accept only the expected UTC timezones in date parsing

HTTP/1.1 hardcodes GMT (RFC 7231 §7.1.1.1) and what is good enough for the
internet must be good enough for us™ as we reuse the implementation
internally to parse (most) dates we encounter in various places like the
Release files with their Date and Valid-Until header fields.

Implementing a fully timezone aware parser just feels too hard for no
effective benefit as it would take 5+ years (= until LTS's are out of
fashion) until a repository could use non-UTC dates and expect it to
work. Not counting non-apt implementations which might or might not
only want to encounter UTC here as well.

As a bonus, this eliminates the use of an instance of setlocale in
libapt.

Closes: 819697
(cherry picked from commit 9febc2b238e1e322dce1f94ecbed46d595893b52)

1 files changed, 15 insertions, 0 deletions

diff --git a/apt-pkg/contrib/strutl.h b/apt-pkg/contrib/strutl.h
index ef1172678..a32aaf06d 100644
--- a/apt-pkg/contrib/strutl.h
+++ b/apt-pkg/contrib/strutl.h
@@ -67,6 +67,21 @@ std::string Base64Encode(const std::string &Str);
 std::string OutputInDepth(const unsigned long Depth, const char* Separator="  ");
 std::string URItoFileName(const std::string &URI);
 std::string TimeRFC1123(time_t Date);
+/** parses time as needed by HTTP/1.1 and Debian files.
+ *
+ * HTTP/1.1 prefers dates in RFC1123 format (but the other two obsolete date formats
+ * are supported to) and e.g. Release files use the same format in Date & Valid-Until
+ * fields.
+ *
+ * Note: datetime strings need to be in UTC timezones (GMT, UTC, Z, +/-0000) to be
+ * parsed. Other timezones will be rejected as invalid. Previous implementations
+ * accepted other timezones, but treated them as UTC.
+ *
+ * @param str is the datetime string to parse
+ * @param[out] time will be the seconds since epoch of the given datetime if
+ *    parsing is successful, undefined otherwise.
+ * @return \b true if parsing was successful, otherwise \b false.
+ */
 bool RFC1123StrToTime(const char* const str,time_t &time) APT_MUSTCHECK;
 bool FTPMDTMStrToTime(const char* const str,time_t &time) APT_MUSTCHECK;
 APT_DEPRECATED_MSG("Use RFC1123StrToTime or FTPMDTMStrToTime as needed instead") bool StrToTime(const std::string &Val,time_t &Result);