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| 1 | +use super::*; |
| 2 | + |
| 3 | +// Return value of `Session::reconstruct_history_from_rollout`, bundling the rebuilt history with |
| 4 | +// the resume/fork hydration metadata derived from the same replay. |
| 5 | +#[derive(Debug)] |
| 6 | +pub(super) struct RolloutReconstruction { |
| 7 | + pub(super) history: Vec<ResponseItem>, |
| 8 | + pub(super) previous_model: Option<String>, |
| 9 | + pub(super) reference_context_item: Option<TurnContextItem>, |
| 10 | +} |
| 11 | + |
| 12 | +#[derive(Debug, Default)] |
| 13 | +enum TurnReferenceContextItem { |
| 14 | + /// No `TurnContextItem` has been seen for this replay span yet. |
| 15 | + /// |
| 16 | + /// This differs from `Cleared`: `NeverSet` means there is no evidence this turn ever |
| 17 | + /// established a baseline, while `Cleared` means a baseline existed and a later compaction |
| 18 | + /// invalidated it. Only the latter must emit an explicit clearing segment for resume/fork |
| 19 | + /// hydration. |
| 20 | + #[default] |
| 21 | + NeverSet, |
| 22 | + /// A previously established baseline was invalidated by later compaction. |
| 23 | + Cleared, |
| 24 | + /// The latest baseline established by this replay span. |
| 25 | + Latest(Box<TurnContextItem>), |
| 26 | +} |
| 27 | + |
| 28 | +#[derive(Debug, Default)] |
| 29 | +struct ActiveReplaySegment<'a> { |
| 30 | + turn_id: Option<String>, |
| 31 | + counts_as_user_turn: bool, |
| 32 | + previous_model: Option<String>, |
| 33 | + reference_context_item: TurnReferenceContextItem, |
| 34 | + base_replacement_history: Option<&'a [ResponseItem]>, |
| 35 | +} |
| 36 | + |
| 37 | +fn turn_ids_are_compatible(active_turn_id: Option<&str>, item_turn_id: Option<&str>) -> bool { |
| 38 | + active_turn_id |
| 39 | + .is_none_or(|turn_id| item_turn_id.is_none_or(|item_turn_id| item_turn_id == turn_id)) |
| 40 | +} |
| 41 | + |
| 42 | +fn finalize_active_segment<'a>( |
| 43 | + active_segment: ActiveReplaySegment<'a>, |
| 44 | + base_replacement_history: &mut Option<&'a [ResponseItem]>, |
| 45 | + previous_model: &mut Option<String>, |
| 46 | + reference_context_item: &mut TurnReferenceContextItem, |
| 47 | + pending_rollback_turns: &mut usize, |
| 48 | +) { |
| 49 | + // Thread rollback drops the newest surviving real user-message boundaries. In replay, that |
| 50 | + // means skipping the next finalized segments that contain a non-contextual |
| 51 | + // `EventMsg::UserMessage`. |
| 52 | + if *pending_rollback_turns > 0 { |
| 53 | + if active_segment.counts_as_user_turn { |
| 54 | + *pending_rollback_turns -= 1; |
| 55 | + } |
| 56 | + return; |
| 57 | + } |
| 58 | + |
| 59 | + // A surviving replacement-history checkpoint is a complete history base. Once we |
| 60 | + // know the newest surviving one, older rollout items do not affect rebuilt history. |
| 61 | + if base_replacement_history.is_none() |
| 62 | + && let Some(segment_base_replacement_history) = active_segment.base_replacement_history |
| 63 | + { |
| 64 | + *base_replacement_history = Some(segment_base_replacement_history); |
| 65 | + } |
| 66 | + |
| 67 | + // `previous_model` comes from the newest surviving user turn that established one. |
| 68 | + if previous_model.is_none() && active_segment.counts_as_user_turn { |
| 69 | + *previous_model = active_segment.previous_model; |
| 70 | + } |
| 71 | + |
| 72 | + // `reference_context_item` comes from the newest surviving user turn baseline, or |
