olšák <maraeo at gmail.com>
Cc: Roland Scheidegger <sroland at vmware.com>
Cc: Ian Romanick <idr at freedesktop.org>
Reported-by: Federico Dossena <info at fdossena.com>
Fixes: 3d81e11b493 ("mesa: remove unnecessary 'sort by year' for the GL
extensions")
---
UNTESTED: Resolved the revert conflicts, but I haven't done any actual
testing. If Federico can do the honours that'll be amazing.

 src/mesa/main/extensions.c | 46 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/src/mesa/main/extensions.c b/src/mesa/main/extensions.c
index 25c3161f7d0..4d95a072793 100644
--- a/src/mesa/main/extensions.c
+++ b/src/mesa/main/extensions.c
@@ -335,6 +335,30 @@ _mesa_extension_supported(const struct gl_context *ctx, extension_index i)
    return (ctx->Version >= ext->version[ctx->API]) && base[ext->offset];
 }
 
+/**
+ * Compare two entries of the extensions table.  Sorts first by year,
+ * then by name.
+ *
+ * Arguments are indices into _mesa_extension_table.
+ */
+static int
+extension_compare(const void *p1, const void *p2)
+{
+   extension_index i1 = * (const extension_index *) p1;
+   extension_index i2 = * (const extension_index *) p2;
+   const struct mesa_extension *e1 = &_mesa_extension_table[i1];
+   const struct mesa_extension *e2 = &_mesa_extension_table[i2];
+   int res;
+
+   res = (int)e1->year - (int)e2->year;
+
+   if (res == 0) {
+      res = strcmp(e1->name, e2->name);
+   }
+
+   return res;
+}
+
 
 /**
  * Construct the GL_EXTENSIONS string.  Called the first time that
@@ -372,8 +396,8 @@ _mesa_make_extension_string(struct gl_context *ctx)
 
       if (i->year <= maxYear &&
           _mesa_extension_supported(ctx, k)) {
-         length += strlen(i->name) + 1; /* +1 for space */
-         extension_indices[count++] = k;
+	 length += strlen(i->name) + 1; /* +1 for space */
+	 ++count;
       }
    }
    for (k = 0; k < MAX_UNRECOGNIZED_EXTENSIONS; k++)
@@ -385,6 +409,24 @@ _mesa_make_extension_string(struct gl_context *ctx)
       return NULL;
    }
 
+   /* Sort extensions in chronological order because idTech 2/3 games
+    * (e.g., Quake3 demo) store the extension list in a fixed size buffer.
+    * Some cases truncate, while others overflow the buffer. Resulting in
+    * misrendering and crashes, respectively.
+    * Address the former here, while the latter will be addressed by setting
+    * the MESA_EXTENSION_MAX_YEAR environment variable.
+    */
+   j = 0;
+   for (k = 0; k < MESA_EXTENSION_COUNT; ++k) {
+      if (_mesa_extension_table[k].year <= maxYear &&
+         _mesa_extension_supported(ctx, k)) {
+         extension_indices[j++] = k;
+      }
+   }
+   assert(j == count);
+   qsort(extension_indices, count,
+         sizeof *extension_indices, extension_compare);
+
    /* Build the extension string.*/
    for (j = 0; j < count; ++j) {
       const struct mesa_extension *i = &_mesa_extension_table[extension_indices[j]];
-- 
2.19.0