[PATCH] migration: Add more error handling to analyze-migration.py

[Fabiano Rosas]

The analyze-migration script was seen failing in s390x in misterious
ways. It seems we're reaching the subsection constructor without any
fields, which would indicate an empty .subsection entry in the vmstate
definition. 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.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 scripts/analyze-migration.py | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/scripts/analyze-migration.py b/scripts/analyze-migration.py
index 8a254a5b6a..1dd98f1d5a 100755
--- a/scripts/analyze-migration.py
+++ b/scripts/analyze-migration.py
@@ -429,6 +429,10 @@ 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 subsection key=%s name=%s" %
+                            (self.section_key, self.vmsd_name))
+
         # When we see compressed array elements, unfold them here
         new_fields = []
         for field in self.desc['struct']['fields']:
@@ -477,6 +481,11 @@ 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()
 
@@ -575,9 +584,8 @@ def __init__(self, filename):
         self.filename = filename
         self.vmsd_desc = None
 
-    def read(self, desc_only = False, dump_memory = False, write_memory = 
False):
-        # Read in the whole file
-        file = MigrationFile(self.filename)
+    def _read(self, file, vmsd_json, desc_only = False, dump_memory = False,
+              write_memory = False):
 
         # File magic
         data = file.read32()
@@ -589,7 +597,7 @@ def read(self, desc_only = False, dump_memory = False, 
write_memory = False):
         if data != self.QEMU_VM_FILE_VERSION:
             raise Exception("Invalid version number %d" % data)
 
-        self.load_vmsd_json(file)
+        self.load_vmsd_json(file, vmsd_json)
 
         # Read sections
         self.sections = collections.OrderedDict()
@@ -632,12 +640,25 @@ def read(self, desc_only = False, dump_memory = False, 
write_memory = False):
                     raise Exception("Mismatched section footer: %x vs %x" % 
(read_section_id, section_id))
             else:
                 raise Exception("Unknown section type: %d" % section_type)
-        file.close()
 
-    def load_vmsd_json(self, file):
+    def read(self, desc_only = False, dump_memory = False,
+             write_memory = False):
+        file = MigrationFile(self.filename)
         vmsd_json = file.read_migration_debug_json()
+
+        try:
+            self._read(file, vmsd_json, desc_only, dump_memory, write_memory)
+        except:
+            raise Exception("Full JSON dump:\n%s", vmsd_json)
+        finally:
+            file.close()
+
+    def load_vmsd_json(self, file, vmsd_json):
         self.vmsd_desc = json.loads(vmsd_json, 
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
-- 
2.35.3