| 73 | + // from a surviving compaction that explicitly cleared that baseline. |
| 74 | + if matches!(reference_context_item, TurnReferenceContextItem::NeverSet) |
| 75 | + && (active_segment.counts_as_user_turn |
| 76 | + || matches!( |
| 77 | + active_segment.reference_context_item, |
| 78 | + TurnReferenceContextItem::Cleared |
| 79 | + )) |
| 80 | + { |
| 81 | + *reference_context_item = active_segment.reference_context_item; |
| 82 | + } |
| 83 | +} |
| 84 | + |
| 85 | +impl Session { |
| 86 | + pub(super) async fn reconstruct_history_from_rollout( |
| 87 | + &self, |
| 88 | + turn_context: &TurnContext, |
| 89 | + rollout_items: &[RolloutItem], |
| 90 | + ) -> RolloutReconstruction { |
| 91 | + // Replay metadata should already match the shape of the future lazy reverse loader, even |
| 92 | + // while history materialization still uses an eager bridge. Scan newest-to-oldest, |
| 93 | + // stopping once a surviving replacement-history checkpoint and the required resume metadata |
| 94 | + // are both known; then replay only the buffered surviving tail forward to preserve exact |
| 95 | + // history semantics. |
| 96 | + let mut base_replacement_history: Option<&[ResponseItem]> = None; |
| 97 | + let mut previous_model = None; |
| 98 | + let mut reference_context_item = TurnReferenceContextItem::NeverSet; |
| 99 | + // Rollback is "drop the newest N user turns". While scanning in reverse, that becomes |
| 100 | + // "skip the next N user-turn segments we finalize". |
| 101 | + let mut pending_rollback_turns = 0usize; |
| 102 | + // Borrowed suffix of rollout items newer than the newest surviving replacement-history |
| 103 | + // checkpoint. If no such checkpoint exists, this remains the full rollout. |
| 104 | + let mut rollout_suffix = rollout_items; |
| 105 | + // Reverse replay accumulates rollout items into the newest in-progress turn segment until |
| 106 | + // we hit its matching `TurnStarted`, at which point the segment can be finalized. |
| 107 | + let mut active_segment: Option<ActiveReplaySegment<'_>> = None; |
| 108 | + |
| 109 | + for (index, item) in rollout_items.iter().enumerate().rev() { |
| 110 | + match item { |
| 111 | + RolloutItem::Compacted(compacted) => { |
| 112 | + let active_segment = |
| 113 | + active_segment.get_or_insert_with(ActiveReplaySegment::default); |
| 114 | + // Looking backward, compaction clears any older baseline unless a newer |
| 115 | + // `TurnContextItem` in this same segment has already re-established it. |
| 116 | + if matches!( |
| 117 | + active_segment.reference_context_item, |
| 118 | + TurnReferenceContextItem::NeverSet |
| 119 | + ) { |
| 120 | + active_segment.reference_context_item = TurnReferenceContextItem::Cleared; |
| 121 | + } |
| 122 | + if active_segment.base_replacement_history.is_none() |
| 123 | + && let Some(replacement_history) = &compacted.replacement_history |
| 124 | + { |
| 125 | + active_segment.base_replacement_history = Some(replacement_history); |
| 126 | + rollout_suffix = &rollout_items[index + 1..]; |
| 127 | + } |
| 128 | + } |
| 129 | + RolloutItem::EventMsg(EventMsg::ThreadRolledBack(rollback)) => { |
| 130 | + pending_rollback_turns = pending_rollback_turns |
| 131 | + .saturating_add(usize::try_from(rollback.num_turns).unwrap_or(usize::MAX)); |
| 132 | + } |
| 133 | + RolloutItem::EventMsg(EventMsg::TurnComplete(event)) => { |
| 134 | + let active_segment = |
| 135 | + active_segment.get_or_insert_with(ActiveReplaySegment::default); |
| 136 | + // Reverse replay often sees `TurnComplete` before any turn-scoped metadata. |
