[2.0.0.rc-1.0] fix(directory): Fallback to previous subdirectory orde…

…ring on mismatch (microsoft#19539)

## Description

Relaxes a consistency assert in SharedDirectory's subdirectory ordering
scheme. This was a recent feature implemented in microsoft#17816.

Production telemetry shows hits for this assert which we don't yet
understand. However, nobody yet relies on correctness of subdirectory
ordering, so falling back to previous behavior while we root cause is a
relatively safe course of action.

Since we haven't been able to reproduce this failure mode using unit
tests even after scrutiny, this also adds a telemetry log (once per
module load) with some basic statistics about the mismatch.

Cherry-pick of microsoft#19536

Co-authored-by: Abram Sanderson <[email protected]>

packages/dds/map/src/directory.ts

@@ -5,7 +5,7 @@
 
 import { assert } from "@fluidframework/core-utils";
 import { TypedEventEmitter } from "@fluid-internal/client-utils";
-import { UsageError } from "@fluidframework/telemetry-utils";
+import { ITelemetryLoggerExt, UsageError } from "@fluidframework/telemetry-utils";
 import { readAndParse } from "@fluidframework/driver-utils";
 import { ISequencedDocumentMessage, MessageType } from "@fluidframework/protocol-definitions";
 import {
@@ -433,6 +433,10 @@ class DirectoryCreationTracker {
 		}, keys);
 		return keys;
 	}
+
+	public get size(): number {
+		return this.keyToIndex.size;
+	}
 }
 
 /**
@@ -498,6 +502,7 @@ export class SharedDirectory
 		this.runtime,
 		this.serializer,
 		posix.sep,
+		this.logger,
 	);
 
 	/**
@@ -810,6 +815,7 @@ export class SharedDirectory
 							this.runtime,
 							this.serializer,
 							posix.join(currentSubDir.absolutePath, subdirName),
+							this.logger,
 						);
 						currentSubDir.populateSubDirectory(subdirName, newSubDir);
 						// Record the newly inserted subdirectory to the creation tracker
@@ -1219,6 +1225,8 @@ function assertNonNullClientId(clientId: string | null): asserts clientId is str
 	assert(clientId !== null, 0x6af /* client id should never be null */);
 }
 
+let hasLoggedDirectoryInconsistency = false;
+
 /**
  * Node of the directory tree.
  * @sealed
@@ -1309,6 +1317,7 @@ class SubDirectory extends TypedEventEmitter<IDirectoryEvents> implements IDirec
 		private readonly runtime: IFluidDataStoreRuntime,
 		private readonly serializer: IFluidSerializer,
 		public readonly absolutePath: string,
+		private readonly logger: ITelemetryLoggerExt,
 	) {
 		super();
 		this.localCreationSeqTracker = new DirectoryCreationTracker();
@@ -1510,10 +1519,24 @@ class SubDirectory extends TypedEventEmitter<IDirectoryEvents> implements IDirec
 
 		const subdirNames = [...ackedSubdirsInOrder, ...localSubdirsInOrder];
 
-		assert(
-			subdirNames.length === this._subdirectories.size,
-			0x85c /* The count of keys for iteration should be consistent with the size of actual data */,
-		);
+		if (subdirNames.length !== this._subdirectories.size) {
+			// TODO: AB#7022: Hitting this block indicates that the eventual consistency scheme for ordering subdirectories
+			// has failed. Fall back to previous directory behavior, which didn't guarantee ordering.
+			// It's not currently clear how to reach this state, so log some diagnostics to help understand the issue.
+			// This whole block should eventually be replaced by an assert that the two sizes align.
+			if (!hasLoggedDirectoryInconsistency) {
+				this.logger.sendTelemetryEvent({
+					eventName: "inconsistentSubdirectoryOrdering",
+					localKeyCount: this.localCreationSeqTracker.size,
+					ackedKeyCount: this.ackedCreationSeqTracker.size,
+					subdirNamesLength: subdirNames.length,
+					subdirectoriesSize: this._subdirectories.size,
+				});
+				hasLoggedDirectoryInconsistency = true;
+			}
+
+			return this._subdirectories.entries();
+		}
 
 		const entriesIterator = {
 			index: 0,
@@ -2596,6 +2619,7 @@ class SubDirectory extends TypedEventEmitter<IDirectoryEvents> implements IDirec
 				this.runtime,
 				this.serializer,
 				absolutePath,
+				this.logger,
 			);
 			/**
 			 * Store the sequnce numbers of newly created subdirectory to the proper creation tracker, based