Reland "Fix the fat rect bug in the threaded backend"

This reverts commit 1b637615144604a2d50cf983ac9aa5aab71cf73b.

Reason for revert: add guard flag

Original change's description:
> Revert "Fix the fat rect bug in the threaded backend"
> 
> This reverts commit c41569a29fde10c6fec43cc4593334e9a4b34b76.
> 
> Reason for revert: maybe break the Chrome layout tests
> 
> Original change's description:
> > Fix the fat rect bug in the threaded backend
> > 
> > Bug: skia:7813
> > Change-Id: I954232be1dccc63ce412ccde92c4f0e4617317b9
> > Reviewed-on: https://skia-review.googlesource.com/121641
> > Reviewed-by: Cary Clark <caryclark@google.com>
> > Commit-Queue: Yuqian Li <liyuqian@google.com>
> 
> TBR=caryclark@google.com,liyuqian@google.com,reed@google.com
> 
> Change-Id: If35617a9774b3367561981e39a2fa89a972684b9
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:7813
> Reviewed-on: https://skia-review.googlesource.com/121820
> Reviewed-by: Yuqian Li <liyuqian@google.com>
> Commit-Queue: Yuqian Li <liyuqian@google.com>

TBR=caryclark@google.com,liyuqian@google.com,reed@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:7813
Change-Id: I8b17be4251b5efc20142295cca787474f32e3a86
Reviewed-on: https://skia-review.googlesource.com/122100
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Yuqian Li <liyuqian@google.com>

3 files changed, 41 insertions, 7 deletions

diff --git a/gm/aaa.cpp b/gm/aaa.cpp
index 910ce19988..b2fc6337d9 100644
--- a/gm/aaa.cpp
+++ b/gm/aaa.cpp
@@ -44,6 +44,7 @@ protected:
 
         y += 200;
 
+        canvas->save();
         canvas->translate(0, y);
         canvas->rotate(1);
         canvas->drawRect({ 20, 20, 20.2f, 200 }, p);
@@ -78,6 +79,7 @@ protected:
 
         // skbug.com/7573
         y += 200;
+        canvas->save();
         canvas->translate(0, y);
         p.setAntiAlias(true);
         path.reset();
@@ -87,6 +89,15 @@ protected:
         path.lineTo(1.93990216f, 10.5837256f);
         canvas->drawPath(path, p);
         canvas->restore();
+
+        // skbug.com/7813
+        // t8888 splits the 800-high canvas into 3 pieces; the boundary is close to 266 and 534
+        path.reset();
+        path.moveTo(700, 266);
+        path.lineTo(710, 266);
+        path.lineTo(710, 534);
+        path.lineTo(700, 534);
+        canvas->drawPath(path, p);
     }
 
 private:
diff --git a/src/core/SkBlitter.cpp b/src/core/SkBlitter.cpp
index 572150a173..d8cea8f17b 100644
--- a/src/core/SkBlitter.cpp
+++ b/src/core/SkBlitter.cpp
@@ -49,7 +49,18 @@ inline static SkAlpha ScalarToAlpha(SkScalar a) {
 
 void SkBlitter::blitFatAntiRect(const SkRect& rect) {
     SkIRect bounds = rect.roundOut();
-    SkASSERT(bounds.width() >= 3 && bounds.height() >= 3);
+    SkASSERT(bounds.width() >= 3);
+
+    // skbug.com/7813
+    // To ensure consistency of the threaded backend (a rect that's considered fat in the init-once
+    // phase must also be considered fat in the draw phase), we have to deal with rects with small
+    // heights because the horizontal tiling in the threaded backend may change the height.
+    //
+    // This also implies that we cannot do vertical tiling unless we can blit any rect (not just the
+    // fat one.)
+    if (bounds.height() == 0) {
+        return;
+    }
 
     int         runSize = bounds.width() + 1; // +1 so we can set runs[bounds.width()] = 0
     void*       storage = this->allocBlitMemory(runSize * (sizeof(int16_t) + sizeof(SkAlpha)));
@@ -66,18 +77,26 @@ void SkBlitter::blitFatAntiRect(const SkRect& rect) {
     SkScalar partialT = bounds.fTop + 1 - rect.fTop;
     SkScalar partialB = rect.fBottom - (bounds.fBottom - 1);
 
+    if (bounds.height() == 1) {
+        partialT = rect.fBottom - rect.fTop;
+    }
+
     alphas[0] = ScalarToAlpha(partialL * partialT);
     alphas[1] = ScalarToAlpha(partialT);
     alphas[bounds.width() - 1] = ScalarToAlpha(partialR * partialT);
     this->blitAntiH(bounds.fLeft, bounds.fTop, alphas, runs);
 
-    this->blitAntiRect(bounds.fLeft, bounds.fTop + 1, bounds.width() - 2, bounds.height() - 2,
-                       ScalarToAlpha(partialL), ScalarToAlpha(partialR));
+    if (bounds.height() > 2) {
+        this->blitAntiRect(bounds.fLeft, bounds.fTop + 1, bounds.width() - 2, bounds.height() - 2,
+                           ScalarToAlpha(partialL), ScalarToAlpha(partialR));
+    }
 
-    alphas[0] = ScalarToAlpha(partialL * partialB);
-    alphas[1] = ScalarToAlpha(partialB);
-    alphas[bounds.width() - 1] = ScalarToAlpha(partialR * partialB);
-    this->blitAntiH(bounds.fLeft, bounds.fBottom - 1, alphas, runs);
+    if (bounds.height() > 1) {
+        alphas[0] = ScalarToAlpha(partialL * partialB);
+        alphas[1] = ScalarToAlpha(partialB);
+        alphas[bounds.width() - 1] = ScalarToAlpha(partialR * partialB);
+        this->blitAntiH(bounds.fLeft, bounds.fBottom - 1, alphas, runs);
+    }
 }
 
 void SkBlitter::blitCoverageDeltas(SkCoverageDeltaList* deltas, const SkIRect& clip,
diff --git a/src/core/SkScanPriv.h b/src/core/SkScanPriv.h
index 3d364988db..f2bb390cec 100644
--- a/src/core/SkScanPriv.h
+++ b/src/core/SkScanPriv.h
@@ -90,7 +90,11 @@ static inline bool TryBlitFatAntiRect(SkBlitter* blitter, const SkPath& path, co
         return true; // The intersection is empty. Hence consider it done.
     }
     SkIRect bounds = rect.roundOut();
+#ifdef SK_SUPPORT_LEGACY_THREADED_DAA_BUGS
     if (bounds.width() < 3 || bounds.height() < 3) {
+#else
+    if (bounds.width() < 3) {
+#endif
         return false; // not fat
     }
     blitter->blitFatAntiRect(rect);