first commit

pigo/internal/session/inherit.go

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+package session
+
+// Context inheritance plumbing for the "infinite context" feature (#483, built
+// on the header fields added by #480). A session created to *continue* another
+// session's collapsed context records two things on its header:
+//
+//   - ContextFrom: the source session id whose checkpoint holds the collapsed
+//     prefix (see runtime.LoadCheckpoint / CheckpointPath).
+//   - ContextWatermark: the message index up to which that prefix was collapsed;
+//     on resume/fork the runtime (#481) can load the source checkpoint and skip
+//     re-summarizing messages [0, ContextWatermark).
+//
+// This is deliberately side-effect-free header plumbing: it never touches the
+// memory dir or the runtime. It sits alongside ParentSession (which records
+// lineage) — inheritance additionally records *where the collapsed context came
+// from* so a forked/continued session need not re-summarize early context.
+
+// SetContextInheritance records on header that this session continues fromID's
+// collapsed context up to watermark. A non-positive watermark or an empty fromID
+// clears inheritance (both fields reset to their zero/omitempty state), so the
+// header round-trips as a session with no inherited checkpoint. This mirrors how
+// ParentSession is a plain header field set at creation time.
+func SetContextInheritance(header *SessionHeader, fromID string, watermark int) {
+	if header == nil {
+		return
+	}
+	if fromID == "" || watermark <= 0 {
+		header.ContextFrom = ""
+		header.ContextWatermark = 0
+		return
+	}
+	header.ContextFrom = fromID
+	header.ContextWatermark = watermark
+}
+
+// ContextInheritance reports the inherited-context source recorded on header:
+// the source session id, the watermark, and whether inheritance is set. It is
+// the read counterpart to SetContextInheritance — a convenience accessor the
+// runtime (#481) uses on resume/fork to decide whether to load the source
+// checkpoint. ok is true only when a non-empty source and a positive watermark
+// are both present, so a header with only one of the two (a malformed or
+// partially written record) reports ok=false rather than a half-configured
+// inheritance.
+func (h SessionHeader) ContextInheritance() (fromID string, watermark int, ok bool) {
+	if h.ContextFrom == "" || h.ContextWatermark <= 0 {
+		return "", 0, false
+	}
+	return h.ContextFrom, h.ContextWatermark, true
+}
+
+// HasContextInheritance reports whether header declares inherited collapsed
+// context. It is shorthand for the ok return of ContextInheritance.
+func (h SessionHeader) HasContextInheritance() bool {
+	_, _, ok := h.ContextInheritance()
+	return ok
+}