| 137 | + // Capture the turn id early so later `TurnContext` / abort items can match it. |
| 138 | + if active_segment.turn_id.is_none() { |
| 139 | + active_segment.turn_id = Some(event.turn_id.clone()); |
| 140 | + } |
| 141 | + } |
| 142 | + RolloutItem::EventMsg(EventMsg::TurnAborted(event)) => { |
| 143 | + if let Some(active_segment) = active_segment.as_mut() { |
| 144 | + if active_segment.turn_id.is_none() |
| 145 | + && let Some(turn_id) = &event.turn_id |
| 146 | + { |
| 147 | + active_segment.turn_id = Some(turn_id.clone()); |
| 148 | + } |
| 149 | + } else if let Some(turn_id) = &event.turn_id { |
| 150 | + active_segment = Some(ActiveReplaySegment { |
| 151 | + turn_id: Some(turn_id.clone()), |
| 152 | + ..Default::default() |
| 153 | + }); |
| 154 | + } |
| 155 | + } |
| 156 | + RolloutItem::EventMsg(EventMsg::UserMessage(_)) => { |
| 157 | + let active_segment = |
| 158 | + active_segment.get_or_insert_with(ActiveReplaySegment::default); |
| 159 | + active_segment.counts_as_user_turn = true; |
| 160 | + } |
| 161 | + RolloutItem::TurnContext(ctx) => { |
| 162 | + let active_segment = |
| 163 | + active_segment.get_or_insert_with(ActiveReplaySegment::default); |
| 164 | + // `TurnContextItem` can attach metadata to an existing segment, but only a |
| 165 | + // real `UserMessage` event should make the segment count as a user turn. |
| 166 | + if active_segment.turn_id.is_none() { |
| 167 | + active_segment.turn_id = ctx.turn_id.clone(); |
| 168 | + } |
| 169 | + if turn_ids_are_compatible( |
| 170 | + active_segment.turn_id.as_deref(), |
| 171 | + ctx.turn_id.as_deref(), |
| 172 | + ) { |
| 173 | + active_segment.previous_model = Some(ctx.model.clone()); |
| 174 | + if matches!( |
| 175 | + active_segment.reference_context_item, |
| 176 | + TurnReferenceContextItem::NeverSet |
| 177 | + ) { |
| 178 | + active_segment.reference_context_item = |
| 179 | + TurnReferenceContextItem::Latest(Box::new(ctx.clone())); |
| 180 | + } |
| 181 | + } |
| 182 | + } |
| 183 | + RolloutItem::EventMsg(EventMsg::TurnStarted(event)) => { |
| 184 | + // `TurnStarted` is the oldest boundary of the active reverse segment. |
| 185 | + if active_segment.as_ref().is_some_and(|active_segment| { |
| 186 | + turn_ids_are_compatible( |
| 187 | + active_segment.turn_id.as_deref(), |
| 188 | + Some(event.turn_id.as_str()), |
| 189 | + ) |
| 190 | + }) && let Some(active_segment) = active_segment.take() |
| 191 | + { |
| 192 | + finalize_active_segment( |
| 193 | + active_segment, |
| 194 | + &mut base_replacement_history, |
| 195 | + &mut previous_model, |
| 196 | + &mut reference_context_item, |
| 197 | + &mut pending_rollback_turns, |
| 198 | + ); |
| 199 | + } |
| 200 | + } |
| 201 | + RolloutItem::ResponseItem(_) |
| 202 | + | RolloutItem::EventMsg(_) |
| 203 | + | RolloutItem::SessionMeta(_) => {} |
| 204 | + } |
| 205 | + |
| 206 | + if base_replacement_history.is_some() |
| 207 | + && previous_model.is_some() |
| 208 | + && !matches!(reference_context_item, TurnReferenceContextItem::NeverSet) |
| 209 | + { |
| 210 | + // At this point we have both eager resume metadata values and the replacement- |
| 211 | + // history base for the surviving tail, so older rollout items cannot affect this |
| 212 | + // result. |
| 213 | + break; |
| 214 | + } |
| 215 | + } |
| 216 | + |
| 217 | + if let Some(active_segment) = active_segment.take() { |
| 218 | + finalize_active_segment( |
| 219 | + active_segment, |
| 220 | + &mut base_replacement_history, |
