ocaml/xenctrl: Make failwith_xc() thread safe

The static error_str[] buffer is not thread-safe, and 1024 bytes is
unreasonably large.  Reduce to 256 bytes (which is still much larger than any
current use), and move it to being a stack variable.

Also, propagate the Noreturn attribute from caml_raise_with_string().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Dave Scott <Dave.Scott@eu.citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>
(cherry picked from commit c8945d5161)
(cherry picked from commit 032673c883)

tools/ocaml/libs/xc/xenctrl_stubs.c

@@ -51,21 +51,22 @@
 	i1 = (uint32_t) Int64_val(Field(input, 0)); \
 	i2 = ((Field(input, 1) == Val_none) ? 0xffffffff : (uint32_t) Int64_val(Field(Field(input, 1), 0)));
 
-#define ERROR_STRLEN 1024
-void failwith_xc(xc_interface *xch)
+static void Noreturn failwith_xc(xc_interface *xch)
 {
-	static char error_str[ERROR_STRLEN];
+	char error_str[256];
 	if (xch) {
 		const xc_error *error = xc_get_last_error(xch);
 		if (error->code == XC_ERROR_NONE)
-                	snprintf(error_str, ERROR_STRLEN, "%d: %s", errno, strerror(errno));
+			snprintf(error_str, sizeof(error_str),
+				 "%d: %s", errno, strerror(errno));
 		else
-			snprintf(error_str, ERROR_STRLEN, "%d: %s: %s",
-				 error->code,
+			snprintf(error_str, sizeof(error_str),
+				 "%d: %s: %s", error->code,
 				 xc_error_code_to_desc(error->code),
 				 error->message);
 	} else {
-		snprintf(error_str, ERROR_STRLEN, "Unable to open XC interface");
+		snprintf(error_str, sizeof(error_str),
+			 "Unable to open XC interface");
 	}
 	caml_raise_with_string(*caml_named_value("xc.error"), error_str);
 }