Make A8 readbacks work from non-A8 surfaces

src/gpu/gl/GrGLCaps.cpp

Index: src/gpu/gl/GrGLCaps.cpp
diff --git a/src/gpu/gl/GrGLCaps.cpp b/src/gpu/gl/GrGLCaps.cpp
index 81d0b4483b0b77cd41cd79672d2e93b9de271987..237412a7fcb7cdf8a5568d120121a74e9dbe0287 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -643,7 +643,11 @@ bool GrGLCaps::readPixelsSupported(GrPixelConfig rtConfig,
                                    GrPixelConfig readConfig,
                                    std::function<void (GrGLenum, GrGLint*)> getIntegerv,
                                    std::function<bool ()> bindRenderTarget) const {
-    SkASSERT(this->isConfigRenderable(rtConfig, false));
+    // If it's not possible to even have a render target of rtConfig then read pixels is
+    // not supported regardless of readConfig.
+    if (!this->isConfigRenderable(rtConfig, false)) {
+        return false;
+    }
 
     GrGLenum readFormat;
     GrGLenum readType;
@@ -652,7 +656,22 @@ bool GrGLCaps::readPixelsSupported(GrPixelConfig rtConfig,
     }
 
     if (kGL_GrGLStandard == fStandard) {
-        // All of our renderable configs can be converted to each other by glReadPixels in OpenGL.
+        // Some OpenGL implementations allow GL_ALPHA as a format to glReadPixels. However,
+        // the manual (https://www.opengl.org/sdk/docs/man/) says only these formats are allowed:
+        // GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_DEPTH_STENCIL, GL_RED, GL_GREEN, GL_BLUE,
+        // GL_RGB, GL_BGR, GL_RGBA, and GL_BGRA. We check for the subset that we would use.
+        if (readFormat != GR_GL_RED && readFormat != GR_GL_RGB && readFormat != GR_GL_RGBA &&
+            readFormat != GR_GL_BGRA) {
+            return false;
+        }
+        // There is also a set of allowed types, but all the types we use are in the set:
+        // GL_UNSIGNED_BYTE, GL_BYTE, GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT,
+        // GL_HALF_FLOAT, GL_FLOAT, GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
+        // GL_UNSIGNED_SHORT_5_6_5, GL_UNSIGNED_SHORT_5_6_5_REV, GL_UNSIGNED_SHORT_4_4_4_4,
+        // GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
+        // GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV,GL_UNSIGNED_INT_10_10_10_2,
+        // GL_UNSIGNED_INT_2_10_10_10_REV, GL_UNSIGNED_INT_24_8, GL_UNSIGNED_INT_10F_11F_11F_REV,
+        // GL_UNSIGNED_INT_5_9_9_9_REV, or GL_FLOAT_32_UNSIGNED_INT_24_8_REV.
         return true;
     }
 
@@ -1070,6 +1089,17 @@ bool GrGLCaps::getExternalFormat(GrPixelConfig surfaceConfig, GrPixelConfig memo
     *externalFormat = fConfigTable[memoryConfig].fFormats.fExternalFormat[usage];
     *externalType = fConfigTable[memoryConfig].fFormats.fExternalType;
 
+    // When GL_RED is supported as a texture format, our alpha-only textures are stored using
+    // GL_RED and we swizzle in order to map all components to 'r'. However, in this case the
+    // surface is not alpha-only and we want alpha to really mean the alpha component of the
+    // texture, not the red component.
+    if (memoryIsAlphaOnly && !surfaceIsAlphaOnly) {
+        if (this->textureRedSupport()) {
+            SkASSERT(GR_GL_RED == *externalFormat);
+            *externalFormat = GR_GL_ALPHA;
+        }
+    }
+
     return true;
 }