TUN-8861: Add session limiter to UDP session manager

## Summary
In order to make cloudflared behavior more predictable and
prevent an exhaustion of resources, we have decided to add
session limits that can be configured by the user. This first
commit introduces the session limiter and adds it to the UDP
handling path. For now the limiter is set to run only in
unlimited mode.

vendor/golang.org/x/mod/modfile/rule.go

@@ -38,6 +38,7 @@ type File struct {
 	Module    *Module
 	Go        *Go
 	Toolchain *Toolchain
+	Godebug   []*Godebug
 	Require   []*Require
 	Exclude   []*Exclude
 	Replace   []*Replace
@@ -65,6 +66,13 @@ type Toolchain struct {
 	Syntax *Line
 }
 
+// A Godebug is a single godebug key=value statement.
+type Godebug struct {
+	Key    string
+	Value  string
+	Syntax *Line
+}
+
 // An Exclude is a single exclude statement.
 type Exclude struct {
 	Mod    module.Version
@@ -289,7 +297,7 @@ func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (parse
 					})
 				}
 				continue
-			case "module", "require", "exclude", "replace", "retract":
+			case "module", "godebug", "require", "exclude", "replace", "retract":
 				for _, l := range x.Line {
 					f.add(&errs, x, l, x.Token[0], l.Token, fix, strict)
 				}
@@ -308,7 +316,9 @@ var laxGoVersionRE = lazyregexp.New(`^v?(([1-9][0-9]*)\.(0|[1-9][0-9]*))([^0-9].
 
 // Toolchains must be named beginning with `go1`,
 // like "go1.20.3" or "go1.20.3-gccgo". As a special case, "default" is also permitted.
-// TODO(samthanawalla): Replace regex with https://pkg.go.dev/go/version#IsValid in 1.23+
+// Note that this regexp is a much looser condition than go/version.IsValid,
+// for forward compatibility.
+// (This code has to be work to identify new toolchains even if we tweak the syntax in the future.)
 var ToolchainRE = lazyregexp.New(`^default$|^go1($|\.)`)
 
 func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
@@ -384,7 +394,7 @@ func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, a
 		if len(args) != 1 {
 			errorf("toolchain directive expects exactly one argument")
 			return
-		} else if strict && !ToolchainRE.MatchString(args[0]) {
+		} else if !ToolchainRE.MatchString(args[0]) {
 			errorf("invalid toolchain version '%s': must match format go1.23.0 or default", args[0])
 			return
 		}
@@ -412,6 +422,22 @@ func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, a
 		}
 		f.Module.Mod = module.Version{Path: s}
 
+	case "godebug":
+		if len(args) != 1 || strings.ContainsAny(args[0], "\"`',") {
+			errorf("usage: godebug key=value")
+			return
+		}
+		key, value, ok := strings.Cut(args[0], "=")
+		if !ok {
+			errorf("usage: godebug key=value")
+			return
+		}
+		f.Godebug = append(f.Godebug, &Godebug{
+			Key:    key,
+			Value:  value,
+			Syntax: line,
+		})
+
 	case "require", "exclude":
 		if len(args) != 2 {
 			errorf("usage: %s module/path v1.2.3", verb)
@@ -654,6 +680,22 @@ func (f *WorkFile) add(errs *ErrorList, line *Line, verb string, args []string,
 		f.Toolchain = &Toolchain{Syntax: line}
 		f.Toolchain.Name = args[0]
 
+	case "godebug":
+		if len(args) != 1 || strings.ContainsAny(args[0], "\"`',") {
+			errorf("usage: godebug key=value")
+			return
+		}
+		key, value, ok := strings.Cut(args[0], "=")
+		if !ok {
+			errorf("usage: godebug key=value")
+			return
+		}
+		f.Godebug = append(f.Godebug, &Godebug{
+			Key:    key,
+			Value:  value,
+			Syntax: line,
+		})
+
 	case "use":
 		if len(args) != 1 {
 			errorf("usage: %s local/dir", verb)
@@ -929,6 +971,15 @@ func (f *File) Format() ([]byte, error) {
 // Cleanup cleans out all the cleared entries.
 func (f *File) Cleanup() {
 	w := 0
+	for _, g := range f.Godebug {
+		if g.Key != "" {
+			f.Godebug[w] = g
+			w++
+		}
+	}
+	f.Godebug = f.Godebug[:w]
+
+	w = 0
 	for _, r := range f.Require {
 		if r.Mod.Path != "" {
 			f.Require[w] = r
@@ -1027,6 +1078,45 @@ func (f *File) AddToolchainStmt(name string) error {
 	return nil
 }
 
