feat: implement irreversible vacuum drop table protection

[dantengsky]

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Summary

🤖 Generated with Claude Code

This PR implements a critical data integrity feature that prevents undrop operations on tables whose data may have been partially or fully cleaned up by vacuum processes. The core principle is: once vacuum has started for a retention period, tables dropped before that period can never be restored, ensuring users cannot accidentally restore tables with incomplete or inconsistent data.

Problem Statement

Previously, there was a dangerous race condition where:

1. User drops a table
2. Vacuum process starts cleaning up the table's data
3. User attempts undrop while vacuum is in progress
4. Undrop succeeds but table data is incomplete/corrupted

This could lead to silent data corruption and inconsistent database state.

Design Philosophy

Tenant-Level Global Watermark Design

We chose a tenant-level global vacuum watermark approach instead of per-table or per-database granularity for several key reasons:

1. Simplicity & Clarity: A single timestamp per tenant is easier to reason about, implement, and maintain
2. Sufficient for Use Case: Vacuum operations are typically tenant-wide administrative tasks, making global coordination natural
3. Consistent Semantics: All undrop operations within a tenant follow the same rules, reducing cognitive overhead
4. Performance: Single KV read/write per tenant vs. potentially thousands for per-table tracking
5. Smooth Evolution Path: The current design can be extended to table/DB-level granularity without breaking changes:
   • Future: VacuumRetentionIdent::new_table(tenant, db_id, table_id)
   • Current: VacuumRetentionIdent::new_global(tenant)
   • The underlying infrastructure supports both patterns seamlessly

Implementation Approach

1. Monotonic Timestamp Mechanism

VacuumWatermark {
    time: DateTime<Utc>, // Monotonically increasing, never decreases
}

• Vacuum Phase: Sets timestamp when vacuum starts for a retention period
• Undrop Phase: Compares table's drop_time against vacuum timestamp
• Safety Rule: drop_time <= vacuum_timestamp → undrop FORBIDDEN

2. Atomic Compare-and-Swap (CAS) Operations

fetch_set_vacuum_timestamp() // Returns OLD value, atomic update

• Uses crud_upsert_with for race-safe timestamp updates
• Handles concurrent vacuum operations safely through atomic CAS
• Only advances timestamp forward (monotonic property)
• Multiple vacuum processes may race, but updates remain consistent

3. Multi-Layer Concurrent Safety

// Transaction conditions handle race conditions safely
txn.condition.push(txn_cond_eq_seq(&vacuum_ident, vacuum_seq));

• Concurrent Operations Handled: Multiple vacuum/undrop operations can race
• Safe Resolution: Atomic transactions ensure consistent outcomes
• Prevents TOCTOU: Time-of-check-time-of-use vulnerabilities eliminated
• Read-Check-Write Pattern: Sequence-based validation prevents timing attacks

4. Fail-Safe Error Handling

• Critical Path Protection: Vacuum ABORTS if timestamp setting fails
• No Silent Failures: All errors are propagated and logged
• Consistency First: Never proceed with data cleanup without protection markers

Key Implementation Details

VacuumWatermark Structure (Rust)

pub struct VacuumWatermark {
    pub time: DateTime<Utc>, // Monotonic timestamp
}

Protobuf Serialization Format

message VacuumWatermark {  // Consistent naming with Rust struct
  uint64 ver = 100;        // Protocol versioning
  uint64 min_reader_ver = 101;
  string time = 1;         // Timestamp string
}

Integration Points

1. Vacuum Trigger: VacuumDropTablesInterpreter::execute2()
   • Sets watermark timestamp BEFORE any data cleanup begins
   • Operation fails if watermark cannot be established
2. Protection Check: handle_undrop_table() with early validation
   • Compares table drop time against vacuum watermark
   • Atomic transaction with sequence validation
3. Metadata Storage: VacuumRetentionIdent for tenant-global coordination

MetaStore Storage Details

The vacuum watermark is stored as a KV pair in the MetaStore:

Key Format:   __fd_vacuum_retention_ts/{tenant_name}
Key Example:  __fd_vacuum_retention_ts/default
Value Type:   VacuumWatermark (protobuf serialized)
Scope:        Global per tenant (one watermark per tenant)

Design Rationale: The tenant-scoped key format enables:

• Tenant Isolation: Each tenant has independent vacuum protection
• Simple Lookup: Single key read for undrop validation
• Future Extension: Key format supports adding database/table specificity later

Backwards Compatibility

• Protobuf v152: New version maintains compatibility with existing deployments
• Optional Semantics: Option<VacuumWatermark> handles "never set" vs "set to value" correctly
• Graceful Fallback: Existing undrop behavior preserved when no vacuum timestamp exists

