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Preserve ordering of flush()es by not letting subsequent flush()es race
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@ -104,8 +104,9 @@ class IndexedDBLogStore {
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this.id = "instance-" + Math.random() + Date.now();
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this.index = 0;
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this.db = null;
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// Promise is not null when a flush is IN PROGRESS
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this.flushPromise = null;
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// set if flush() is called whilst one is ongoing
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this.flushAgainPromise = null;
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}
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/**
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@ -165,10 +166,8 @@ class IndexedDBLogStore {
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* - If B doesn't wait for A's flush to complete, B will be missing the
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* contents of A's flush.
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* To protect against this, we set 'flushPromise' when a flush is ongoing.
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* Subsequent calls to flush() during this period will chain another flush.
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* This guarantees that we WILL do a brand new flush at some point in the
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* future. Once the flushes have finished, it's safe to clobber the promise
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* with a new one to prevent very deep promise chains from building up.
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* Subsequent calls to flush() during this period will chain another flush,
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* then keep returning that same chained flush.
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*
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* This guarantees that we will always eventually do a flush when flush() is
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* called.
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@ -178,14 +177,17 @@ class IndexedDBLogStore {
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flush() {
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// check if a flush() operation is ongoing
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if (this.flushPromise && this.flushPromise.isPending()) {
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// chain a flush operation after this one has completed to guarantee
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// that a complete flush() is done. This does mean that if there are
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// 3 calls to flush() in one go, the 2nd and 3rd promises will run
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// concurrently, but this is fine since they can safely race when
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// collecting logs.
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return this.flushPromise.then(() => {
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if (this.flushAgainPromise && this.flushAgainPromise.isPending()) {
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// this is the 3rd+ time we've called flush() : return the same
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// promise.
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return this.flushAgainPromise;
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}
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// queue up a flush to occur immediately after the pending one
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// completes.
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this.flushAgainPromise = this.flushPromise.then(() => {
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return this.flush();
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});
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return this.flushAgainPromise;
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}
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// there is no flush promise or there was but it has finished, so do
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// a brand new one, destroying the chain which may have been built up.
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