Add get_max_sockets() and remove dead code

The control port was using set_max_file_descriptors() with a limit set to 0
to query the number of maximum socket Tor can use. With the recent changes
to that function, a check was introduced to make sure a user can not set a
value below the amount we reserved for non socket.

This commit adds get_max_sockets() that returns the value of max_sockets so
we can stop using that "setter" function to get the current value.

Finally, the dead code is removed that is the code that checked for limit
equal to 0. From now on, set_max_file_descriptors() should never be used
with a limit set to 0 for a valid use case.

Fixes #16697

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

changes/bug16697

@@ -0,0 +1,10 @@
+  o Minor bugfixes
+    - Control port was using set_max_file_descriptors() with a limit set to
+      0 to get the max value. A recent fix made this use case return an
+      error and introduced dead code in that function. This triggered a
+      warning that our limit (ConnLimit) was invalid but in reality it was
+      not.
+      
+      Now, to the control port uses a specific getter function to query the
+      value and set_max_file_descriptors() should never be used again for
+      that purpose. Fixes #16697; bugfix on 0.2.7.2-alpha.

src/common/compat.c

@@ -1589,6 +1589,13 @@ tor_ersatz_socketpair(int family, int type, int protocol, tor_socket_t fd[2])
 }
 #endif
 
+/* Return the maximum number of allowed sockets. */
+int
+get_max_sockets(void)
+{
+  return max_sockets;
+}
+
 /** Number of extra file descriptors to keep in reserve beyond those that we
  * tell Tor it's allowed to use. */
 #define ULIMIT_BUFFER 32 /* keep 32 extra fd's beyond ConnLimit_ */
@@ -1601,8 +1608,6 @@ tor_ersatz_socketpair(int family, int type, int protocol, tor_socket_t fd[2])
  * If we can't find a number greater than or equal to <b>limit</b>,
  * then we fail: return -1.
  *
- * If <b>limit</b> is 0, then do not adjust the current maximum.
- *
  * Otherwise, return 0 and store the maximum we found inside <b>max_out</b>.*/
 int
 set_max_file_descriptors(rlim_t limit, int *max_out)
@@ -1647,17 +1652,6 @@ set_max_file_descriptors(rlim_t limit, int *max_out)
              strerror(errno));
     return -1;
   }
-  if (limit == 0) {
-    /* XXXX DEAD CODE We can't reach this point, since the first "if" in this
-     * function increases limit if it started out less than ULIMIT_BUFFER */
-
-    /* If limit == 0, return the maximum value without setting it. */
-    limit = rlim.rlim_max;
-    if (limit > INT_MAX)
-      limit = INT_MAX;
-    *max_out = max_sockets = (int)limit - ULIMIT_BUFFER;
-    return 0;
-  }
   if (rlim.rlim_max < limit) {
     log_warn(LD_CONFIG,"We need %lu file descriptors available, and we're "
              "limited to %lu. Please change your ulimit -n.",

src/common/compat.h

@@ -617,6 +617,7 @@ set_uint8(void *cp, uint8_t v)
 #if !defined(HAVE_RLIM_T)
 typedef unsigned long rlim_t;
 #endif
+int get_max_sockets(void);
 int set_max_file_descriptors(rlim_t limit, int *max);
 int tor_disable_debugger_attach(void);
 int switch_id(const char *user);

src/or/control.c

@@ -1541,8 +1541,7 @@ getinfo_helper_misc(control_connection_t *conn, const char *question,
       }
     #endif
   } else if (!strcmp(question, "process/descriptor-limit")) {
-    int max_fds=-1;
-    set_max_file_descriptors(0, &max_fds);
+    int max_fds = get_max_sockets();
     tor_asprintf(answer, "%d", max_fds);
   } else if (!strcmp(question, "limits/max-mem-in-queues")) {
     tor_asprintf(answer, U64_FORMAT,