[#xx] Add support for SELECT-FILTER expressions

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

netmap/policy.go

@@ -560,6 +560,44 @@ func (p *PlacementPolicy) DecodeString(s string) error {
 	return nil
 }
 
+// SelectFilterExpr is an expression containing only selectors and filters.
+// It's useful to evaluate their effect before being used in a policy.
+type SelectFilterExpr struct {
+	cbf      uint32
+	selector *netmap.Selector
+	filters  []netmap.Filter
+}
+
+// DecodeString decodes a string into a SelectFilterExpr.
+// Returns an error if s is malformed.
+func DecodeSelectFilterString(s string) (*SelectFilterExpr, error) {
+	var v policyVisitor
+
+	input := antlr.NewInputStream(s)
+	lexer := parser.NewQueryLexer(input)
+	lexer.RemoveErrorListeners()
+	lexer.AddErrorListener(&v)
+	stream := antlr.NewCommonTokenStream(lexer, 0)
+
+	pp := parser.NewQuery(stream)
+	pp.BuildParseTrees = true
+
+	pp.RemoveErrorListeners()
+	pp.AddErrorListener(&v)
+	sfExpr := pp.SelectFilterExpr().Accept(&v)
+
+	if len(v.errors) != 0 {
+		return nil, v.errors[0]
+	}
+
+	parsed, ok := sfExpr.(*SelectFilterExpr)
+	if !ok {
+		return nil, fmt.Errorf("unexpected parsed instance type %T", sfExpr)
+	}
+
+	return parsed, nil
+}
+
 var (
 	// errUnknownFilter is returned when a value of FROM in a query is unknown.
 	errUnknownFilter = errors.New("filter not found")
@@ -636,6 +674,39 @@ func (p *policyVisitor) VisitPolicy(ctx *parser.PolicyContext) any {
 	return pl
 }
 
+func (p *policyVisitor) VisitSelectFilterExpr(ctx *parser.SelectFilterExprContext) any {
+	if len(p.errors) != 0 {
+		return nil
+	}
+
+	sfExpr := new(SelectFilterExpr)
+
+	if cbfStmt := ctx.CbfStmt(); cbfStmt != nil {
+		cbf, ok := cbfStmt.(*parser.CbfStmtContext).Accept(p).(uint32)
+		if !ok {
+			return nil
+		}
+		sfExpr.cbf = cbf
+	}
+
+	if selStmt := ctx.SelectStmt(); selStmt != nil {
+		sel, ok := selStmt.Accept(p).(*netmap.Selector)
+		if !ok {
+			return nil
+		}
+		sfExpr.selector = sel
+	}
+
+	filtStmts := ctx.AllFilterStmt()
+	sfExpr.filters = make([]netmap.Filter, 0, len(filtStmts))
+
+	for _, f := range filtStmts {
+		sfExpr.filters = append(sfExpr.filters, *f.Accept(p).(*netmap.Filter))
+	}
+
+	return sfExpr
+}
+
 func (p *policyVisitor) VisitCbfStmt(ctx *parser.CbfStmtContext) any {
 	cbf, err := strconv.ParseUint(ctx.GetBackupFactor().GetText(), 10, 32)
 	if err != nil {