Using test names as sandbox dirs is now optional

Given that test names can be long _and_ some platforms insist upon a
short maximum path length (eg, Windows), we may not want to
indiscriminately use path names as the sandbox directory name.

However, this has high utility in some situations, so allow it as an
opt-in. By default, we'll just use an 8 hexadigit "random" string.
(We'll use the rand(3) mechanism, which is not be particularly random,
but since the sandbox directories are sequestered in a unique tempdir,
a clar process need only attempt to avoid collisions between itself,
not with other processes.)

clar/sandbox.h

@@ -187,6 +187,12 @@ static void clar_tempdir_init(void)
 	if (chdir(_clar_tempdir) != 0)
 		clar_abort("Failed to change into tempdir '%s': %s.\n",
 			   _clar_tempdir, strerror(errno));
+
+#if !defined(CLAR_SANDBOX_TEST_NAMES) && defined(_WIN32)
+	srand(clock() ^ (unsigned int)time(NULL) ^ GetCurrentProcessId() ^ GetCurrentThreadId());
+#elif !defined(CLAR_SANDBOX_TEST_NAMES)
+	srand(clock() ^ time(NULL) ^ (getpid() << 16));
+#endif
 }
 
 static void append(char *dst, const char *src)
@@ -208,9 +214,20 @@ static void append(char *dst, const char *src)
 
 static int clar_sandbox_create(const char *suite_name, const char *test_name)
 {
+#ifndef CLAR_SANDBOX_TEST_NAMES
+	char alpha[] = "0123456789abcdef";
+	int num = rand();
+#endif
+
 	cl_assert(_clar_sandbox[0] == '\0');
 
-	cl_assert(strlen(_clar_tempdir) + strlen(suite_name) + strlen(test_name) + 2 < CLAR_MAX_PATH);
+	/*
+	 * We may want to use test names as sandbox directory names for
+	 * readability, _however_ on platforms with restrictions for short
+	 * file / folder names (eg, Windows), this may be too long.
+	 */
+#ifdef CLAR_SANDBOX_TEST_NAMES
+	cl_assert(strlen(_clar_tempdir) + strlen(suite_name) + strlen(test_name) + 3 < CLAR_MAX_PATH);
 
 	strcpy(_clar_sandbox, _clar_tempdir);
 	_clar_sandbox[_clar_tempdir_len] = '/';
@@ -219,6 +236,26 @@ static int clar_sandbox_create(const char *suite_name, const char *test_name)
 	append(_clar_sandbox, suite_name);
 	append(_clar_sandbox, "__");
 	append(_clar_sandbox, test_name);
+#else
+	((void)suite_name);
+	((void)test_name);
+	((void)append);
+
+	cl_assert(strlen(_clar_tempdir) + 9 < CLAR_MAX_PATH);
+
+	strcpy(_clar_sandbox, _clar_tempdir);
+	_clar_sandbox[_clar_tempdir_len] = '/';
+
+	_clar_sandbox[_clar_tempdir_len + 1] = alpha[(num & 0xf0000000) >> 28];
+	_clar_sandbox[_clar_tempdir_len + 2] = alpha[(num & 0x0f000000) >> 24];
+	_clar_sandbox[_clar_tempdir_len + 3] = alpha[(num & 0x00f00000) >> 20];
+	_clar_sandbox[_clar_tempdir_len + 4] = alpha[(num & 0x000f0000) >> 16];
+	_clar_sandbox[_clar_tempdir_len + 5] = alpha[(num & 0x0000f000) >> 12];
+	_clar_sandbox[_clar_tempdir_len + 6] = alpha[(num & 0x00000f00) >> 8];
+	_clar_sandbox[_clar_tempdir_len + 7] = alpha[(num & 0x000000f0) >> 4];
+	_clar_sandbox[_clar_tempdir_len + 8] = alpha[(num & 0x0000000f) >> 0];
+	_clar_sandbox[_clar_tempdir_len + 9] = '\0';
+#endif
 
 	if (mkdir(_clar_sandbox, 0700) != 0)
 		return -1;