internal/sockstate: Restore connection checking on Linux.

internal/sockstate/netstat_darwin.go

@@ -32,9 +32,10 @@ func tcpSocks(accept AcceptFn) ([]sockTabEntry, error) {
 }
 
 func GetConnInode(c *net.TCPConn) (uint64, error) {
-	_, err := getConnFd(c)
+	_, finalize, err := getConnFd(c)
 	if err != nil {
 		return 0, err
 	}
+	defer finalize()
 	return 0, ErrSocketCheckNotImplemented.New()
 }

internal/sockstate/netstat_freebsd.go

@@ -32,9 +32,10 @@ func tcpSocks(accept AcceptFn) ([]sockTabEntry, error) {
 }
 
 func GetConnInode(c *net.TCPConn) (uint64, error) {
-	_, err := getConnFd(c)
+	_, finalize, err := getConnFd(c)
 	if err != nil {
 		return 0, err
 	}
+	defer finalize()
 	return 0, ErrSocketCheckNotImplemented.New()
 }

internal/sockstate/netstat_linux.go

@@ -279,10 +279,11 @@ func tcpSocks(accept AcceptFn) ([]sockTabEntry, error) {
 
 // GetConnInode returns the inode number of an fd.
 func GetConnInode(conn *net.TCPConn) (n uint64, err error) {
-	fd, err := getConnFd(conn)
+	fd, finalize, err := getConnFd(conn)
 	if err != nil {
 		return 0, err
 	}
+	defer finalize()
 
 	if isWSL || isProcBlocked {
 		return 0, ErrSocketCheckNotImplemented.New()

internal/sockstate/netstat_unix.go

@@ -22,10 +22,10 @@ import (
 	"syscall"
 )
 
-func getConnFd(c *net.TCPConn) (fd uintptr, err error) {
+func getConnFd(c *net.TCPConn) (fd uintptr, finalize func() error, err error) {
 	f, err := c.File()
 	if err != nil {
-		return 0, err
+		return 0, nil, err
 	}
 
 	fd = f.Fd()
@@ -34,7 +34,5 @@ func getConnFd(c *net.TCPConn) (fd uintptr, err error) {
 	// blocking Close() in some cases.
 	syscall.SetNonblock(int(fd), true)
 
-	f.Close()
-
-	return fd, nil
+	return fd, f.Close, nil
 }