Fix parsing of filtered files

This bug was originally triggered by the test suite, because the
error message in the code had a typo. While looking for that I noticed
that the code that was parsing the code seemed fishy.

This is now cleaner. First of all, I split the error messages for
"unknown tag" and "no tag". Then I rewrote the loop that parses
so it is robust against error situations. Then I tweaked the yarn
test so it now works.

2 files changed, 22 insertions, 7 deletions

diff --git a/obnamlib/hooks.py b/obnamlib/hooks.py
index 91baed3b..a8a98a00 100644
--- a/obnamlib/hooks.py
+++ b/obnamlib/hooks.py
@@ -73,7 +73,12 @@ class Hook(object):
 
 class MissingFilterError(obnamlib.ObnamError):
 
-    msg = 'Unknown filter tag enountered: {tagname}'
+    msg = 'Unknown filter tag: {tagname}'
+
+
+class NoFilterTagError(obnamlib.ObnamError):
+
+    msg = 'No filter tag found'
 
 
 class FilterHook(Hook):
@@ -108,13 +113,23 @@ class FilterHook(Hook):
         raise NotImplementedError()
 
     def run_filter_read(self, data, *args, **kwargs):
-        tag, content = data.split("\0", 1)
-        while tag != "":
+
+        def filter_next_tag(data):
+            split = data.split('\0', 1)
+            if len(split) == 1:
+                raise NoFilterTagError()
+            tag, remaining = split
+            if tag == '':
+                return False, remaining
             if tag not in self.bytag:
                 raise MissingFilterError(tagname=repr(tag))
-            data = self.bytag[tag].filter_read(content, *args, **kwargs)
-            tag, content = data.split("\0", 1)
-        return content
+            callback = self.bytag[tag]
+            return True, callback.filter_read(remaining, *args, **kwargs)
+
+        more = True
+        while more:
+            more, data = filter_next_tag(data)
+        return data
 
     def run_filter_write(self, data, *args, **kwargs):
         tracing.trace('called')
diff --git a/yarns/0060-encryption.yarn b/yarns/0060-encryption.yarn
index dbdc4c1d..384d7a70 100644
--- a/yarns/0060-encryption.yarn
+++ b/yarns/0060-encryption.yarn
@@ -99,7 +99,7 @@ client, should not be able to access the data.
     GIVEN a user U3 calling themselves U1
     WHEN user U3 attempts to restore their latest generation in repository R into X3
     THEN the attempt failed with exit code 1
-    AND the error message matches "Unknown filter tag encountered\|need more than 1 value to unpack"
+    AND the error message matches "R169C6X: Unknown filter tag\|R173AEX: No filter tag found"
 
 (The error message above indicates that there's a bug in Obnam, which
 is that the error message, when an encryption is not provided but the