Report an error if sk_LastFragColor is referenced without fbFetchSupport

Adjusted default caps in skslc to be consistent with runtime behavior,
and added optional settings mode to enable the feature. Tests for both
scenarios. (The error test crashed prior to the fix).

Bug: oss-fuzz:38726
Change-Id: I5270d4837ac982085d7baf5abd4b361f7bfb8562
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449062
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

gn/sksl_tests.gni

@@ -69,6 +69,7 @@ sksl_error_tests = [
   "/sksl/errors/InvalidOutParams.sksl",
   "/sksl/errors/InvalidToken.sksl",
   "/sksl/errors/InvalidUnary.sksl",
+  "/sksl/errors/LastFragColorWithoutCaps.sksl",
   "/sksl/errors/LayoutInFunctions.sksl",
   "/sksl/errors/LayoutRepeatedQualifiers.sksl",
   "/sksl/errors/MatrixToVectorCast3x3.sksl",
@@ -161,6 +162,7 @@ sksl_error_tests = [
 sksl_glsl_tests = [
   "/sksl/glsl/ForceHighPrecision.sksl",
   "/sksl/glsl/IncompleteShortIntPrecision.sksl",
+  "/sksl/glsl/LastFragColor.sksl",
   "/sksl/glsl/LayoutQualifiers.sksl",
   "/sksl/glsl/ShortIntPrecision.sksl",
   "/sksl/glsl/TextureSharpenVersion110.sksl",

resources/sksl/errors/LastFragColorWithoutCaps.sksl

@@ -0,0 +1,3 @@
+void main() {
+    sk_FragColor = sk_LastFragColor;
+}

resources/sksl/glsl/LastFragColor.sksl

@@ -0,0 +1,5 @@
+/*#pragma settings FramebufferFetchSupport*/
+
+void main() {
+    sk_FragColor = sk_LastFragColor;
+}

src/sksl/SkSLMain.cpp

@@ -139,6 +139,10 @@ static bool detect_shader_settings(const SkSL::String& text,
                     static auto s_emulateAbsIntCaps = Factory::EmulateAbsIntFunction();
                     *caps = s_emulateAbsIntCaps.get();
                 }
+                if (settingsText.consumeSuffix(" FramebufferFetchSupport")) {
+                    static auto s_fbFetchSupport = Factory::FramebufferFetchSupport();
+                    *caps = s_fbFetchSupport.get();
+                }
                 if (settingsText.consumeSuffix(" IncompleteShortIntPrecision")) {
                     static auto s_incompleteShortIntCaps = Factory::IncompleteShortIntPrecision();
                     *caps = s_incompleteShortIntCaps.get();

src/sksl/SkSLUtil.h

@@ -89,7 +89,7 @@ public:
         return fGLSLGeneration > k110_GrGLSLGeneration;
     }
 
-    bool fFBFetchSupport = true;
+    bool fFBFetchSupport = false;
     bool fbFetchSupport() const {
         return fFBFetchSupport;
     }
@@ -346,6 +346,13 @@ public:
         return result;
     }
 
+    static ShaderCapsPointer FramebufferFetchSupport() {
+        ShaderCapsPointer result = MakeShaderCaps();
+        result->fFBFetchSupport = true;
+        result->fFBFetchColorName = "gl_LastFragData[0]";
+        return result;
+    }
+
     static ShaderCapsPointer IncompleteShortIntPrecision() {
         ShaderCapsPointer result = MakeShaderCaps();
         result->fVersionDeclString = "#version 310es";

src/sksl/codegen/SkSLGLSLCodeGenerator.cpp

@@ -777,7 +777,12 @@ void GLSLCodeGenerator::writeVariableReference(const VariableReference& ref) {
             this->write("gl_InstanceID");
             break;
         case SK_LASTFRAGCOLOR_BUILTIN:
-            this->write(this->caps().fbFetchColorName());
+            if (this->caps().fbFetchSupport()) {
+                this->write(this->caps().fbFetchColorName());
+            } else {
+                fContext.fErrors->error(ref.fOffset,
+                                        "sk_LastFragColor requires framebuffer fetch support");
+            }
             break;
         default:
             this->write(ref.variable()->name());

tests/sksl/errors/LastFragColorWithoutCaps.glsl

@@ -0,0 +1,4 @@
+### Compilation failed:
+
+error: 2: sk_LastFragColor requires framebuffer fetch support
+1 error

tests/sksl/glsl/LastFragColor.glsl

@@ -0,0 +1,5 @@
+
+out vec4 sk_FragColor;
+void main() {
+    sk_FragColor = gl_LastFragData[0];
+}

tests/sksl/shared/Caps.asm.frag

@@ -42,7 +42,6 @@ OpStore %x %int_0
 OpStore %y %int_0
 OpStore %z %int_0
 OpStore %x %int_1
-OpStore %y %int_1
 OpStore %z %int_1
 %20 = OpLoad %int %x
 %21 = OpConvertSToF %float %20

tests/sksl/shared/Caps.glsl

@@ -5,7 +5,6 @@ void main() {
     int y = 0;
     int z = 0;
     x = 1;
-    y = 1;
     z = 1;
     sk_FragColor.xyz = vec3(float(x), float(y), float(z));
 }

tests/sksl/shared/Caps.metal

@@ -13,7 +13,6 @@ fragment Outputs fragmentMain(Inputs _in [[stage_in]], bool _frontFacing [[front
     int y = 0;
     int z = 0;
     x = 1;
-    y = 1;
     z = 1;
     _out.sk_FragColor.xyz = float3(float(x), float(y), float(z));
     return _out;