Add a "bisect" mode to dm for debugging path drawing

Bug: skia:
Change-Id: Idc841545dfe3d33f43c1c8a3cf23199c322f7b11
Reviewed-on: https://skia-review.googlesource.com/156929
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

dm/DM.cpp

@@ -95,6 +95,12 @@ DEFINE_int32(shard,  0, "Which shard do I run?");
 DEFINE_string(mskps, "", "Directory to read mskps from, or a single mskp file.");
 DEFINE_bool(forceRasterPipeline, false, "sets gSkForceRasterPipelineBlitter");
 
+DEFINE_string(bisect, "",
+        "Pair of: SKP file to bisect, followed by an l/r bisect trail string (e.g., 'lrll'). The "
+        "l/r trail specifies which half to keep at each step of a binary search through the SKP's "
+        "paths. An empty string performs no bisect. Only the SkPaths are bisected; all other draws "
+        "are thrown out. This is useful for finding a reduced repo case for path drawing bugs.");
+
 DEFINE_bool(ignoreSigInt, false, "ignore SIGINT signals during test execution");
 
 DEFINE_string(dont_write, "", "File extensions to skip writing to --writePath.");  // See skia:6821
@@ -793,6 +799,11 @@ static bool gather_srcs() {
 #if defined(SK_XML)
     gather_file_srcs<SVGSrc>(FLAGS_svgs, "svg");
 #endif
+    if (!FLAGS_bisect.isEmpty()) {
+        // An empty l/r trail string will draw all the paths.
+        push_src("bisect", "",
+                 new BisectSrc(FLAGS_bisect[0], FLAGS_bisect.count() > 1 ? FLAGS_bisect[1] : ""));
+    }
 
     SkTArray<SkString> images;
     if (!CollectImages(FLAGS_images, &images)) {

dm/DMSrcSink.cpp

@@ -1140,6 +1140,58 @@ Name SKPSrc::name() const { return SkOSPath::Basename(fPath.c_str()); }
 
 /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
 
+BisectSrc::BisectSrc(Path path, const char* trail) : INHERITED(path), fTrail(trail) {}
+
+Error BisectSrc::draw(SkCanvas* canvas) const {
+    struct FoundPath {
+        SkPath fPath;
+        SkPaint fPaint;
+        SkMatrix fViewMatrix;
+    };
+
+    // This subclass of SkCanvas just extracts all the SkPaths (drawn via drawPath) from an SKP.
+    class PathFindingCanvas : public SkCanvas {
+    public:
+        PathFindingCanvas(int width, int height) : SkCanvas(width, height, nullptr) {}
+        const SkTArray<FoundPath>& foundPaths() const { return fFoundPaths; }
+
+    private:
+        void onDrawPath(const SkPath& path, const SkPaint& paint) override {
+            fFoundPaths.push_back() = {path, paint, this->getTotalMatrix()};
+        }
+
+        SkTArray<FoundPath> fFoundPaths;
+    };
+
+    PathFindingCanvas pathFinder(canvas->getBaseLayerSize().width(),
+                                 canvas->getBaseLayerSize().height());
+    Error err = this->INHERITED::draw(&pathFinder);
+    if (!err.isEmpty()) {
+        return err;
+    }
+
+    int start = 0, end = pathFinder.foundPaths().count();
+    for (const char* ch = fTrail.c_str(); *ch; ++ch) {
+        int midpt = (start + end) / 2;
+        if ('l' == *ch) {
+            start = midpt;
+        } else if ('r' == *ch) {
+            end = midpt;
+        }
+    }
+
+    for (int i = start; i < end; ++i) {
+        const FoundPath& path = pathFinder.foundPaths()[i];
+        SkAutoCanvasRestore acr(canvas, true);
+        canvas->concat(path.fViewMatrix);
+        canvas->drawPath(path.fPath, path.fPaint);
+    }
+
+    return "";
+}
+
+/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
+
 #if defined(SK_ENABLE_SKOTTIE)
 SkottieSrc::SkottieSrc(Path path) : fPath(std::move(path)) {}
 

dm/DMSrcSink.h

@@ -248,6 +248,21 @@ private:
     Path fPath;
 };
 
+// This class extracts all the paths from an SKP and then removes unwanted paths according to the
+// provided l/r trail. It then just draws the remaining paths. (Non-path draws are thrown out.) It
+// is useful for finding a reduced repo case for path drawing bugs.
+class BisectSrc : public SKPSrc {
+public:
+    explicit BisectSrc(Path path, const char* trail);
+
+    Error draw(SkCanvas*) const override;
+
+private:
+    SkString fTrail;
+
+    typedef SKPSrc INHERITED;
+};
+
 
 #if defined(SK_ENABLE_SKOTTIE)
 class SkottieSrc final : public Src {