Set the open file limit to the current value before changing it

If setrlimit() failed, max_out wasn't set in set_max_file_descriptors()
ending in a state where we don't use ULIMIT_BUFFER for things like tor
private key files.

Also fix the set_max_file_descriptors() documentation.

Fixes #16274

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

changes/bug16274

@@ -0,0 +1,5 @@
+  o Minor bugfix (open file limit):
+    - Fix set_max_file_descriptors() to set by default the max open file
+      limit to the current limit in case setrlimit() fails so we at least
+      have a usable value; Fixes #16274; bugfix on tor-0.2.0.10-alpha~71;
+      Patch by dgoulet.

src/common/compat.c

@@ -1600,15 +1600,23 @@ get_max_sockets(void)
  * tell Tor it's allowed to use. */
 #define ULIMIT_BUFFER 32 /* keep 32 extra fd's beyond ConnLimit_ */
 
-/** Learn the maximum allowed number of file descriptors, and tell the system
- * we want to use up to that number. (Some systems have a low soft limit, and
- * let us set it higher.)
+/** Learn the maximum allowed number of file descriptors, and tell the
+ * system we want to use up to that number. (Some systems have a low soft
+ * limit, and let us set it higher.)  We compute this by finding the largest
+ * number that we can use.
  *
- * We compute this by finding the largest number that we can use.
- * If we can't find a number greater than or equal to <b>limit</b>,
- * then we fail: return -1.
+ * If the limit is below the reserved file descriptor value (ULIMIT_BUFFER),
+ * return -1 and <b>max_out</b> is untouched.
  *
- * Otherwise, return 0 and store the maximum we found inside <b>max_out</b>.*/
+ * If we can't find a number greater than or equal to <b>limit</b>, then we
+ * fail by returning -1 and <b>max_out</b> is untouched.
+ *
+ * If we are unable to set the limit value because of setrlimit() failing,
+ * return -1 and <b>max_out</b> is set to the current maximum value returned
+ * by getrlimit().
+ *
+ * Otherwise, return 0 and store the maximum we found inside <b>max_out</b>
+ * and set <b>max_sockets</b> with that value as well.*/
 int
 set_max_file_descriptors(rlim_t limit, int *max_out)
 {
@@ -1665,7 +1673,7 @@ set_max_file_descriptors(rlim_t limit, int *max_out)
   }
   /* Set the current limit value so if the attempt to set the limit to the
    * max fails at least we'll have a valid value of maximum sockets. */
-  max_sockets = (int)rlim.rlim_cur - ULIMIT_BUFFER;
+  *max_out = max_sockets = (int)rlim.rlim_cur - ULIMIT_BUFFER;
   rlim.rlim_cur = rlim.rlim_max;
 
   if (setrlimit(RLIMIT_NOFILE, &rlim) != 0) {