SkRWBuffer for thread-safe 'stream' sharing

src/core/SkRWBuffer.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkRWBuffer.h"
+#include "SkStream.h"
+
+// Force small chunks to be a page's worth
+static const size_t kMinAllocSize = 4096;
+
+struct SkBufferBlock {
+    SkBufferBlock*  fNext;
+    size_t          fUsed;
+    size_t          fCapacity;
+    
+    const void* startData() const { return this + 1; };
+    
+    size_t avail() const { return fCapacity - fUsed; }
+    void* availData() { return (char*)this->startData() + fUsed; }
+    
+    static SkBufferBlock* Alloc(size_t length) {
+        size_t capacity = LengthToCapacity(length);
+        SkBufferBlock* block = (SkBufferBlock*)sk_malloc_throw(sizeof(SkBufferBlock) + capacity);
+        block->fNext = NULL;
+        block->fUsed = 0;
+        block->fCapacity = capacity;
+        return block;
+    }
+
+    // Return number of bytes actually appended
+    size_t append(const void* src, size_t length) {
+        this->validate();
+        size_t amount = SkTMin(this->avail(), length);
+        memcpy(this->availData(), src, amount);
+        fUsed += amount;
+        this->validate();
+        return amount;
+    }
+
+    void validate() const {
+#ifdef SK_DEBUG
+        SkASSERT(fCapacity > 0);
+        SkASSERT(fUsed <= fCapacity);
+#endif
+    }
+
+private:
+    static size_t LengthToCapacity(size_t length) {
+        const size_t minSize = kMinAllocSize - sizeof(SkBufferBlock);
+        return SkTMax(length, minSize);
+    }
+};
+
+struct SkBufferHead {
+    mutable int32_t fRefCnt;
+    SkBufferBlock   fBlock;
+
+    static size_t LengthToCapacity(size_t length) {

[scroggo, 2015/04/28 17:27:39]

Could be private?

[reed2, 2015/04/29 00:43:49]

Possibly, tho this struct is private just to this impl file.

+        const size_t minSize = kMinAllocSize - sizeof(SkBufferHead);
+        return SkTMax(length, minSize);
+    }
+
+    static SkBufferHead* Alloc(size_t length) {
+        size_t capacity = LengthToCapacity(length);
+        size_t size = sizeof(SkBufferHead) + capacity;
+        SkBufferHead* head = (SkBufferHead*)sk_malloc_throw(size);
+        head->fRefCnt = 1;
+        head->fBlock.fNext = NULL;
+        head->fBlock.fUsed = 0;
+        head->fBlock.fCapacity = capacity;
+        return head;
+    }
+    
+    void ref() const {
+        SkASSERT(fRefCnt > 0);
+        sk_atomic_inc(&fRefCnt);
+    }
+    
+    void unref() const {
+        SkASSERT(fRefCnt > 0);
+        // A release here acts in place of all releases we "should" have been doing in ref().
+        if (1 == sk_atomic_fetch_add(&fRefCnt, -1, sk_memory_order_acq_rel)) {

[scroggo, 2015/04/28 17:27:39]

It would be nice to share this code with the other two places (so far) that we
use it.

[reed2, 2015/04/29 00:43:49]

Agreed, but I haven't seen how to do that sharing yet.

+            // Like unique(), the acquire is only needed on success.
+            SkBufferBlock* block = fBlock.fNext;
+            sk_free((void*)this);
+            while (block) {
+                SkBufferBlock* next = block->fNext;
+                sk_free(block);
+                block = next;
+            }
+            SkDebugf("done\n");

[scroggo, 2015/04/28 17:27:39]

Do you want to check this in?

[reed2, 2015/04/29 00:43:49]

Doh! Removed.

