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Commit e95203e0 authored by Hans Boehm's avatar Hans Boehm
Browse files

Make onSaveInstanceState wait for database writes 

Bug: 34051652

Add a mechanism to wait for (asynchronous) database writes to
complete.

Invoke it in Calculator.onSaveInstanceState(). This avoids a potential
situation in which the saved instance state bundle includes a reference
to an expression that never made it to the database. This may be more
likely than it appears, since we run all background tasks on a single
thread. A database save operation can get hung up behind a computation.

Note that in almost all cases, the wait operation will return
immediately, as evidenced by the fact that this very rarely
leads to crashes without the wait operation.

Change-Id: I7c2bdcd9bf19f6cb4bd653fb9244bca0f6b25435
parent c3a0b31c

src/com/android/calculator2/Calculator.java

+3 −0
Original line number Diff line number Diff line
@@ -496,6 +496,9 @@ public class Calculator extends Activity 
        outState.putByteArray(KEY_EVAL_STATE, byteArrayStream.toByteArray());

        outState.putBoolean(KEY_INVERSE_MODE, mInverseToggle.isSelected());

        outState.putBoolean(KEY_SHOW_TOOLBAR, mDisplayView.isToolbarVisible());

        // We must wait for asynchronous writes to complete, since outState may contain

        // references to expressions being written.

        mEvaluator.waitForWrites();

    }



    // Set the state, updating delete label and display colors.

src/com/android/calculator2/Evaluator.java

+7 −0
Original line number Diff line number Diff line
@@ -1889,6 +1889,13 @@ public class Evaluator implements CalculatorExpr.ExprResolver { 
        return sb.toString();

    }



    /**

     * Wait for pending writes to the database to complete.

     */

    public void waitForWrites() {

        mExprDB.waitForWrites();

    }



    /**

     * Destroy the current evaluator, forcing getEvaluator to allocate a new one.

     * This is needed for testing, since Robolectric apparently doesn't let us preserve

src/com/android/calculator2/ExpressionDB.java

+44 −0
Original line number Diff line number Diff line
@@ -394,6 +394,48 @@ public class ExpressionDB { 
        eraser.executeOnExecutor(AsyncTask.SERIAL_EXECUTOR);

    }



    // We track the number of outstanding writes to prevent onSaveInstanceState from

    // completing with in-flight database writes.



    private int mIncompleteWrites = 0;

    private Object mWriteCountsLock = new Object();  // Protects the preceding field.



    private void writeCompleted() {

        synchronized(mWriteCountsLock) {

            if (--mIncompleteWrites == 0) {

                mWriteCountsLock.notifyAll();

            }

        }

    }



    private void writeStarted() {

        synchronized(mWriteCountsLock) {

            ++mIncompleteWrites;

        }

    }



    /**

     * Wait for in-flight writes to complete.

     * This is not safe to call from one of our background tasks, since the writing

     * tasks may be waiting for the same underlying thread that we're using, resulting

     * in deadlock.

     */

    public void waitForWrites() {

        synchronized(mWriteCountsLock) {

            boolean caught = false;

            while (mIncompleteWrites != 0) {

                try {

                    mWriteCountsLock.wait();

                } catch (InterruptedException e) {

                    caught = true;

                }

            }

            if (caught) {

                Thread.currentThread().interrupt();

            }

        }

    }



    /**

     * Insert the given row in the database without blocking the UI thread.

     * These tasks must be executed on a serial executor to avoid reordering writes.
@@ -403,6 +445,7 @@ public class ExpressionDB { 
        protected Long doInBackground(ContentValues... cvs) {

            long index = cvs[0].getAsLong(ExpressionEntry._ID);

            long result = mExpressionDB.insert(ExpressionEntry.TABLE_NAME, null, cvs[0]);

            writeCompleted();

            // Return 0 on success, row id on failure.

            if (result == -1) {

                return index;
@@ -454,6 +497,7 @@ public class ExpressionDB { 
                // to just include values between mMinAccessible and mMaxAccessible.

                return newIndex;

            }

            writeStarted();

            ContentValues cvs = data.toContentValues();

            cvs.put(ExpressionEntry._ID, newIndex);

            AsyncWriter awriter = new AsyncWriter();