Write special debug app to slow down parts of ownCloud

[PVince81]

Since race conditions are difficult to test, one idea would be to write a special app that makes the following possible:

1. Create artificial delays in specific parts of the code:

• in pre/post hooks
• storage wrapper methods
• stream wrapper methods (ex: delay stream_write)
• cache wrapper that slows down cache updates (ex: rename case)
• implement a special external storage backend based on Local, but that slows down operations

2. Trigger exceptions

• trigger pre-configured exceptions in hooks (to simulate failures)
• return specific failure codes for file named "errortest-404" (returns 404), "errortest-403" (returns 403), etc (can be useful to test the sync client's response to specific errors) CC @dragotin

3. Additional ideas:

• react to special file names "lock-XXX" and keeps them locked
• configurable through public APIs ? (so the app can be enabled and driven by tests)

@jnfrmarks @nickvergessen @icewind1991

[PVince81]

This is because some race conditions can be reproduced consistently by manually adding sleep() operations in the ownCloud code, like here: owncloud/core#13391 (comment)

[PVince81]

• locking provider that stores locks in "privatedata", makes it possible to lock resources permanently for testing by using the " privatedata" OCS calls
• or alternatively the app provides its own OCS calls to permalock files for testing
• API call.to check locks or return special attribute in PROPFIND

[jnfrmarks]

@jvillafanez

[nickvergessen]

Things can get added to https://github.com/owncloud/core/tree/master/apps/testing