+// AddGodebug sets the first godebug line for key to value,
+// preserving any existing comments for that line and removing all
+// other godebug lines for key.
+//
+// If no line currently exists for key, AddGodebug adds a new line
+// at the end of the last godebug block.
+func (f *File) AddGodebug(key, value string) error {
+	need := true
+	for _, g := range f.Godebug {
+		if g.Key == key {
+			if need {
+				g.Value = value
+				f.Syntax.updateLine(g.Syntax, "godebug", key+"="+value)
+				need = false
+			} else {
+				g.Syntax.markRemoved()
+				*g = Godebug{}
+			}
+		}
+	}
+
+	if need {
+		f.addNewGodebug(key, value)
+	}
+	return nil
+}
+
+// addNewGodebug adds a new godebug key=value line at the end
+// of the last godebug block, regardless of any existing godebug lines for key.
+func (f *File) addNewGodebug(key, value string) {
+	line := f.Syntax.addLine(nil, "godebug", key+"="+value)
+	g := &Godebug{
+		Key:    key,
+		Value:  value,
+		Syntax: line,
+	}
+	f.Godebug = append(f.Godebug, g)
+}
+
 // AddRequire sets the first require line for path to version vers,
 // preserving any existing comments for that line and removing all
 // other lines for path.
@@ -1334,6 +1424,16 @@ func (f *File) SetRequireSeparateIndirect(req []*Require) {
 	f.SortBlocks()
 }
 
+func (f *File) DropGodebug(key string) error {
+	for _, g := range f.Godebug {
+		if g.Key == key {
+			g.Syntax.markRemoved()
+			*g = Godebug{}
+		}
+	}
+	return nil
+}
+
 func (f *File) DropRequire(path string) error {
 	for _, r := range f.Require {
 		if r.Mod.Path == path {

vendor/golang.org/x/mod/modfile/work.go

@@ -14,6 +14,7 @@ import (
 type WorkFile struct {
 	Go        *Go
 	Toolchain *Toolchain
+	Godebug   []*Godebug
 	Use       []*Use
 	Replace   []*Replace
 
@@ -68,7 +69,7 @@ func ParseWork(file string, data []byte, fix VersionFixer) (*WorkFile, error) {
 					Err:      fmt.Errorf("unknown block type: %s", strings.Join(x.Token, " ")),
 				})
 				continue
-			case "use", "replace":
+			case "godebug", "use", "replace":
 				for _, l := range x.Line {
 					f.add(&errs, l, x.Token[0], l.Token, fix)
 				}
@@ -184,6 +185,55 @@ func (f *WorkFile) DropToolchainStmt() {
 	}
 }
 
+// AddGodebug sets the first godebug line for key to value,
+// preserving any existing comments for that line and removing all
+// other godebug lines for key.
+//
+// If no line currently exists for key, AddGodebug adds a new line
+// at the end of the last godebug block.
+func (f *WorkFile) AddGodebug(key, value string) error {
+	need := true
+	for _, g := range f.Godebug {
+		if g.Key == key {
+			if need {
+				g.Value = value
+				f.Syntax.updateLine(g.Syntax, "godebug", key+"="+value)
+				need = false
+			} else {
+				g.Syntax.markRemoved()
+				*g = Godebug{}
+			}
+		}
+	}
+
+	if need {
+		f.addNewGodebug(key, value)
+	}
+	return nil
+}
+
+// addNewGodebug adds a new godebug key=value line at the end
+// of the last godebug block, regardless of any existing godebug lines for key.
+func (f *WorkFile) addNewGodebug(key, value string) {
+	line := f.Syntax.addLine(nil, "godebug", key+"="+value)
+	g := &Godebug{
+		Key:    key,
+		Value:  value,
+		Syntax: line,
+	}
+	f.Godebug = append(f.Godebug, g)
+}
+
+func (f *WorkFile) DropGodebug(key string) error {
+	for _, g := range f.Godebug {
+		if g.Key == key {
+			g.Syntax.markRemoved()
+			*g = Godebug{}
+		}
+	}
+	return nil
+}
+
 func (f *WorkFile) AddUse(diskPath, modulePath string) error {
 	need := true
 	for _, d := range f.Use {

vendor/golang.org/x/mod/module/module.go

@@ -506,6 +506,7 @@ var badWindowsNames = []string{
 	"PRN",
 	"AUX",
 	"NUL",
+	"COM0",
 	"COM1",
 	"COM2",
 	"COM3",
@@ -515,6 +516,7 @@ var badWindowsNames = []string{
 	"COM7",
 	"COM8",
 	"COM9",
+	"LPT0",
 	"LPT1",
 	"LPT2",
 	"LPT3",