+        }
+    }
+    
+    void validate(size_t minUsed, SkBufferBlock* tail = NULL) const {
+#ifdef SK_DEBUG
+        SkASSERT(fRefCnt > 0);
+        size_t totalUsed = 0;
+        const SkBufferBlock* block = &fBlock;
+        const SkBufferBlock* lastBlock = block;
+        while (block) {
+            block->validate();
+            totalUsed += block->fUsed;
+            lastBlock = block;
+            block = block->fNext;
+        }
+        SkASSERT(minUsed <= totalUsed);
+        if (tail) {
+            SkASSERT(tail == lastBlock);
+        }
+#endif
+    }
+};
+
+SkROBuffer::SkROBuffer(const SkBufferHead* head, size_t used) : fHead(SkRef(head)), fUsed(used) {

[scroggo, 2015/04/28 17:27:39]

This should be SkSafeRef(head), since head may be NULL.

Alternatively, an SkROBuffer is useless with no data. Since this is created by a
factory method, maybe that should return NULL if head is NULL?

[reed2, 2015/04/29 00:43:49]

Done.

+    if (head) {
+        SkASSERT(used > 0);
+        head->validate(used);
+    } else {
+        SkASSERT(0 == used);
+    }
+}
+
+SkROBuffer::~SkROBuffer() {
+    if (fHead) {
+        fHead->validate(fUsed);
+        fHead->unref();
+    }
+}
+
+SkROBuffer::Iter::Iter(const SkROBuffer* buffer) {
+    this->reset(buffer);
+}
+
+void SkROBuffer::Iter::reset(const SkROBuffer* buffer) {
+    if (buffer) {
+        fBlock = &buffer->fHead->fBlock;
+        fRemaining = buffer->fUsed;
+    } else {
+        fBlock = NULL;
+        fRemaining = 0;
+    }
+}
+
+const void* SkROBuffer::Iter::data() const {
+    return fRemaining ? fBlock->startData() : NULL;
+}
+
+size_t SkROBuffer::Iter::size() const {
+    return SkTMin(fBlock->fUsed, fRemaining);
+}
+
+bool SkROBuffer::Iter::next() {
+    if (fRemaining) {
+        fRemaining -= this->size();
+        fBlock = fBlock->fNext;
+    }
+    return fRemaining != 0;
+}
+
+SkRWBuffer::SkRWBuffer(size_t initialCapacity) : fHead(NULL), fTail(NULL), fTotalUsed(0) {}
+
+SkRWBuffer::~SkRWBuffer() {
+    this->validate();
+    fHead->unref();
+}
+
+void SkRWBuffer::append(const void* src, size_t length) {
+    this->validate();
+    if (0 == length) {
+        return;
+    }
+
+    fTotalUsed += length;
+
+    if (NULL == fHead) {
+        fHead = SkBufferHead::Alloc(length);
+        fTail = &fHead->fBlock;
+    }
+
+    size_t written = fTail->append(src, length);
+    SkASSERT(written <= length);
+    src = (const char*)src + written;
+    length -= written;
+
+    if (length) {
+        SkBufferBlock* block = SkBufferBlock::Alloc(length);
+        fTail->fNext = block;
+        fTail = block;
+        written = fTail->append(src, length);
+        SkASSERT(written == length);
+    }
+    this->validate();
+}
+
+void* SkRWBuffer::append(size_t length) {
+    this->validate();
+    if (0 == length) {
+        return NULL;
+    }
+
+    fTotalUsed += length;
+    
+    if (NULL == fHead) {
+        fHead = SkBufferHead::Alloc(length);
+        fTail = &fHead->fBlock;
+    } else if (fTail->avail() < length) {
+        SkBufferBlock* block = SkBufferBlock::Alloc(length);
+        fTail->fNext = block;
+        fTail = block;
+    }
+
+    fTail->fUsed += length;
+    this->validate();
+    return (char*)fTail->availData() - length;
+}
+
+#ifdef SK_DEBUG
+void SkRWBuffer::validate() const {
+    if (fHead) {
+        fHead->validate(fTotalUsed, fTail);
+    } else {
+        SkASSERT(NULL == fTail);
+        SkASSERT(0 == fTotalUsed);
+    }
+}
+#endif
+
+SkROBuffer* SkRWBuffer::newRBufferSnapshot() const {
+    return SkNEW_ARGS(SkROBuffer, (fHead, fTotalUsed));
+}
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+class SkROBufferStreamAsset : public SkStreamAsset {
+    void validate() const {
+#ifdef SK_DEBUG
+        SkASSERT(fGlobalOffset <= fBuffer->size());
+        SkASSERT(fLocalOffset <= fIter.size());
+        SkASSERT(fLocalOffset <= fGlobalOffset);
+#endif
+    }
+
+    class AutoValidate {

[scroggo, 2015/04/28 17:27:39]

Only needed in SK_DEBUG?

