Add SkCachedData and use it for SkMipMap

src/core/SkCachedData.cpp

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkCachedData_priv.h"
+#include "SkRefCnt.h"
+#include "SkDiscardableMemory.h"
+
+SkCachedData::SkCachedData(void* data, size_t size)
+    : fData(data)
+    , fSize(size)
+    , fRefCnt(1)
+    , fStorageType(kMalloc_StorageType)
+    , fInCache(false)
+    , fIsLocked(true)
+{
+    fStorage.fMalloc = data;
+}
+
+SkCachedData::SkCachedData(size_t size, SkDiscardableMemory* dm)
+    : fData(dm->data())
+    , fSize(size)
+    , fRefCnt(1)
+    , fStorageType(kDiscardableMemory_StorageType)
+    , fInCache(false)
+    , fIsLocked(true)
+{
+    fStorage.fDM = dm;
+}
+
+SkCachedData::~SkCachedData() {
+    switch (fStorageType) {
+        case kMalloc_StorageType:
+            sk_free(fStorage.fMalloc);
+            break;
+        case kDiscardableMemory_StorageType:
+            SkDELETE(fStorage.fDM);
+            break;
+    }
+}
+
+class SkCachedData::AutoMutexWritable {
+public:
+    AutoMutexWritable(const SkCachedData* cd) : fCD(const_cast<SkCachedData*>(cd)) {
+        fCD->get_mutex().acquire();
+        fCD->validate();
+    }
+    ~AutoMutexWritable() {
+        fCD->validate();
+        fCD->get_mutex().release();
+    }
+
+    SkCachedData* get() { return fCD; }
+    SkCachedData* operator->() { return fCD; }
+
+private:
+    SkCachedData* fCD;
+};
+
+void SkCachedData::internalRef(bool fromCache) const {
+    AutoMutexWritable writable(this);

[mtklein, 2014/09/27 14:47:49]

I really like this pattern.

+
+    SkASSERT(!fromCache || !writable->fInCache);    // we can only be in a cache once
+
+    if ((1 == writable->fRefCnt) && fInCache) {
+        writable->in_mutex_lock();
+    }
+    
+    writable->fRefCnt += 1;
+    writable->fInCache |= fromCache;

[mtklein, 2014/09/27 14:47:49]

Just a nit, but I'd go with ||= here.  It really is logical bools, not bits.

[reed1, 2014/09/27 17:25:58]

||= isn't legal afaik

However, I did rewrite to use an "if"

+}
+
+bool SkCachedData::doInternalUnref(bool fromCache) const {
+    AutoMutexWritable writable(this);
+
+    switch (writable->fRefCnt) {

[mtklein, 2014/09/27 14:47:49]

These guys might read better if you decrement first. 

switch (--writable->fRefCnt) {
  case 0:  // we're going to be deleted
  case 1:  // if we're down to one owner
}
return 0 == writable->fRefCnt;

[reed1, 2014/09/27 17:25:58]

Done.

+        case 1:
+            // we're going to be deleted, so we need to be unlocked (for DiscardableMemory)
+            if (writable->fIsLocked) {
+                writable->in_mutex_unlock();
+            }
+            break;
+        case 2:
+            if (fInCache && !fromCache) {
+                // If we're down to 1 owner, and that owner is the cache, this it is safe
+                // to unlock (and mutate fData) even if the cache is in a different thread,
+                // as the cache is NOT allowed to inspect or use fData.
+                writable->in_mutex_unlock();
+            }
+            break;
+        default:
+            break;
+    }
+
+    if (fromCache) {
+        SkASSERT(writable->fInCache);
+        writable->fInCache = false;
+    }
+
+    // return true when we need to be deleted
+    return 0 == --writable->fRefCnt;
+}
+
+void SkCachedData::internalUnref(bool fromCache) const {
+    if (this->doInternalUnref(fromCache)) {
+        // can't delete inside doInternalUnref, since it is locking a mutex (which we own)
+        SkDELETE(this);
+    }
+}
+
+void SkCachedData::in_mutex_lock() {
+    this->assert_in_mutex();
+
+    SkASSERT(!fIsLocked);
+    fIsLocked = true;
+
+    switch (fStorageType) {
+        case kMalloc_StorageType:
+            this->setData(fStorage.fMalloc);
+            break;
+        case kDiscardableMemory_StorageType:
+            if (fStorage.fDM->lock()) {
+                this->setData(fStorage.fDM->data());
+            } else {
+                this->setData(NULL);   // signal failure to lock, contents are gone
+            }
+            break;
+    }
+}
+
+void SkCachedData::in_mutex_unlock() {
+    this->assert_in_mutex();
+
+    SkASSERT(fIsLocked);
+    fIsLocked = false;
+
+    switch (fStorageType) {
+        case kMalloc_StorageType:
+            // nothing to do/check
+            break;
+        case kDiscardableMemory_StorageType:
+            if (fData) {    // did the previous lock succeed?
+                fStorage.fDM->unlock();
+            }
+            break;
+    }
+    this->setData(NULL);   // signal that we're in an unlocked state
+}
+
+#ifdef SK_DEBUG
+void SkCachedData::validate() const {
+    if (fIsLocked) {
+        SkASSERT((fInCache && fRefCnt > 1) || !fInCache);
+        switch (fStorageType) {
+            case kMalloc_StorageType:
+                SkASSERT(fData == fStorage.fMalloc);
+                break;
+            case kDiscardableMemory_StorageType:
+                // fData can be null or the actual value, depending if DM's lock succeeded
+                break;
+        }
+    } else {
+        SkASSERT((fInCache && 1 == fRefCnt) || (0 == fRefCnt));
+        SkASSERT(NULL == fData);
+    }
+}
+#endif