feat(libtor): prevent concurrent calls

Closes ooni/probe#2623

internal/libtor/enabled.go

@@ -84,6 +84,7 @@ import (
 	"os"
 	"runtime"
 	"sync"
+	"sync/atomic"
 
 	"github.com/cretz/bine/process"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
@@ -163,6 +164,10 @@ func (p *torProcess) Wait() (err error) {
 	return
 }
 
+// ErrConcurrentCalls indicates there have been concurrent libtor calls, which
+// would lead to memory corruption inside of libtor.a.
+var ErrConcurrentCalls = errors.New("libtor: another thread is already running tor")
+
 // ErrTooManyArguments indicates that p.args contains too many arguments
 var ErrTooManyArguments = errors.New("libtor: too many arguments")
 
@@ -172,6 +177,9 @@ var ErrCannotCreateControlSocket = errors.New("libtor: cannot create a control s
 // ErrNonzeroExitCode indicates that tor returned a nonzero exit code
 var ErrNonzeroExitCode = errors.New("libtor: command completed with nonzero exit code")
 
+// concurrentCalls prevents concurrent libtor.a calls.
+var concurrentCalls = &atomic.Int64{}
+
 // runtor runs tor until completion and ensures that tor exits when
 // the given ctx is cancelled or its deadline expires.
 func (p *torProcess) runtor(ctx context.Context, cc net.Conn, args ...string) {
@@ -191,6 +199,13 @@ func (p *torProcess) runtor(ctx context.Context, cc net.Conn, args ...string) {
 		return
 	}
 
+	// make sure we're not going to have actual concurrent calls.
+	if !concurrentCalls.CompareAndSwap(0, 1) {
+		p.startErr <- ErrConcurrentCalls // nonblocking channel
+		return
+	}
+	defer concurrentCalls.Store(0)
+
 	// Note: when writing this code I was wondering whether I needed to
 	// use unsafe.Pointer to track pointers that matter to C code. Reading
 	// this message[1] has been useful to understand that the most likely