Introduce a sync.Pool for ANTLR Lexer / Parser instances (#348)

* Introduce a sync.Pool for ANTLR Parser instances
* Introduce sync.Pool instances for the Lexer as well.

Note, the change brings in github.com/antlr/antlr4/pulls/2816 which updates the
ANTLR Go runtime. Other applications using vendored code, or alternate sources
of truth besides the main ANTLR GitHub repository will also need to be udpated
for this change to work.

WORKSPACE

@@ -30,7 +30,7 @@ go_repository(
 
 go_repository(
   name = "com_github_antlr",
-  tag = "4.7.2",
+  commit = "621b933c7a7f01c67ae9de15103151fa0f9d6d90",
   importpath = "github.com/antlr/antlr4",
 )
 

go.mod

@@ -3,7 +3,7 @@ module github.com/google/cel-go
 go 1.12
 
 require (
-	github.com/antlr/antlr4 v0.0.0-20190819145818-b43a4c3a8015
+	github.com/antlr/antlr4 v0.0.0-20200503195918-621b933c7a7f
 	github.com/golang/protobuf v1.3.4
 	github.com/google/cel-spec v0.4.0
 	golang.org/x/text v0.3.2

go.sum

@@ -2,8 +2,8 @@ cloud.google.com/go v0.26.0 h1:e0WKqKTd5BnrG8aKH3J3h+QvEIQtSUcf2n5UZ5ZgLtQ=
 cloud.google.com/go v0.26.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
 github.com/BurntSushi/toml v0.3.1 h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
-github.com/antlr/antlr4 v0.0.0-20190819145818-b43a4c3a8015 h1:StuiJFxQUsxSCzcby6NFZRdEhPkXD5vxN7TZ4MD6T84=
-github.com/antlr/antlr4 v0.0.0-20190819145818-b43a4c3a8015/go.mod h1:T7PbCXFs94rrTttyxjbyT5+/1V8T2TYDejxUfHJjw1Y=
+github.com/antlr/antlr4 v0.0.0-20200503195918-621b933c7a7f h1:0cEys61Sr2hUBEXfNV8eyQP01oZuBgoMeHunebPirK8=
+github.com/antlr/antlr4 v0.0.0-20200503195918-621b933c7a7f/go.mod h1:T7PbCXFs94rrTttyxjbyT5+/1V8T2TYDejxUfHJjw1Y=
 github.com/census-instrumentation/opencensus-proto v0.2.1/go.mod h1:f6KPmirojxKA12rnyqOA5BBL4O983OfeGPqjHWSTneU=
 github.com/client9/misspell v0.3.4 h1:ta993UF76GwbvJcIo3Y68y/M3WxlpEHPWIGDkJYwzJI=
 github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=

parser/parser.go

@@ -19,6 +19,7 @@ package parser
 import (
 	"fmt"
 	"strconv"
+	"sync"
 
 	"github.com/antlr/antlr4/runtime/Go/antlr"
 
@@ -69,12 +70,30 @@ type parser struct {
 	macros map[string]Macro
 }
 
-var _ gen.CELVisitor = (*parser)(nil)
+var (
+	_ gen.CELVisitor = (*parser)(nil)
+
+	lexerPool *sync.Pool = &sync.Pool{
+		New: func() interface{} {
+			return gen.NewCELLexer(nil)
+		},
+	}
+
+	parserPool *sync.Pool = &sync.Pool{
+		New: func() interface{} {
+			return gen.NewCELParser(nil)
+		},
+	}
+)
 
 func (p *parser) parse(expression string) *exprpb.Expr {
-	stream := antlr.NewInputStream(expression)
-	lexer := gen.NewCELLexer(stream)
-	prsr := gen.NewCELParser(antlr.NewCommonTokenStream(lexer, 0))
+	lexer := lexerPool.Get().(*gen.CELLexer)
+	lexer.SetInputStream(antlr.NewInputStream(expression))
+	defer lexerPool.Put(lexer)
+
+	prsr := parserPool.Get().(*gen.CELParser)
+	prsr.SetInputStream(antlr.NewCommonTokenStream(lexer, 0))
+	defer parserPool.Put(prsr)
 
 	lexer.RemoveErrorListeners()
 	prsr.RemoveErrorListeners()