shaderapi: Fix AttachShader error

Detect a duplicate Shader type as and error instead of silently allowing
it, restrict to ES2 API.

v2: Tapani Pälli <tapani.palli@intel.com>
    - make the check run time instead of compile time

v3: chadv
    - Quote spec on which error to generate.

Signed-off-by: bma <Bo.Ma@windriver.com>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-and-tested-by: Chad Versace <chad.versace@linux.intel.com>

src/mesa/main/shaderapi.c

@@ -207,6 +207,8 @@ attach_shader(struct gl_context *ctx, GLuint program, GLuint shader)
    struct gl_shader *sh;
    GLuint i, n;
 
+   const bool same_type_disallowed = _mesa_is_gles(ctx);
+
    shProg = _mesa_lookup_shader_program_err(ctx, program, "glAttachShader");
    if (!shProg)
       return;
@@ -227,6 +229,18 @@ attach_shader(struct gl_context *ctx, GLuint program, GLuint shader)
           */
          _mesa_error(ctx, GL_INVALID_OPERATION, "glAttachShader");
          return;
+      } else if (same_type_disallowed &&
+                 shProg->Shaders[i]->Type == sh->Type) {
+        /* Shader with the same type is already attached to this program,
+         * OpenGL ES 2.0 and 3.0 specs say:
+         *
+         *      "Multiple shader objects of the same type may not be attached
+         *      to a single program object. [...] The error INVALID_OPERATION
+         *      is generated if [...] another shader object of the same type
+         *      as shader is already attached to program."
+         */
+         _mesa_error(ctx, GL_INVALID_OPERATION, "glAttachShader");
+         return;
       }
    }