test(dhcptest): write "no boot file" as absent, which is what a reply carries

Four scenarios asserted `bootfile matches ^$` for the cases where the point
is that the client is handed nothing to fetch: OPAL-v3 and s390x, which get
a conf-file instead, a petitboot node, and the architecture whose loader was
taken off disk.

That assertion cannot pass. A server naming no boot file sends an empty
BOOTP file header and no option 67, and an empty value is absent -- so the
run reported "bootfile is not present in the reply" against both backends,
in every case, whatever the server had done. Four failures that said nothing
about either implementation.

`bootfile absent` is the assertion those scenarios wanted. A unit test pins
it, so the unsatisfiable form cannot come back unnoticed.

xCAT-test/dhcptest/README.md

@@ -140,6 +140,10 @@ One per line under a single multi-line `assert` key, as `target op value`.
 | `matches` `contains` `starts-with` `ends-with` | text |
 | `<` `<=` `>` `>=` | numeric |
 
+"Nothing to fetch" is `bootfile absent`, not a match against an empty string:
+a server that names no boot file sends an empty `file` header and no option 67,
+which is an absent value, and `matches ^$` can never hold against it.
+
 Assert on `bootfile`, not on `file`, unless the header itself is the point.
 Servers genuinely differ — ISC dhcpd fills the BOOTP header, dnsmasq answers in
 option 67 once the client has asked for it — and firmware reads whichever

xCAT-test/dhcptest/conf/loader-absent.conf

@@ -33,7 +33,7 @@ client_arch  = 0x0000
 assert =
     msgtype  == OFFER
     yiaddr   in %(pool)s
-    bootfile matches ^$
+    bootfile absent
 
 [scenario present-loader-is-still-named]
 description = S-12: An architecture whose loader is still there is unaffected

xCAT-test/dhcptest/conf/netboot-methods.conf

@@ -160,7 +160,7 @@ client_arch = 0x000e
 assert =
     msgtype    == OFFER
     option:209 == %(petitboot_conf)s
-    bootfile   matches ^$
+    bootfile   absent
 
 [scenario noip-interface-is-denied]
 description = S-07: An interface marked *NOIP* is not answered at all

xCAT-test/dhcptest/conf/pxe-arch-matrix.conf

@@ -177,7 +177,7 @@ client_arch  = 0x000e
 assert =
     msgtype    == OFFER
     option:209 == %(opal_conf)s
-    bootfile   matches ^$
+    bootfile   absent
 
 [scenario s390x-qemu]
 description = S-16: A QEMU s390x client is given a conf-file and no loader
@@ -188,7 +188,7 @@ client_arch = 0x001f
 assert =
     msgtype    == OFFER
     option:209 == %(s390x_conf)s
-    bootfile   matches ^$
+    bootfile   absent
 
 [scenario vendor-class-etherboot]
 description = S-18: A client that identifies itself by vendor class alone still boots

xCAT-test/dhcptest/tests/test_assertions.py

@@ -72,6 +72,17 @@ class Evaluation(unittest.TestCase):
         self.assertTrue(self.check("siaddr == 10.0.0.1").ok)
         self.assertFalse(self.check("siaddr == 10.0.0.2").ok)
 
+    def test_a_reply_naming_no_boot_file_is_absent_not_empty(self):
+        # A server that hands out no boot file sends an empty `file` header and
+        # no option 67. "Nothing to fetch" is therefore written `bootfile
+        # absent`: a regex against an empty string can never hold, so a
+        # scenario written that way would fail whatever the server did.
+        nothing = offer(file="", options={54: "10.0.0.1"})
+        self.assertTrue(self.check("bootfile absent", nothing).ok)
+        self.assertFalse(self.check("bootfile present", nothing).ok)
+        self.assertFalse(self.check("bootfile matches ^$", nothing).ok)
+        self.assertTrue(self.check("bootfile present").ok)
+
     def test_options_by_number_and_name(self):
         self.assertTrue(self.check("option:54 == 10.0.0.1").ok)
         self.assertTrue(self.check("option:server_id == 10.0.0.1").ok)