Inbox Database ("write-only" mode)
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                 Key: COUCHDB-1287
                 URL: https://issues.apache.org/jira/browse/COUCHDB-1287
             Project: CouchDB
          Issue Type: New Feature
          Components: HTTP Interface
    Affects Versions: 2.0
            Reporter: Jason Smith
            Priority: Minor


Currently, we can only grant combined read+write access in the _security object "members" section. A user can either do both or neither. This prevents a very common requirement for couch apps: sending private information from less-privileged users to more-privileged users.

There is no (reasonable) way to make an "inbox" where anybody may create a doc for me, but only I may read it. An inbox database allows user-to-user, or user-to-admin private messages. (Not only chat messages, but asynchronous notifications--with a per-user inbox, perhaps even service requests and responses.)

There is no reason _security.members (formerly .readers) should control write access. validate_doc_update() functions do this better.

I propose a boolean flag, _security.members.allow_anonymous_writes. If it is true, then CouchDB will allow document updates from non-members, giving validate_doc_update() the final word on accepting or rejecting the update.

Requirements:

1. Everything about _security stays the same (backward-compatible)
2. If members.allow_anonymous_writes === true, then most PUT and POSTs may proceed
3. All updates are still subject to approval by all validate_doc_update functions, same as before.

The following unit tests cover as much of the functionality as I can think of. (My patch is unfinished but X indicates that I have it working.)

X Set a database with validate_doc_update, members != []
X member can write
X non-member cannot read
X non-member cannot write
X non-member cannot write even with .is_ok = true
X Set inbox mode
For non-member:
  X cannot update with .is_ok = false (still subject to validator)
  X can create with .is_ok = true
  X can update with .is_ok = true
  X Can store an attachment with "_attachments"
  X Can store attachments via direct query
  X Can delete an attachment via direct query
  X can delete the doc
  X can create via an _update function
  X can update via an _update function
  * None of these should work:
    X POST a temp view
    X POST a view with {"keys":["keys", "which", "exist", "and some which don't"]
    * POST /db/exist X-HTTP-Method-Override: GET
    * POST /db/_all_docs
    * POST /db/_changes
    * For _show and _list:
      * POST
      * OPTIONS
      * VARIOUS, NONSTANDARD, METHODS (in case Couch allows them later)
  * These syntax/semantic errors in _security should all fail:
    * .members.required_to_write = null, [missing], "", 0, true, 1, "false", [false], {false:false}
    * .required_to_write = false

These are the known changes to the security model. I consider these all to be either very unlikely in practice, or worth the trade-off.

* If you write to an inbox DB, you know, for a time, a subset of its documents (but that's the point)
* An _update function could reveal a document to the user, with or without changing it. However, an admin must install such a misguided update function.
* You can launch timing attacks to learn information about validate_doc_update
  * You might discover whether doc IDs exist in the DB or not
  * You might discover a well-known open source validation function. You can look for bugs in its source code.
* Zero or more things which Jason can't think of

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