Fix uninitialized memory bug in the SkGlyphCache.

The core of the problem is that the system is asked to lookup the metrics for a character with id == 0. This causes a hit in the fCharToGlyphHash matching the sentinel glyph. This happens because fCharToGlpyhHash is initialized with all zeros, therefore, the fID is zero matching the char with id == 0. The fAdvanceX field of the sentinel glyph is in fact not initialized.

The bigger question is now did a zero character get passed to getUnicharMetrics?

The breaking code is basically as follows:
wchar_t glyph = L'S';
paint.measureText(&glyph, 2);

This get mischaracterized as a utf8 string instead of a utf16(?) string. Because of the little endian ordering, this is the character string 'L' '\0'. Since the size of the original string is two bytes (but a single character) the '\0' is treated as its own character and past to getUnicharMetrics.

TEST:
On windows failed using DrMemory. With this change does not fail.

BUG=463204

Review URL: https://codereview.chromium.org/977063002

1 files changed, 6 insertions, 4 deletions

diff --git a/src/core/SkGlyphCache.h b/src/core/SkGlyphCache.h
index 200655bb41..d0b792f273 100644
--- a/src/core/SkGlyphCache.h
+++ b/src/core/SkGlyphCache.h
@@ -207,13 +207,15 @@ private:
     enum {
         kHashBits           = 8,
         kHashCount          = 1 << kHashBits,
-        kHashMask           = kHashCount - 1,
-        kSentinelGlyphIndex = 0,
-        kSentinelGlyphID    = ~0
+        kHashMask           = kHashCount - 1
     };
-    
+
     // A quick lookup to avoid the binary search looking for glyphs in fGlyphArray.
     uint16_t             fGlyphHash[kHashCount];
+    // Contains the SkGlyphs that are used by fGlyphHash and fCharToGlyphHash. The zero element
+    // is reserved for a sentinel SkGlyph that reduces the logic to check for collisions in the
+    // hash arrays. The zero element has an fID of SkGlyph::kImpossibleID which never matches
+    // any combined id generated for a char or a glyph.
     SkTDArray<SkGlyph>   fGlyphArray;
     SkChunkAlloc         fGlyphAlloc;