Implement back-tracking when searching for attributes in XML or resource bag

Shared libraries have their package ID assigned at run-time, so some
of the guarantees we used to have about sort order of attributes in
bags or XML elements no longer hold.

This CL adds back-tracking and can jump to the nearest attribute with the
same package ID and continue searching.

This means that attributes with the same package ID must be sorted by increasing
resource ID, as was the case before.

Attributes with the same package ID must be grouped together, but the groups can
be in any order. Ex: 0x02010001, 0x02010002, 0x01010000, 0x01010010, 0x7f010032

Bug:17666947
Change-Id: I9c198bbb6ca788849aac85b6323606ea5d9550d6

1 files changed, 201 insertions, 0 deletions

diff --git a/include/androidfw/AttributeFinder.h b/include/androidfw/AttributeFinder.h
new file mode 100644
index 0000000..a0ffeb3
--- /dev/null
+++ b/include/androidfw/AttributeFinder.h
@@ -0,0 +1,201 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef H_ATTRIBUTE_FINDER
+#define H_ATTRIBUTE_FINDER
+
+#include <stdint.h>
+#include <utils/KeyedVector.h>
+
+namespace android {
+
+static inline uint32_t getPackage(uint32_t attr) {
+    return attr >> 24;
+}
+
+/**
+ * A helper class to search linearly for the requested
+ * attribute, maintaining it's position and optimizing for
+ * the case that subsequent searches will involve an attribute with
+ * a higher attribute ID.
+ *
+ * In the case that a subsequent attribute has a different package ID,
+ * its resource ID may not be larger than the preceding search, so
+ * back tracking is supported for this case. This
+ * back tracking requirement is mainly for shared library
+ * resources, whose package IDs get assigned at runtime
+ * and thus attributes from a shared library may
+ * be out of order.
+ *
+ * We make two assumptions about the order of attributes:
+ * 1) The input has the same sorting rules applied to it as
+ *    the attribute data contained by this class.
+ * 2) Attributes are grouped by package ID.
+ * 3) Among attributes with the same package ID, the attributes are
+ *    sorted by increasing resource ID.
+ *
+ * Ex: 02010000, 02010001, 010100f4, 010100f5, 0x7f010001, 07f010003
+ *
+ * The total order of attributes (including package ID) can not be linear
+ * as shared libraries get assigned dynamic package IDs at runtime, which
+ * may break the sort order established at build time.
+ */
+template <typename Derived, typename Iterator>
+class BackTrackingAttributeFinder {
+public:
+    BackTrackingAttributeFinder(const Iterator& begin, const Iterator& end);
+
+    Iterator find(uint32_t attr);
+
+private:
+    void jumpToClosestAttribute(uint32_t packageId);
+    void markCurrentPackageId(uint32_t packageId);
+
+    Iterator mBegin;
+    Iterator mEnd;
+    Iterator mCurrent;
+    Iterator mLargest;
+    uint32_t mLastPackageId;
+    uint32_t mCurrentAttr;
+
+    // Package Offsets (best-case, fast look-up).
+    Iterator mFrameworkStart;
+    Iterator mAppStart;
+
+    // Worst case, we have shared-library resources.
+    KeyedVector<uint32_t, Iterator> mPackageOffsets;
+};
+
+template <typename Derived, typename Iterator> inline
+BackTrackingAttributeFinder<Derived, Iterator>::BackTrackingAttributeFinder(const Iterator& begin, const Iterator& end)
+    : mBegin(begin)
+    , mEnd(end)
+    , mCurrent(begin)
+    , mLargest(begin)
+    , mLastPackageId(0)
+    , mCurrentAttr(0)
+    , mFrameworkStart(end)
+    , mAppStart(end) {
+}
+
+template <typename Derived, typename Iterator>
+void BackTrackingAttributeFinder<Derived, Iterator>::jumpToClosestAttribute(const uint32_t packageId) {
+    switch (packageId) {
+        case 0x01:
+            mCurrent = mFrameworkStart;
+            break;
+        case 0x7f:
+            mCurrent = mAppStart;
+            break;
+        default: {
+            ssize_t idx = mPackageOffsets.indexOfKey(packageId);
+            if (idx >= 0) {
+                // We have seen this package ID before, so jump to the first
+                // attribute with this package ID.
+                mCurrent = mPackageOffsets[idx];
+            } else {
+                mCurrent = mEnd;
+            }
+            break;
+        }
+    }
+
+    // We have never seen this package ID yet, so jump to the
+    // latest/largest index we have processed so far.
+    if (mCurrent == mEnd) {
+        mCurrent = mLargest;
+    }
+
+    if (mCurrent != mEnd) {
+        mCurrentAttr = static_cast<const Derived*>(this)->getAttribute(mCurrent);
+    }
+}
+
+template <typename Derived, typename Iterator>
+void BackTrackingAttributeFinder<Derived, Iterator>::markCurrentPackageId(const uint32_t packageId) {
+    switch (packageId) {
+        case 0x01:
+            mFrameworkStart = mCurrent;
+            break;
+        case 0x7f:
+            mAppStart = mCurrent;
+            break;
+        default:
+            mPackageOffsets.add(packageId, mCurrent);
+            break;
+    }
+}
+
+template <typename Derived, typename Iterator>
+Iterator BackTrackingAttributeFinder<Derived, Iterator>::find(uint32_t attr) {
+    if (!(mBegin < mEnd)) {
+        return mEnd;
+    }
+
+    if (mCurrentAttr == 0) {
+        // One-time initialization.
+        mCurrentAttr = static_cast<const Derived*>(this)->getAttribute(mBegin);
+        mLastPackageId = getPackage(mCurrentAttr);
+        markCurrentPackageId(mLastPackageId);
+    }
+
+    // Looking for the needle (attribute we're looking for)
+    // in the haystack (the attributes we're searching through)
+    const uint32_t needlePackageId = getPackage(attr);
+    if (mLastPackageId != needlePackageId) {
+        jumpToClosestAttribute(needlePackageId);
+        mLastPackageId = needlePackageId;
+    }
+
+    // Walk through the xml attributes looking for the requested attribute.
+    while (mCurrent != mEnd) {
+        const uint32_t haystackPackageId = getPackage(mCurrentAttr);
+        if (needlePackageId == haystackPackageId && attr < mCurrentAttr) {
+            // The attribute we are looking was not found.
+            break;
+        }
+        const uint32_t prevAttr = mCurrentAttr;
+
+        // Move to the next attribute in the XML.
+        ++mCurrent;
+        if (mCurrent != mEnd) {
+            mCurrentAttr = static_cast<const Derived*>(this)->getAttribute(mCurrent);
+            const uint32_t newHaystackPackageId = getPackage(mCurrentAttr);
+            if (haystackPackageId != newHaystackPackageId) {
+                // We've moved to the next group of attributes
+                // with a new package ID, so we should record
+                // the offset of this new package ID.
+                markCurrentPackageId(newHaystackPackageId);
+            }
+        }
+
+        if (mCurrent > mLargest) {
+            // We've moved past the latest attribute we've
+            // seen.
+            mLargest = mCurrent;
+        }
+
+        if (attr == prevAttr) {
+            // We found the attribute we were looking for.
+            return mCurrent - 1;
+        }
+    }
+    return mEnd;
+}
+
+} // namespace android
+
+#endif // H_ATTRIBUTE_FINDER