ssl: use locking

When using in a multithreaded context, OpenSSL needs to lock, and leaves
it up to application to provide said locks.

We were not doing this, and it's just luck that's kept us from crashing
up to now.

src/netops.c

@@ -35,6 +35,11 @@
 #include "http_parser.h"
 #include "global.h"
 
+#if defined(GIT_SSL) && defined(GIT_THREADS)
+/* OpenSSL wants us to keep an array of locks */
+static git_mutex *openssl_locks;
+#endif
+
 #ifdef GIT_WIN32
 static void net_set_error(const char *str)
 {
@@ -387,6 +392,24 @@ cert_fail_name:
 	return -1;
 }
 
+#ifdef GIT_THREADS
+void openssl_locking_function(int mode, int n, const char *file, int line)
+{
+	int lock;
+
+	GIT_UNUSED(file);
+	GIT_UNUSED(line);
+
+	lock = mode & CRYPTO_LOCK;
+
+	if (lock) {
+		git_mutex_lock(&openssl_locks[n]);
+	} else {
+		git_mutex_unlock(&openssl_locks[n]);
+	}
+}
+#endif
+
 /**
  * The OpenSSL init functions are not reentrant so we need to init
  * them under lock.
@@ -409,6 +432,25 @@ static int init_ssl(void)
 	SSL_library_init();
 	SSL_load_error_strings();
 
+#ifdef GIT_THREADS
+	{
+		int num_locks, i;
+
+
+		CRYPTO_set_locking_callback(openssl_locking_function);
+
+		num_locks = CRYPTO_num_locks();
+		openssl_locks = git__calloc(num_locks, sizeof(git_mutex));
+		for (i = 0; i < num_locks; i++) {
+			if (git_mutex_init(&openssl_locks[i]) < 0) {
+				git_mutex_unlock(&git__ssl_mutex);
+				giterr_set(GITERR_SSL, "failed to init lock %d", i);
+				return -1;
+			}
+		}
+	}
+#endif
+
 	git_atomic_set(&git__ssl_init, 1);
 	git_mutex_unlock(&git__ssl_mutex);