archivmail/internal/index/manticore_snippet.go

package index

import (
	"fmt"
	"log"
	"strings"
)

// matchFieldOrder defines the priority in which fields are probed when
// determining which one of them caused a hit. Order is taken from PROJ-44:
// subject > body > attachment_text > attachment_names > from_addr > to_addr.
//
// The list is intentionally small — the per-hit cost is one extra Manticore
// SELECT per probed field until a match is found. With typical page sizes
// (<=50) and at most 6 probes per hit this stays well below 300 cheap queries
// per request.
var matchFieldOrder = []string{
	"subject",
	"body",
	"attachment_text",
	"attachment_names",
	"from_addr",
	"to_addr",
}

// enrichHitsWithSnippets fills Hit.Snippet and Hit.MatchField for each hit in
// place. Errors are logged but never propagated — a hit without snippet is
// still a valid hit (PROJ-44 edge case: "Snippet-Generierung schlägt fehl …
// kein Hard-Error").
func (idx *manticoreIndex) enrichHitsWithSnippets(hits []Hit, query string) {
	for i := range hits {
		field := idx.detectMatchField(hits[i].ID, query)
		if field != "" {
			hits[i].MatchField = field
		}

		snip, err := idx.buildSnippet(hits[i].ID, query, field)
		if err != nil {
			log.Printf("manticore snippet: mail=%s err=%v", hits[i].ID, err)
			continue
		}
		hits[i].Snippet = snip
	}
}

// detectMatchField runs a small MATCH() probe per field in matchFieldOrder
// until one returns a row. Returns "" when no field matches — that can happen
// for filter-only matches (e.g. date range only) or when the query terms
// require multiple fields combined.
func (idx *manticoreIndex) detectMatchField(mailID, query string) string {
	rowID := hashMailID(mailID)
	escaped := escapeManticoreMatch(query)

	for _, field := range matchFieldOrder {
		// SAFETY: field is from a static allow-list above, never user input.
		matchExpr := fmt.Sprintf("@%s %s", field, escaped)
		q := fmt.Sprintf(
			`SELECT id FROM %s WHERE id = ? AND MATCH(?) LIMIT 1`,
			idx.table,
		)
		var got int64
		err := idx.db.QueryRow(q, rowID, matchExpr).Scan(&got)
		if err == nil {
			return field
		}
	}
	return ""
}

// buildSnippet returns an excerpt of the matched field with <b>...</b>
// markers around match words via Manticore's CALL SNIPPETS(...) function.
// When matchField is empty, "body" is used as a sensible default.
//
// Manticore's SNIPPETS expects the source text as the first argument, the
// table name as the second, and the query as the third. We fetch the source
// column for the hit first (small SELECT) and then call SNIPPETS in a second
// query. Two roundtrips per hit is acceptable for typical page sizes.
func (idx *manticoreIndex) buildSnippet(mailID, query, matchField string) (string, error) {
	field := matchField
	if field == "" {
		field = "body"
	}
	// Whitelist guard — never interpolate user-provided field names.
	allowed := false
	for _, f := range matchFieldOrder {
		if f == field {
			allowed = true
			break
		}
	}
	if !allowed {
		return "", fmt.Errorf("manticore snippet: invalid field %q", field)
	}

	rowID := hashMailID(mailID)
	source, err := idx.fetchFieldText(field, rowID)
	if err != nil {
		return "", err
	}
	source = strings.TrimSpace(source)
	if source == "" {
		return "", nil
	}

	// SELECT SNIPPET(text, query) FROM table.
	// Manticore 25+ supports SNIPPET() as a SELECT expression; CALL SNIPPETS
	// returns a different packet type that the Go MySQL driver mishandles
	// ("malformed packet"). Default markers are <b>/<b/>, no options needed.
	q := fmt.Sprintf(`SELECT SNIPPET(?, ?) FROM %s LIMIT 1`, idx.table)
	row := idx.db.QueryRow(q, source, query)
	var snippet string
	if err := row.Scan(&snippet); err != nil {
		return "", fmt.Errorf("call snippets %s: %w", idx.table, err)
	}
	return snippet, nil
}

// fetchFieldText loads a single text column for one row. field must already
// be validated against matchFieldOrder by the caller.
func (idx *manticoreIndex) fetchFieldText(field string, rowID int64) (string, error) {
	q := fmt.Sprintf(`SELECT %s FROM %s WHERE id = ? LIMIT 1`, field, idx.table)
	var text string
	if err := idx.db.QueryRow(q, rowID).Scan(&text); err != nil {
		return "", fmt.Errorf("fetch %s: %w", field, err)
	}
	return text, nil
}