Define DocumentDB-specific specs for engine-dependent features

[eerxuan]

Problem

The functional test repo currently validates behavior against MongoDB by default, since DocumentDB doesn't have official specs for all features yet. Most features aim for compatibility, but several features are inherently engine-specific and produce different output. These need DocumentDB-specific specs.

Running the $collStats tests (PR #144) against DocumentDB (open-source, via Docker) shows 178/252 failures — not because of bugs, but because the output structure differs.

Engine-Specific Features Needing DocumentDB Specs

$collStats (PR #144)

• host and localTime fields: present in MongoDB, absent in DocumentDB
• storageStats.capped: present in MongoDB, absent in DocumentDB
• storageStats.wiredTiger: MongoDB-specific storage engine details
• latencyStats: works in MongoDB, returns error 115 in DocumentDB
• queryExecStats: works in MongoDB, returns error 115 in DocumentDB
• null sub-option handling: MongoDB treats as "omitted", DocumentDB rejects with type error

$indexStats

• Output fields (host, accesses, shard) likely differ
• Needs verification against DocumentDB

explain plans

• Query plan stages (IXSCAN, COLLSCAN, etc.) are engine-specific
• Execution stats fields differ
• Index usage reporting differs

$currentOp

• Operation details are engine-specific
• Connection/lock information differs

$planCacheStats

• Plan cache implementation is engine-specific

What's Needed

For each feature above:

1. Run tests against DocumentDB to capture current behavior
2. Engine team confirms for each divergence: intentional behavior, not yet implemented, or bug
3. Define the DocumentDB spec based on confirmed intended behavior
4. Update tests with engine-specific expectations for confirmed divergences

Context

• The functional tests define the DocumentDB spec (ground truth)
• Current behavior can serve as draft spec, but needs engine team confirmation
• Related: CI design discussion in Functional test infrastructure: baseline tracking, CI gating, and developer workflow documentdb#567

[eerxuan]

Updated clarification: For each divergence found when running tests against DocumentDB, the engine team needs to confirm one of:

1. Intentional — this is the desired DocumentDB behavior, tests should expect it
2. Not yet implemented — this behavior will be added, track as a known gap
3. Bug — current behavior is incorrect, should be fixed

The functional tests should only codify confirmed intended behavior as the spec.

[danielfrankcom]

This is a blocker for $collMod, we will hold off working on any tests for that operator until we can resolve this.

Ref: #26