[reed2, 2015/04/29 00:43:49]

Done.

+        SkROBufferStreamAsset* fStream;
+    public:
+        AutoValidate(SkROBufferStreamAsset* stream) : fStream(stream) { stream->validate(); }
+        ~AutoValidate() { fStream->validate(); }
+    };
+
+#ifdef SK_DEBUG
+    #define AUTO_VALIDATE   AutoValidate av(this);
+#else
+    #define AUTO_VALIDATE
+#endif
+
+public:
+    SkROBufferStreamAsset(const SkROBuffer* buffer) : fBuffer(SkRef(buffer)), fIter(buffer) {
+        fGlobalOffset = fLocalOffset = 0;
+    }
+
+    virtual ~SkROBufferStreamAsset() { fBuffer->unref(); }
+
+    size_t getLength() const override { return fBuffer->size(); }
+
+    bool rewind() override {
+        AUTO_VALIDATE
+        fIter.reset(fBuffer);
+        fGlobalOffset = fLocalOffset = 0;
+        return true;
+    }
+
+    size_t read(void* dst, size_t request) override {
+        AUTO_VALIDATE
+        size_t bytesRead = 0;
+        for (;;) {
+            size_t size = fIter.size();

[scroggo, 2015/04/28 17:27:39]

const?

+            SkASSERT(fLocalOffset <= size);
+            size_t avail = SkTMin(size - fLocalOffset, request - bytesRead);

[scroggo, 2015/04/28 17:27:39]

const?

+            if (dst) {
+                memcpy(dst, (const char*)fIter.data() + fLocalOffset, avail);
+                dst = (char*)dst + avail;
+            }
+            bytesRead += avail;
+            fLocalOffset += avail;
+            SkASSERT(bytesRead <= request);
+            if (bytesRead == request) {
+                break;
+            }
+            // If we get here, we've exhausted the current iter
+            SkASSERT(fLocalOffset == size);
+            fLocalOffset = 0;
+            if (!fIter.next()) {
+                break;   // ran out of data
+            }
+        }
+        fGlobalOffset += bytesRead;
+        SkASSERT(fGlobalOffset <= fBuffer->size());
+        return bytesRead;
+    }
+
+    bool isAtEnd() const override {
+        return fBuffer->size() == fGlobalOffset;
+    }
+    
+    SkStreamAsset* duplicate() const override {
+        return SkNEW_ARGS(SkROBufferStreamAsset, (fBuffer));
+    }
+    
+    size_t getPosition() const {
+        return fGlobalOffset;
+    }
+    
+    bool seek(size_t position) {
+        AUTO_VALIDATE
+        if (position < fGlobalOffset) {
+            this->rewind();
+        }
+        (void)this->skip(position - fGlobalOffset);
+        return true;
+    }
+    
+    bool move(long offset) {
+        AUTO_VALIDATE
+        offset += fGlobalOffset;
+        if (offset <= 0) {
+            this->rewind();
+        } else {
+            (void)this->seek(SkToSizeT(offset));
+        }
+        return true;
+    }
+    
+    SkStreamAsset* fork() const override {
+        SkStreamAsset* clone = this->duplicate();
+        clone->seek(this->getPosition());
+        return clone;
+    }
+    
+
+private:
+    const SkROBuffer*   fBuffer;
+    SkROBuffer::Iter    fIter;
+    size_t              fLocalOffset;
+    size_t              fGlobalOffset;
+};
+
+SkStreamAsset* SkRWBuffer::newStreamSnapshot() const {
+    SkAutoTUnref<SkROBuffer> buffer(this->newRBufferSnapshot());
+    return SkNEW_ARGS(SkROBufferStreamAsset, (buffer));
+}