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/tools/internal/imports/fix.go

@@ -104,7 +104,10 @@ type packageInfo struct {
 
 // parseOtherFiles parses all the Go files in srcDir except filename, including
 // test files if filename looks like a test.
-func parseOtherFiles(fset *token.FileSet, srcDir, filename string) []*ast.File {
+//
+// It returns an error only if ctx is cancelled. Files with parse errors are
+// ignored.
+func parseOtherFiles(ctx context.Context, fset *token.FileSet, srcDir, filename string) ([]*ast.File, error) {
 	// This could use go/packages but it doesn't buy much, and it fails
 	// with https://golang.org/issue/26296 in LoadFiles mode in some cases.
 	considerTests := strings.HasSuffix(filename, "_test.go")
@@ -112,11 +115,14 @@ func parseOtherFiles(fset *token.FileSet, srcDir, filename string) []*ast.File {
 	fileBase := filepath.Base(filename)
 	packageFileInfos, err := os.ReadDir(srcDir)
 	if err != nil {
-		return nil
+		return nil, ctx.Err()
 	}
 
 	var files []*ast.File
 	for _, fi := range packageFileInfos {
+		if ctx.Err() != nil {
+			return nil, ctx.Err()
+		}
 		if fi.Name() == fileBase || !strings.HasSuffix(fi.Name(), ".go") {
 			continue
 		}
@@ -132,7 +138,7 @@ func parseOtherFiles(fset *token.FileSet, srcDir, filename string) []*ast.File {
 		files = append(files, f)
 	}
 
-	return files
+	return files, ctx.Err()
 }
 
 // addGlobals puts the names of package vars into the provided map.
@@ -557,12 +563,7 @@ func (p *pass) addCandidate(imp *ImportInfo, pkg *packageInfo) {
 
 // fixImports adds and removes imports from f so that all its references are
 // satisfied and there are no unused imports.
-//
-// This is declared as a variable rather than a function so goimports can
-// easily be extended by adding a file with an init function.
-var fixImports = fixImportsDefault
-
-func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) error {
+func fixImports(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) error {
 	fixes, err := getFixes(context.Background(), fset, f, filename, env)
 	if err != nil {
 		return err
@@ -592,7 +593,10 @@ func getFixes(ctx context.Context, fset *token.FileSet, f *ast.File, filename st
 		return fixes, nil
 	}
 
-	otherFiles := parseOtherFiles(fset, srcDir, filename)
+	otherFiles, err := parseOtherFiles(ctx, fset, srcDir, filename)
+	if err != nil {
+		return nil, err
+	}
 
 	// Second pass: add information from other files in the same package,
 	// like their package vars and imports.
@@ -1192,7 +1196,7 @@ func addExternalCandidates(ctx context.Context, pass *pass, refs references, fil
 	if err != nil {
 		return err
 	}
-	if err = resolver.scan(context.Background(), callback); err != nil {
+	if err = resolver.scan(ctx, callback); err != nil {
 		return err
 	}
 
@@ -1203,7 +1207,7 @@ func addExternalCandidates(ctx context.Context, pass *pass, refs references, fil
 	}
 	results := make(chan result, len(refs))
 
-	ctx, cancel := context.WithCancel(context.TODO())
+	ctx, cancel := context.WithCancel(ctx)
 	var wg sync.WaitGroup
 	defer func() {
 		cancel()