migration: Add more error handling to analyze-migration.py

The analyze-migration script was seen failing in s390x in misterious
ways. It seems we're reaching the VMSDFieldStruct constructor without
any fields, which would indicate an empty .subsection entry, a
VMSTATE_STRUCT with no fields or a vmsd with no fields. We don't have
any of those, at least not without the unmigratable flag set, so this
should never happen.

Add some debug statements so that we can see what's going on the next
time the issue happens.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20250109185249.23952-2-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

scripts/analyze-migration.py

@@ -429,6 +429,9 @@ def __init__(self, desc, file):
         super(VMSDFieldStruct, self).__init__(desc, file)
         self.data = collections.OrderedDict()
 
+        if 'fields' not in self.desc['struct']:
+            raise Exception("No fields in struct. VMSD:\n%s" % self.desc)
+
         # When we see compressed array elements, unfold them here
         new_fields = []
         for field in self.desc['struct']['fields']:
@@ -477,6 +480,10 @@ def read(self):
                     raise Exception("Subsection %s not found at offset %x" % ( subsection['vmsd_name'], self.file.tell()))
                 name = self.file.readstr()
                 version_id = self.file.read32()
+
+                if not subsection:
+                    raise Exception("Empty description for subsection: %s" % name)
+
                 self.data[name] = VMSDSection(self.file, version_id, subsection, (name, 0))
                 self.data[name].read()
 
@@ -574,10 +581,13 @@ def __init__(self, filename):
         }
         self.filename = filename
         self.vmsd_desc = None
+        self.vmsd_json = ""
 
-    def read(self, desc_only = False, dump_memory = False, write_memory = False):
+    def read(self, desc_only = False, dump_memory = False,
+             write_memory = False):
         # Read in the whole file
         file = MigrationFile(self.filename)
+        self.vmsd_json = file.read_migration_debug_json()
 
         # File magic
         data = file.read32()
@@ -635,9 +645,11 @@ def read(self, desc_only = False, dump_memory = False, write_memory = False):
         file.close()
 
     def load_vmsd_json(self, file):
-        vmsd_json = file.read_migration_debug_json()
+        self.vmsd_desc = json.loads(self.vmsd_json,
-        self.vmsd_desc = json.loads(vmsd_json, object_pairs_hook=collections.OrderedDict)
+                                    object_pairs_hook=collections.OrderedDict)
         for device in self.vmsd_desc['devices']:
+            if 'fields' not in device:
+                raise Exception("vmstate for device %s has no fields" % device['name'])
             key = (device['name'], device['instance_id'])
             value = ( VMSDSection, device )
             self.section_classes[key] = value
@@ -666,31 +678,34 @@ def default(self, o):
 
 jsonenc = JSONEncoder(indent=4, separators=(',', ': '))
 
-if args.extract:
+if not any([args.extract, args.dump == "state", args.dump == "desc"]):
-    dump = MigrationDump(args.file)
-
-    dump.read(desc_only = True)
-    print("desc.json")
-    f = open("desc.json", "w")
-    f.truncate()
-    f.write(jsonenc.encode(dump.vmsd_desc))
-    f.close()
-
-    dump.read(write_memory = True)
-    dict = dump.getDict()
-    print("state.json")
-    f = open("state.json", "w")
-    f.truncate()
-    f.write(jsonenc.encode(dict))
-    f.close()
-elif args.dump == "state":
-    dump = MigrationDump(args.file)
-    dump.read(dump_memory = args.memory)
-    dict = dump.getDict()
-    print(jsonenc.encode(dict))
-elif args.dump == "desc":
-    dump = MigrationDump(args.file)
-    dump.read(desc_only = True)
-    print(jsonenc.encode(dump.vmsd_desc))
-else:
     raise Exception("Please specify either -x, -d state or -d desc")
+
+try:
+    dump = MigrationDump(args.file)
+
+    if args.extract:
+        dump.read(desc_only = True)
+
+        print("desc.json")
+        f = open("desc.json", "w")
+        f.truncate()
+        f.write(jsonenc.encode(dump.vmsd_desc))
+        f.close()
+
+        dump.read(write_memory = True)
+        dict = dump.getDict()
+        print("state.json")
+        f = open("state.json", "w")
+        f.truncate()
+        f.write(jsonenc.encode(dict))
+        f.close()
+    elif args.dump == "state":
+        dump.read(dump_memory = args.memory)
+        dict = dump.getDict()
+        print(jsonenc.encode(dict))
+    elif args.dump == "desc":
+        dump.read(desc_only = True)
+        print(jsonenc.encode(dump.vmsd_desc))
+except Exception:
+    raise Exception("Full JSON dump:\n%s", dump.vmsd_json)