The documentation of the "slot_testv_and_readv_and_writev" method in interfaces.py requires clarification.

[zancas]

There are several ambiguities, or errors, in the old version [ or my understanding thereof ;-) ].

First: It wasn't clear that the read operation occurs regardless of the outcome of the test operation.

Second: It ambiguously states that the write vectors are applied to "those shares" (in fact [iiuc] the write vector shares may be overlapping, or even disjoint, from the existing shares).

It _is_ correct that the shares which may be written will only be written in the case that a call to:

SOMESHARE.check_testv(testv)  

returns "True" for all shares in the test_and_write_vectors, but the SOMESHARE will be EmptyShare if the share to be written is not already on the server.

Here's how I changed it in a local git repo:

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       modified:   interfaces.py
#
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#       modified:   mutable/layout.py
#
diff --git a/src/allmydata/interfaces.py b/src/allmydata/interfaces.py
index a73ce86..b15863b 100644
--- a/src/allmydata/interfaces.py
+++ b/src/allmydata/interfaces.py
@@ -159,10 +159,13 @@ class RIStorageServer(RemoteInterface):
                                         tw_vectors=TestAndWriteVectorsForShares,
                                         r_vector=ReadVector,
                                         ):
-        """General-purpose test-and-set operation for mutable slots. Perform
-        a bunch of comparisons against the existing shares. If they all pass,
-        then apply a bunch of write vectors to those shares. Then use the
-        read vectors to extract data from all the shares and return the data.
+        """
+        General-purpose atomic test-read-and-set operation for mutable slots.
+        (1) Perform comparisons tests against the extant shares. 
+        (2) Use the read vectors to extract "old data" from all extant shares.
+        (3) If all tests in (1) passed: then write tw_vectors shares. 
+        (4) Return whether the tests passed and the "old data", which does not 
+        include any modifications made by the writes.

         This method is, um, large. The goal is to allow clients to update all
         the shares associated with a mutable file in a single round trip.
@@ -232,9 +235,11 @@ class RIStorageServer(RemoteInterface):
         than the size of the data after applying all write vectors.

         The read vector is used to extract data from all known shares,
-        *before* any writes have been applied. The same vector is used for
-        all shares. This captures the state that was tested by the test
-        vector.
+        *before* any writes have been applied. The same read vector is used 
+        for all shares. This captures the state that was tested by the test
+        vector, for extant shares. Of course, the extracted data contains no
+        information about shares written to new shares that did not previously
+        exist.

         This method returns two values: a boolean and a dict. The boolean is
         True if the write vectors were applied, False if not. The dict is