pathmtu/pathmtu.c: Fix search algorithm.

Turns out I can't write a binary search algorithm.  Fiddle with it so
that it doesn't get stuck sometimes.  Some of the bounds management is a
little tricky, so add some documentation and assertions to make sure
that everything is as expected.

diff --git a/pathmtu/pathmtu.c b/pathmtu/pathmtu.c
index d347b0dddf20070b938e9301bfba7026c9bde7bf..8dac7b1153293f80822539adab2877ff1128e7eb 100644 (file)
--- a/pathmtu/pathmtu.c
+++ b/pathmtu/pathmtu.c
@@ -32,6 +32,7 @@
 
 #include "config.h"
 
+#include <assert.h>
 #include <errno.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -238,6 +239,8 @@ static int pathmtu(const struct param *pp)
    * try to hug that initially.
    */
   for (;;) {
+    assert(lo <= mtu && mtu <= hi);
+    if (pp->f & F_VERBOSE) moan("probe: %d <= %d <= %d", lo, mtu, hi);
     rc = probe(&ps, st, mtu);
     switch (rc) {
 
@@ -295,19 +298,36 @@ static int pathmtu(const struct param *pp)
          if (pp->f & F_VERBOSE) moan("probe returned: found correct MTU");
          goto done;
        }
-       if (pp->f & F_VERBOSE) moan("probe returned: guessing higher");
        lo = mtu;
+
+       /* Now we must make a new guess, between lo and hi.  We know that lo
+        * is good; but we're not so sure about hi here.  We know that hi >
+        * lo, so this will find an approximate midpoint, greater than lo and
+        * no more than hi.
+        */
+       if (pp->f & F_VERBOSE) moan("probe returned: guessing higher");
        mtu += (hi - lo + 1)/2;
        break;
 
       case RC_LOWER:
       lower:
-       if (mtu == lo) {
+       /* If this didn't work, and we're already at the bottom of our
+        * possible range, then something has gone horribly wrong.
+        */
+       assert(lo < mtu);
+       hi = mtu - 1;
+       if (lo == hi) {
          if (pp->f & F_VERBOSE) moan("error returned: found correct MTU");
+         mtu = lo;
          goto done;
        }
+
+       /* We must make a new guess, between lo and hi.  We're probably
+        * fairly sure that lo will succeed, since either it's the minimum
+        * MTU or we've tested it already; but we're not quite sure about hi,
+        * so we want to aim high.
+        */
        if (pp->f & F_VERBOSE) moan("error returned: guessing lower");
-       hi = mtu - 1;
        mtu -= (hi - lo + 1)/2;
        break;