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    The error INVALID_OPERATION is generated by
    BeginTransformFeedbackEXT if any transform feedback buffer object
    binding point used in transform feedback mode does not have a
    buffer object bound.

This required adding a new NumBuffers field to the
gl_transform_feedback_info struct, to keep track of how many transform
feedback buffers are required by the current program.

Fixes Piglit tests:
- EXT_transform_feedback/api-errors interleaved_unbound
- EXT_transform_feedback/api-errors separate_unbound_0_1
- EXT_transform_feedback/api-errors separate_unbound_0_2
- EXT_transform_feedback/api-errors separate_unbound_1_2
---
 src/glsl/linker.cpp               |    7 +++++--
 src/mesa/main/mtypes.h            |    5 +++++
 src/mesa/main/transformfeedback.c |   12 ++++++++++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index ed9a5d7..43be205 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -1889,11 +1889,14 @@ store_tfeedback_info(struct gl_context *ctx, struct gl_shader_program *prog,
                      tfeedback_decl *tfeedback_decls)
 {
    unsigned total_tfeedback_components = 0;
+   bool separate_attribs_mode =
+      prog->TransformFeedback.BufferMode == GL_SEPARATE_ATTRIBS;
    memset(&prog->LinkedTransformFeedback, 0,
           sizeof(prog->LinkedTransformFeedback));
+   prog->LinkedTransformFeedback.NumBuffers =
+      separate_attribs_mode ? num_tfeedback_decls : 1;
    for (unsigned i = 0; i < num_tfeedback_decls; ++i) {
-      unsigned buffer =
-         prog->TransformFeedback.BufferMode == GL_SEPARATE_ATTRIBS ? i : 0;
+      unsigned buffer = separate_attribs_mode ? i : 0;
       if (!tfeedback_decls[i].store(prog, &prog->LinkedTransformFeedback,
                                     buffer))
          return false;
diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index d520f98..456131f 100644
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -1821,6 +1821,11 @@ struct gl_uniform_list;
 struct gl_transform_feedback_info {
    unsigned NumOutputs;
 
+   /**
+    * Number of transform feedback buffers in use by this program.
+    */
+   unsigned NumBuffers;
+
    struct {
       unsigned OutputRegister;
       unsigned OutputBuffer;
diff --git a/src/mesa/main/transformfeedback.c b/src/mesa/main/transformfeedback.c
index 305589d..6e93b3b 100644
--- a/src/mesa/main/transformfeedback.c
+++ b/src/mesa/main/transformfeedback.c
@@ -342,9 +342,12 @@ void GLAPIENTRY
 _mesa_BeginTransformFeedback(GLenum mode)
 {
    struct gl_transform_feedback_object *obj;
+   struct gl_transform_feedback_info *info;
+   int i;
    GET_CURRENT_CONTEXT(ctx);
 
    obj = ctx->TransformFeedback.CurrentObject;
+   info = &ctx->Shader.CurrentVertexProgram->LinkedTransformFeedback;
 
    switch (mode) {
    case GL_POINTS:
@@ -363,6 +366,15 @@ _mesa_BeginTransformFeedback(GLenum mode)
       return;
    }
 
+   for (i = 0; i < info->NumBuffers; ++i) {
+      if (obj->BufferNames[i] == 0) {
+         _mesa_error(ctx, GL_INVALID_OPERATION,
+                     "glBeginTransformFeedback(binding point %d does not have "
+                     "a buffer object bound)", i);
+         return;
+      }
+   }
+
    FLUSH_VERTICES(ctx, _NEW_TRANSFORM_FEEDBACK);
    obj->Active = GL_TRUE;
    ctx->TransformFeedback.Mode = mode;
-- 
1.7.6.4