add explicit API to know if the writebuffer is using its external storage

To help with https://skia-review.googlesource.com/c/skia/+/114262

Bug: skia:
Change-Id: I83fcccdec0dbaef28d0e557ea6fb9bd0ed59ed44
Reviewed-on: https://skia-review.googlesource.com/114284
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>

src/core/SkWriteBuffer.cpp

@@ -33,6 +33,10 @@ SkBinaryWriteBuffer::~SkBinaryWriteBuffer() {
     SkSafeUnref(fTFSet);
 }
 
+bool SkBinaryWriteBuffer::usingInitialStorage() const {
+    return fWriter.usingInitialStorage();
+}
+
 void SkBinaryWriteBuffer::writeByteArray(const void* data, size_t size) {
     fWriter.write32(SkToU32(size));
     fWriter.writePad(data, size);

src/core/SkWriteBuffer.h

@@ -92,6 +92,10 @@ public:
 
     size_t bytesWritten() const { return fWriter.bytesWritten(); }
 
+    // Returns true iff all of the bytes written so far are stored in the initial storage
+    // buffer provided in the constructor or the most recent call to reset.
+    bool usingInitialStorage() const;
+
     void writeByteArray(const void* data, size_t size) override;
     void writeBool(bool value) override;
     void writeScalar(SkScalar value) override;

src/core/SkWriter32.h

@@ -39,6 +39,10 @@ public:
     // return the current offset (will always be a multiple of 4)
     size_t bytesWritten() const { return fUsed; }
 
+    // Returns true iff all of the bytes written so far are stored in the initial storage
+    // buffer provided in the constructor or the most recent call to reset.
+    bool usingInitialStorage() const { return fData == fExternal; }
+
     SK_ATTR_DEPRECATED("use bytesWritten")
     size_t size() const { return this->bytesWritten(); }
 

tests/SerializationTest.cpp

@@ -637,3 +637,32 @@ DEF_TEST(Annotations, reporter) {
     TestAnnotationCanvas canvas(reporter, recs, SK_ARRAY_COUNT(recs));
     canvas.drawPicture(pict1);
 }
+
+DEF_TEST(WriteBuffer_storage, reporter) {
+    enum {
+        kSize = 32
+    };
+    int32_t storage[kSize/4];
+    char src[kSize];
+    sk_bzero(src, kSize);
+
+    SkBinaryWriteBuffer writer(storage, kSize);
+    REPORTER_ASSERT(reporter, writer.usingInitialStorage());
+    REPORTER_ASSERT(reporter, writer.bytesWritten() == 0);
+    writer.write(src, kSize - 4);
+    REPORTER_ASSERT(reporter, writer.usingInitialStorage());
+    REPORTER_ASSERT(reporter, writer.bytesWritten() == kSize - 4);
+    writer.writeInt(0);
+    REPORTER_ASSERT(reporter, writer.usingInitialStorage());
+    REPORTER_ASSERT(reporter, writer.bytesWritten() == kSize);
+
+    writer.reset(storage, kSize-4);
+    REPORTER_ASSERT(reporter, writer.usingInitialStorage());
+    REPORTER_ASSERT(reporter, writer.bytesWritten() == 0);
+    writer.write(src, kSize - 4);
+    REPORTER_ASSERT(reporter, writer.usingInitialStorage());
+    REPORTER_ASSERT(reporter, writer.bytesWritten() == kSize - 4);
+    writer.writeInt(0);
+    REPORTER_ASSERT(reporter, !writer.usingInitialStorage());   // this is the change
+    REPORTER_ASSERT(reporter, writer.bytesWritten() == kSize);
+}