parser: Allow parsing arbitrary functions

In Thanos we would like to start experimenting with custom functions that are
currently not part of the PromQL spec. We would do this by adding an implementation
for those functions in the Thanos engine: https://github.com/thanos-community/promql-engine and allow
users to decide which engine they want to use on a per-query basis.

Since we use the PromQL parser from Prometheus, injecting functions in the global `Functions` variable
would mean they also become available for the Prometheus engine. To avoid this side-effect, this commit
exposes a Parser interface in which the supported functions can be injected as an option. If not functions
are injected, the parser implementation will default to the functions defined in the global Functions variable.

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

promql/parser/functions.go

@@ -387,7 +387,7 @@ var Functions = map[string]*Function{
 }
 
 // getFunction returns a predefined Function object for the given name.
-func getFunction(name string) (*Function, bool) {
-	function, ok := Functions[name]
+func getFunction(name string, functions map[string]*Function) (*Function, bool) {
+	function, ok := functions[name]
 	return function, ok
 }

promql/parser/generated_parser.y

@@ -339,7 +339,7 @@ grouping_label  : maybe_label
 
 function_call   : IDENTIFIER function_call_body
                         {
-                        fn, exist := getFunction($1.Val)
+                        fn, exist := getFunction($1.Val, yylex.(*parser).functions)
                         if !exist{
                                 yylex.(*parser).addParseErrf($1.PositionRange(),"unknown function with name %q", $1.Val)
                         }

promql/parser/generated_parser.y.go

@@ -1210,7 +1210,7 @@ yydefault:
 		yyDollar = yyS[yypt-2 : yypt+1]
 //line promql/parser/generated_parser.y:341
 		{
-			fn, exist := getFunction(yyDollar[1].item.Val)
+			fn, exist := getFunction(yyDollar[1].item.Val, yylex.(*parser).functions)
 			if !exist {
 				yylex.(*parser).addParseErrf(yyDollar[1].item.PositionRange(), "unknown function with name %q", yyDollar[1].item.Val)
 			}

promql/parser/parse.go

@@ -37,12 +37,20 @@ var parserPool = sync.Pool{
 	},
 }
 
+type Parser interface {
+	ParseExpr() (Expr, error)
+	Close()
+}
+
 type parser struct {
 	lex Lexer
 
 	inject    ItemType
 	injecting bool
 
+	// functions contains all functions supported by the parser instance.
+	functions map[string]*Function
+
 	// Everytime an Item is lexed that could be the end
 	// of certain expressions its end position is stored here.
 	lastClosing Pos
@@ -53,6 +61,62 @@ type parser struct {
 	parseErrors           ParseErrors
 }
 
+type Opt func(p *parser)
+
+func WithFunctions(functions map[string]*Function) Opt {
+	return func(p *parser) {
+		p.functions = functions
+	}
+}
+
+// NewParser returns a new parser.
+func NewParser(input string, opts ...Opt) *parser {
+	p := parserPool.Get().(*parser)
+
+	p.functions = Functions
+	p.injecting = false
+	p.parseErrors = nil
+	p.generatedParserResult = nil
+
+	// Clear lexer struct before reusing.
+	p.lex = Lexer{
+		input: input,
+		state: lexStatements,
+	}
+
+	// Apply user define options.
+	for _, opt := range opts {
+		opt(p)
+	}
+
+	return p
+}
+
+func (p *parser) ParseExpr() (expr Expr, err error) {
+	defer p.recover(&err)
+
+	parseResult := p.parseGenerated(START_EXPRESSION)
+
+	if parseResult != nil {
+		expr = parseResult.(Expr)
+	}
+
+	// Only typecheck when there are no syntax errors.
+	if len(p.parseErrors) == 0 {
+		p.checkAST(expr)
+	}
+
+	if len(p.parseErrors) != 0 {
+		err = p.parseErrors
+	}
+
+	return expr, err
+}
+
+func (p *parser) Close() {
+	defer parserPool.Put(p)
+}
+
 // ParseErr wraps a parsing error with line and position context.
 type ParseErr struct {
 	PositionRange PositionRange
@@ -105,32 +169,15 @@ func (errs ParseErrors) Error() string {
 