Safety Guarantees

1. ✅ Primary Guarantee: Tables with potentially incomplete data cannot be restored
2. ✅ Temporal Consistency: Vacuum timestamp creates a clear "point of no return"
3. ✅ Concurrent Safety: Race conditions handled safely under high concurrency
4. ✅ Atomic Operations: Partial state updates impossible
5. ✅ Monotonic Progress: Protection level can only increase, never decrease
6. ✅ Fail-Safe Operation: Vacuum aborts if protection cannot be established

Critical Safety Flow

1. User initiates: VACUUM DROP TABLE
2. System sets: vacuum_timestamp = retention_boundary
3. ❌ If step 2 fails → ABORT (no data cleanup)
4. ✅ If step 2 succeeds → proceed with data cleanup
5. Later: User attempts UNDROP TABLE
6. System checks: table.drop_time <= vacuum_timestamp?
7. ❌ If true → REJECT (data may be incomplete)
8. ✅ If false → allow undrop

Undrop Behavior Matrix (Sample Scenarios)

Scenario	Vacuum Watermark	Table Drop Time	Undrop Result	Reason
Pre-vacuum State	None (never set)	Any time	✅ ALLOWED	No vacuum has run yet - data guaranteed safe
Post-vacuum Risk	Set to 2023-12-01 (example)	Dropped 2023-11-15 (example)	❌ BLOCKED	Drop predates vacuum - data may be cleaned
Post-vacuum Safe	Set to 2023-12-01 (example)	Dropped 2023-12-10 (example)	✅ ALLOWED	Drop postdates vacuum - data guaranteed intact

Key Insight: When no vacuum watermark exists, undrop operations are both safe and correct - it means no vacuum cleanup has ever been performed on this tenant, so all dropped table data remains intact. This also ensures smooth migration from older versions without breaking existing workflows.

State Flow Diagram

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│                 │    │                  │    │                 │
│   Table Active  │───▶│  Table Dropped   │───▶│ Vacuum Started  │
│                 │    │  (safe to undrop)│    │ (sets watermark)│
└─────────────────┘    └──────────────────┘    └─────────────────┘
         ▲                        │                       │
         │                        │                       ▼
         │                        │            ┌─────────────────┐
         │                        │            │                 │
         │                        │            │ Data Cleaning   │
         │                        │            │   In Progress   │
         │                        │            │                 │
         │                        │            └─────────────────┘
         │                        │                       │
         │                        ▼                       ▼
┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│                 │    │                  │    │                 │
│ Table Restored  │◀───│  Undrop Request  │───▶│ Undrop Blocked  │
│                 │    │                  │    │ (Data Unsafe)   │
└─────────────────┘    └──────────────────┘    └─────────────────┘
         ▲                        │
         │                        │
         └─── Vacuum watermark ───┘
              check passes
          (drop_time > watermark)


Vacuum Watermark Timeline Example:
═══════════════════════════════════════════════════════════════════

Timeline (Scenario: data_retention_time_in_days = 30):

Oct-15        Nov-01       Nov-20       Dec-01       Jan-05
│             │            │            │            │
TableA        │            TableB       VACUUM       UNDROP
Dropped       │            Dropped     EXECUTION    Requests
│             │                         (sets        │
│             │                         watermark    │
│             │                         = Nov-01)    │
│             │                                      │
│             │                                      └─ TableA: ❌ BLOCKED
│             │                                         (Oct-15 ≤ Nov-01)
│             │                          
│             │                                      └─ TableB: ✅ ALLOWED
│             │                                         (Nov-20 > Nov-01)
│             │                          
│             └─ Watermark boundary      
│                (retention cutoff)      
│                                        
└─ TableA dropped before watermark       
   (data potentially cleaned)            

Key insight: Watermark = vacuum_execution_time - data_retention_time_in_days

Legend:
  ❌ BLOCKED  = Undrop rejected (table dropped before vacuum, data potentially cleaned)
  ✅ ALLOWED  = Undrop permitted (table dropped after vacuum watermark, data safe)

Note: The watermark at Dec-01 creates a temporal protection boundary - any table
dropped before this time may have had its data cleaned by vacuum operations.

