WebGL compatibility: add stencil mask and ref restriction

BUG=angleproject:1523
BUG=chromium:668223

Change-Id: I0726769c938fdfd50af0fad1cef1746d4af2a589
Reviewed-on: https://chromium-review.googlesource.com/422084
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/tests/gl_tests/WebGLCompatibilityTest.cpp b/src/tests/gl_tests/WebGLCompatibilityTest.cpp
index 8464111..86f4954 100644
--- a/src/tests/gl_tests/WebGLCompatibilityTest.cpp
+++ b/src/tests/gl_tests/WebGLCompatibilityTest.cpp
@@ -212,6 +212,66 @@
     EXPECT_GL_ERROR(GL_INVALID_OPERATION);
 }
 
+// Tests the WebGL requirement of having the same stencil mask, writemask and ref for fron and back
+TEST_P(WebGLCompatibilityTest, RequiresSameStencilMaskAndRef)
+{
+    // Run the test in an FBO to make sure we have some stencil bits.
+    GLRenderbuffer renderbuffer;
+    glBindRenderbuffer(GL_RENDERBUFFER, renderbuffer.get());
+    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH24_STENCIL8, 32, 32);
+
+    GLFramebuffer framebuffer;
+    glBindFramebuffer(GL_FRAMEBUFFER, framebuffer.get());
+    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER,
+                              renderbuffer.get());
+
+    ANGLE_GL_PROGRAM(program, "void main() { gl_Position = vec4(0, 0, 0, 1); }",
+                     "void main() { gl_FragColor = vec4(0, 1, 0, 1); }")
+    glUseProgram(program.get());
+    ASSERT_GL_NO_ERROR();
+
+    // Having ref and mask the same for front and back is valid.
+    glStencilMask(255);
+    glStencilFunc(GL_ALWAYS, 0, 255);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    ASSERT_GL_NO_ERROR();
+
+    // Having a different front - back write mask generates an error.
+    glStencilMaskSeparate(GL_FRONT, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    EXPECT_GL_ERROR(GL_INVALID_OPERATION);
+
+    // Setting both write masks separately to the same value is valid.
+    glStencilMaskSeparate(GL_BACK, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    ASSERT_GL_NO_ERROR();
+
+    // Having a different stencil front - back mask generates an error
+    glStencilFuncSeparate(GL_FRONT, GL_ALWAYS, 0, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    EXPECT_GL_ERROR(GL_INVALID_OPERATION);
+
+    // Setting both masks separately to the same value is valid.
+    glStencilFuncSeparate(GL_BACK, GL_ALWAYS, 0, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    ASSERT_GL_NO_ERROR();
+
+    // Having a different stencil front - back reference generates an error
+    glStencilFuncSeparate(GL_FRONT, GL_ALWAYS, 255, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    EXPECT_GL_ERROR(GL_INVALID_OPERATION);
+
+    // Setting both references separately to the same value is valid.
+    glStencilFuncSeparate(GL_BACK, GL_ALWAYS, 255, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    ASSERT_GL_NO_ERROR();
+
+    // Using different stencil funcs, everything being equal is valid.
+    glStencilFuncSeparate(GL_BACK, GL_NEVER, 255, 1);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    ASSERT_GL_NO_ERROR();
+}
+
 // Use this to select which configurations (e.g. which renderer, which GLES major version) these
 // tests should be run against.
 ANGLE_INSTANTIATE_TEST(WebGLCompatibilityTest,