 // ParseExpr returns the expression parsed from the input.
 func ParseExpr(input string) (expr Expr, err error) {
-	p := newParser(input)
-	defer parserPool.Put(p)
-	defer p.recover(&err)
-
-	parseResult := p.parseGenerated(START_EXPRESSION)
-
-	if parseResult != nil {
-		expr = parseResult.(Expr)
-	}
-
-	// Only typecheck when there are no syntax errors.
-	if len(p.parseErrors) == 0 {
-		p.checkAST(expr)
-	}
-
-	if len(p.parseErrors) != 0 {
-		err = p.parseErrors
-	}
-
-	return expr, err
+	p := NewParser(input)
+	defer p.Close()
+	return p.ParseExpr()
 }
 
 // ParseMetric parses the input into a metric
 func ParseMetric(input string) (m labels.Labels, err error) {
-	p := newParser(input)
-	defer parserPool.Put(p)
+	p := NewParser(input)
+	defer p.Close()
 	defer p.recover(&err)
 
 	parseResult := p.parseGenerated(START_METRIC)
@@ -148,8 +195,8 @@ func ParseMetric(input string) (m labels.Labels, err error) {
 // ParseMetricSelector parses the provided textual metric selector into a list of
 // label matchers.
 func ParseMetricSelector(input string) (m []*labels.Matcher, err error) {
-	p := newParser(input)
-	defer parserPool.Put(p)
+	p := NewParser(input)
+	defer p.Close()
 	defer p.recover(&err)
 
 	parseResult := p.parseGenerated(START_METRIC_SELECTOR)
@@ -164,22 +211,6 @@ func ParseMetricSelector(input string) (m []*labels.Matcher, err error) {
 	return m, err
 }
 
-// newParser returns a new parser.
-func newParser(input string) *parser {
-	p := parserPool.Get().(*parser)
-
-	p.injecting = false
-	p.parseErrors = nil
-	p.generatedParserResult = nil
-
-	// Clear lexer struct before reusing.
-	p.lex = Lexer{
-		input: input,
-		state: lexStatements,
-	}
-	return p
-}
-
 // SequenceValue is an omittable value in a sequence of time series values.
 type SequenceValue struct {
 	Value   float64
@@ -200,10 +231,10 @@ type seriesDescription struct {
 
 // ParseSeriesDesc parses the description of a time series.
 func ParseSeriesDesc(input string) (labels labels.Labels, values []SequenceValue, err error) {
-	p := newParser(input)
+	p := NewParser(input)
 	p.lex.seriesDesc = true
 
-	defer parserPool.Put(p)
+	defer p.Close()
 	defer p.recover(&err)
 
 	parseResult := p.parseGenerated(START_SERIES_DESCRIPTION)
@@ -799,7 +830,7 @@ func MustLabelMatcher(mt labels.MatchType, name, val string) *labels.Matcher {
 }
 
 func MustGetFunction(name string) *Function {
-	f, ok := getFunction(name)
+	f, ok := getFunction(name, Functions)
 	if !ok {
 		panic(fmt.Errorf("function %q does not exist", name))
 	}

promql/parser/parse_test.go

@@ -3714,7 +3714,7 @@ func TestParseSeries(t *testing.T) {
 }
 
 func TestRecoverParserRuntime(t *testing.T) {
-	p := newParser("foo bar")
+	p := NewParser("foo bar")
 	var err error
 
 	defer func() {
@@ -3728,7 +3728,7 @@ func TestRecoverParserRuntime(t *testing.T) {
 }
 
 func TestRecoverParserError(t *testing.T) {
-	p := newParser("foo bar")
+	p := NewParser("foo bar")
 	var err error
 
 	e := errors.New("custom error")
@@ -3776,3 +3776,20 @@ func TestExtractSelectors(t *testing.T) {
 		require.Equal(t, expected, ExtractSelectors(expr))
 	}
 }
+
+func TestParseCustomFunctions(t *testing.T) {
+	funcs := Functions
+	funcs["custom_func"] = &Function{
+		Name:       "custom_func",
+		ArgTypes:   []ValueType{ValueTypeMatrix},
+		ReturnType: ValueTypeVector,
+	}
+	input := "custom_func(metric[1m])"
+	p := NewParser(input, WithFunctions(funcs))
+	expr, err := p.ParseExpr()
+	require.NoError(t, err)
+
+	call, ok := expr.(*Call)
+	require.True(t, ok)
+	require.Equal(t, "custom_func", call.Func.Name)
+}