| 221 | + &mut previous_model, |
| 222 | + &mut reference_context_item, |
| 223 | + &mut pending_rollback_turns, |
| 224 | + ); |
| 225 | + } |
| 226 | + |
| 227 | + let mut history = ContextManager::new(); |
| 228 | + let mut saw_legacy_compaction_without_replacement_history = false; |
| 229 | + if let Some(base_replacement_history) = base_replacement_history { |
| 230 | + history.replace(base_replacement_history.to_vec()); |
| 231 | + } |
| 232 | + // Materialize exact history semantics from the replay-derived suffix. The eventual lazy |
| 233 | + // design should keep this same replay shape, but drive it from a resumable reverse source |
| 234 | + // instead of an eagerly loaded `&[RolloutItem]`. |
| 235 | + for item in rollout_suffix { |
| 236 | + match item { |
| 237 | + RolloutItem::ResponseItem(response_item) => { |
| 238 | + history.record_items( |
| 239 | + std::iter::once(response_item), |
| 240 | + turn_context.truncation_policy, |
| 241 | + ); |
| 242 | + } |
| 243 | + RolloutItem::Compacted(compacted) => { |
| 244 | + if let Some(replacement_history) = &compacted.replacement_history { |
| 245 | + // This should actually never happen, because the reverse loop above (to build rollout_suffix) |
| 246 | + // should stop before any compaction that has Some replacement_history |
| 247 | + history.replace(replacement_history.clone()); |
| 248 | + } else { |
| 249 | + saw_legacy_compaction_without_replacement_history = true; |
| 250 | + // Legacy rollouts without `replacement_history` should rebuild the |
| 251 | + // historical TurnContext at the correct insertion point from persisted |
| 252 | + // `TurnContextItem`s. These are rare enough that we currently just clear |
| 253 | + // `reference_context_item`, reinject canonical context at the end of the |
| 254 | + // resumed conversation, and accept the temporary out-of-distribution |
| 255 | + // prompt shape. |
| 256 | + // TODO(ccunningham): if we drop support for None replacement_history compaction items, |
| 257 | + // we can get rid of this second loop entirely and just build `history` directly in the first loop. |
| 258 | + let user_messages = collect_user_messages(history.raw_items()); |
| 259 | + let rebuilt = compact::build_compacted_history( |
| 260 | + Vec::new(), |
| 261 | + &user_messages, |
| 262 | + &compacted.message, |
| 263 | + ); |
| 264 | + history.replace(rebuilt); |
| 265 | + } |
| 266 | + } |
| 267 | + RolloutItem::EventMsg(EventMsg::ThreadRolledBack(rollback)) => { |
| 268 | + history.drop_last_n_user_turns(rollback.num_turns); |
| 269 | + } |
| 270 | + RolloutItem::EventMsg(_) |
| 271 | + | RolloutItem::TurnContext(_) |
| 272 | + | RolloutItem::SessionMeta(_) => {} |
| 273 | + } |
| 274 | + } |
| 275 | + |
| 276 | + let reference_context_item = match reference_context_item { |
| 277 | + TurnReferenceContextItem::NeverSet | TurnReferenceContextItem::Cleared => None, |
| 278 | + TurnReferenceContextItem::Latest(turn_reference_context_item) => { |
| 279 | + Some(*turn_reference_context_item) |
| 280 | + } |
| 281 | + }; |
| 282 | + let reference_context_item = if saw_legacy_compaction_without_replacement_history { |
| 283 | + None |
| 284 | + } else { |
| 285 | + reference_context_item |
| 286 | + }; |
| 287 | + |
| 288 | + RolloutReconstruction { |
| 289 | + history: history.raw_items().to_vec(), |
| 290 | + previous_model, |
| 291 | + reference_context_item, |
| 292 | + } |
| 293 | + } |
| 294 | +} |
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