Implementation Logic Flow

VACUUM DROP TABLE Operation:
┌─────────────────────────────────────────────────────────────┐
│ VacuumDropTablesInterpreter::execute2()                    │
├─────────────────────────────────────────────────────────────┤
│ 1. Calculate retention_time = now() - retention_days       │
│ 2. if (!dry_run) {                                         │
│      try: fetch_set_vacuum_timestamp(tenant, retention_time)│
│      catch: ABORT vacuum (fail-safe)                       │
│    }                                                        │
│ 3. Proceed with data cleanup...                            │
└─────────────────────────────────────────────────────────────┘
                                │
                                ▼
┌─────────────────────────────────────────────────────────────┐
│ fetch_set_vacuum_timestamp() - Meta CRUD Operation         │
├─────────────────────────────────────────────────────────────┤
│ Conditions:                                                 │
│   - Atomic read-modify-write operation                     │
│   - Monotonic timestamp validation                         │
│                                                             │
│ Operations:                                                 │
│   - Read: current = kv_api.get(VacuumRetentionIdent)       │
│   - Check: new_time > current.time OR current.is_none()    │
│   - Write: kv_api.upsert(new_timestamp) if should_update   │
│                                                             │
│ Result:                                                     │
│   - Returns: Some(old_value) if updated, None if no change │
│   - Guarantees: Watermark only advances forward            │
│                                                             │
│ Note: Implementation details simplified for clarity         │
│       Uses MetaStore's CRUD operations                     │
└─────────────────────────────────────────────────────────────┘

UNDROP TABLE Operation:
┌─────────────────────────────────────────────────────────────┐
│ handle_undrop_table()                                       │
├─────────────────────────────────────────────────────────────┤
│ 1. Get table metadata (drop_time)                          │
│ 2. vacuum_watermark = kv_api.get_pb(VacuumRetentionIdent)  │
│ 3. Early validation:                                       │
│    match vacuum_watermark {                                │
│      None => continue,           // ✅ Legacy compatibility │
│      Some(wm) => {                                         │
│        if drop_time <= wm.time {                           │
│          return ERROR_RETENTION_GUARD  // ❌ Data unsafe   │
│        }                                                   │
│      }                                                     │
│    }                                                       │
│ 4. Begin atomic transaction with seq check...              │
└─────────────────────────────────────────────────────────────┘
                                │
                                ▼
┌─────────────────────────────────────────────────────────────┐
│ Atomic Transaction - Mini KV Transaction (Concurrent Safe) │
├─────────────────────────────────────────────────────────────┤
│ Conditions:                                                 │
│   - table_meta.seq == expected_seq                         │
│   - vacuum_watermark.seq == expected_seq  // Key Safety!   │
│   - database_meta.seq == expected_seq                      │
│                                                             │
│ Operations:                                                 │
│   - table_meta.drop_on = None                              │
│   - Update database references                             │
│   - Restore table name mappings                            │
│                                                             │
│ Result:                                                     │
│   - SUCCESS: Table restored safely                         │
│   - FAILURE: Concurrent vacuum detected, retry/abort       │
│                                                             │
│ Note: Uses MetaStore KV transactions                       │
│       All conditions must pass for transaction to succeed  │
└─────────────────────────────────────────────────────────────┘

Concurrent Safety Example

Timeline: T1 → T2 → T3 → T4 → T5

T1: Table dropped
T2: Vacuum process A reads current watermark (None)
T3: Vacuum process B reads current watermark (None)
T4: Process A updates watermark to retention_time → SUCCESS (first update)
T5: Process B attempts same retention_time → NO UPDATE NEEDED (timestamp unchanged)
    ↳ Both operations succeed: A sets the watermark, B detects no change needed

Alternative scenario - Different timestamps:
T4: Process A updates watermark to time_1 → SUCCESS
T5: Process B updates watermark to time_2 > time_1 → SUCCESS (monotonic advance)
    ↳ Watermark advances monotonically: None → time_1 → time_2

Later: Undrop reads watermark with seq, includes seq check in transaction
       If concurrent vacuum updates watermark during undrop → transaction fails

Test Coverage

• Unit Tests: Core API behavior and monotonic property validation
• Integration Tests: End-to-end vacuum-undrop workflows
• Concurrency Tests: Race condition handling validation
• Compatibility Tests: Protobuf serialization/deserialization (v152)
• Error Handling: Failure mode validation and fail-safe behavior
• Timing Tests: Protection boundary verification

Files Modified

• Core Logic: garbage_collection_api.rs, schema_api.rs
• Integration: interpreter_vacuum_drop_tables.rs
• Data Model: vacuum_watermark.rs, vacuum_retention_ident.rs
• Serialization: vacuum_retention_from_to_protobuf_impl.rs
• Protobuf: vacuum_watermark.proto
• Error Handling: app_error.rs, exception_code.rs
• Tests: schema_api_test_suite.rs, v152_vacuum_retention.rs

This implementation provides rock-solid data integrity guarantees while maintaining performance and operational simplicity. The tenant-level design offers the right balance of safety, simplicity, and future extensibility.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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