glsl: Use a consistent technique for tracking link success/failure.

This patch changes link_shaders() so that it sets prog->LinkStatus to
true when it starts, and then relies on linker_error() to set it to
false if a link failure occurs.

Previously, link_shaders() would set prog->LinkStatus to true halfway
through its execution; as a result, linker functions that executed
during the first half of link_shaders() would have to do their own
success/failure tracking; if they didn't, then calling linker_error()
would add an error message to the log, but not cause the link to fail.
Since it wasn't always obvious from looking at a linker function
whether it was called before or after link_shaders() set
prog->LinkStatus to true, this carried a high risk of bugs.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/link_interface_blocks.cpp

@@ -31,7 +31,7 @@
 #include "linker.h"
 #include "main/macros.h"
 
-bool
+void
 validate_intrastage_interface_blocks(struct gl_shader_program *prog,
                                      const gl_shader **shader_list,
                                      unsigned num_shaders)
@@ -65,16 +65,15 @@ validate_intrastage_interface_blocks(struct gl_shader_program *prog,
          } else if (old_iface_type != iface_type) {
             linker_error(prog, "definitions of interface block `%s' do not"
                          " match\n", iface_type->name);
-            return false;
+            return;
          }
       }
    }
-
-   return true;
 }
 
-bool
-validate_interstage_interface_blocks(const gl_shader *producer,
+void
+validate_interstage_interface_blocks(struct gl_shader_program *prog,
+                                     const gl_shader *producer,
                                      const gl_shader *consumer)
 {
    glsl_symbol_table interfaces;
@@ -105,9 +104,9 @@ validate_interstage_interface_blocks(const gl_shader *producer,
       if (expected_type == NULL)
          continue;
 
-      if (var->interface_type != expected_type)
-         return false;
+      if (var->interface_type != expected_type) {
+         linker_error(prog, "interface block mismatch between shader stages\n");
+         return;
+      }
    }
-
-   return true;
 }