fix incorrect access to tzname[] by strptime %Z conversion specifier

there are three issues here:

1. if tzset has not been called (explicitly or implicitly), the
tzname[] array will contain null pointers, and the dereference to
compare against them has undefined behavior (and will fault).

2. access to tzname[] was performed without the timezone lock held.
this resulted in a data race if the timezone is concurrently changed
from another thread.

3. due to unintended signedness of the types, the open-coded isalpha
in the non-matching case was wrong and would continue past null
termination.

to fix the first two issues, the body of the %Z conversion is moved to
__tz.c where it has access to locking, and null checks are added.

there is probably an argument to be made that the equivalent of tzset
should happen here, but POSIX does not specify that to happen, so in
the absence of an interpretation adding such an allowance or
requirement, it is not done.

the third issue is fixed just by using the existing isalpha macro.

src/time/__tz.c

@@ -436,3 +436,22 @@ const char *__tm_to_tzname(const struct tm *tm)
 	UNLOCK(lock);
 	return p;
 }
+
+int __tzname_to_isdst(const char *restrict *s)
+{
+	size_t len;
+	int isdst = -1;
+	LOCK(lock);
+	if (tzname[0] && !strncmp(*s, tzname[0], len = strlen(tzname[0]))) {
+		isdst = 0;
+		*s += len;
+	} else if (tzname[1] && !strncmp(*s, tzname[1], len=strlen(tzname[1]))) {
+		isdst = 1;
+		*s += len;
+	} else {
+		/* FIXME: is this supposed to be an error? */
+		while (isalpha(**s)) ++*s;
+	}
+	UNLOCK(lock);
+	return isdst;
+}

src/time/strptime.c

@@ -5,6 +5,7 @@
 #include <stddef.h>
 #include <string.h>
 #include <strings.h>
+#include "time_impl.h"
 
 char *strptime(const char *restrict s, const char *restrict f, struct tm *restrict tm)
 {
@@ -207,16 +208,8 @@ char *strptime(const char *restrict s, const char *restrict f, struct tm *restri
 			s += 5;
 			break;
 		case 'Z':
-			if (!strncmp(s, tzname[0], len = strlen(tzname[0]))) {
-				tm->tm_isdst = 0;
-				s += len;
-			} else if (!strncmp(s, tzname[1], len=strlen(tzname[1]))) {
-				tm->tm_isdst = 1;
-				s += len;
-			} else {
-				/* FIXME: is this supposed to be an error? */
-				while ((*s|32)-'a' <= 'z'-'a') s++;
-			}
+			i = __tzname_to_isdst(&s);
+			if (i>=0) tm->tm_isdst = i;
 			break;
 		case '%':
 			if (*s++ != '%') return 0;

src/time/time_impl.h

@@ -5,6 +5,7 @@ hidden int __month_to_secs(int, int);
 hidden long long __year_to_secs(long long, int *);
 hidden long long __tm_to_secs(const struct tm *);
 hidden const char *__tm_to_tzname(const struct tm *);
+hidden int __tzname_to_isdst(const char *restrict *);
 hidden int __secs_to_tm(long long, struct tm *);
 hidden void __secs_to_zone(long long, int, int *, long *, long *, const char **);
 hidden const char *__strftime_fmt_1(char (*)[100], size_t *, int, const struct tm *, locale_t, int);