No longer exit for missing required protocolversions on an old consensus

Specifically, if the consensus is older than the (estimted or
measured) release date for this version of tor, we assume that the
required versions may have changed in between that consensus and
this release.

Implements ticket 27735 and proposal 297.

changes/prop297

@@ -0,0 +1,7 @@
+  o Minor features (required protocols):
+    - Tor no longer exits if it is missing a required protocol, if the
+      consensus that requires the protocol predates the release date of the
+      version of Tor. This change prevents Tor releases from exiting because
+      of an old cached consensus, on the theory that a newer cached
+      consensus might not require the protocol.  Implements proposal 297;
+      closes ticket 27735.

src/core/or/versions.c

@@ -16,6 +16,25 @@
 
 #include "core/or/tor_version_st.h"
 
+/**
+ * Return the approximate date when this release came out, or was
+ * scheduled to come out, according to the APPROX_RELEASE_DATE set in
+ * configure.ac
+ **/
+time_t
+tor_get_approx_release_date(void)
+{
+  char tbuf[ISO_TIME_LEN+1];
+  tor_snprintf(tbuf, sizeof(tbuf),
+               "%s 00:00:00", APPROX_RELEASE_DATE);
+  time_t result = 0;
+  int r = parse_iso_time(tbuf, &result);
+  if (BUG(r < 0)) {
+    result = 0;
+  }
+  return result;
+}
+
 /** Return VS_RECOMMENDED if <b>myversion</b> is contained in
  * <b>versionlist</b>.  Else, return VS_EMPTY if versionlist has no
  * entries. Else, return VS_OLD if every member of

src/core/or/versions.h

@@ -26,6 +26,8 @@ typedef enum version_status_t {
   VS_UNKNOWN, /**< We have no idea. */
 } version_status_t;
 
+time_t tor_get_approx_release_date(void);
+
 version_status_t tor_version_is_obsolete(const char *myversion,
                                          const char *versionlist);
 int tor_version_parse_platform(const char *platform,

src/feature/nodelist/networkstatus.c

@@ -2684,6 +2684,9 @@ networkstatus_check_required_protocols(const networkstatus_t *ns,
   const char *required, *recommended;
   char *missing = NULL;
 
+  const bool consensus_postdates_this_release =
+    ns->valid_after >= tor_get_approx_release_date();
+
   tor_assert(warning_out);
 
   if (client_mode) {
@@ -2701,7 +2704,7 @@ networkstatus_check_required_protocols(const networkstatus_t *ns,
                  "%s on the Tor network. The missing protocols are: %s",
                  func, missing);
     tor_free(missing);
-    return 1;
+    return consensus_postdates_this_release ? 1 : 0;
   }
 
   if (! protover_all_supported(recommended, &missing)) {