I'll give it a try and we'll see if it's problematic.

https://codereview.chromium.org/2866283002

The following revision refers to this bug:
  https://chromium.googlesource.com/external/github.com/luci/luci-py.git/+/d1ab255eac4e9ef88d437d476a586d22cda15952

commit d1ab255eac4e9ef88d437d476a586d22cda15952
Author: nodir <nodir@chromium.org>
Date: Wed May 10 19:18:44 2017

named caches: move instead of symlinking

Moving named caches is better than symlinking because
- C++ compiler used in swarming tasks reads symlinks =>
  compiler output contains absolute paths to named caches =>
  compiler cache key contains CWD =>
  compiler cache is useless
- if a bot dies, there is a higher chance that a cache is corrupted =>
  better to discard the cache by not moving it back
- simpler

Move named cache directories to the task rundir before running a task,
and move them back when task completes (and bot did not die).

Revisit NamedCacheManager design, simplify its API.
install() leaves the cache manager in consistent state: the entries are removed
from the cache. uninstall() puts them back.

R=maruel@chromium.org
BUG= 715326 

Review-Url: https://codereview.chromium.org/2866283002

[modify] https://crrev.com/d1ab255eac4e9ef88d437d476a586d22cda15952/client/named_cache.py
[modify] https://crrev.com/d1ab255eac4e9ef88d437d476a586d22cda15952/client/run_isolated.py
[modify] https://crrev.com/d1ab255eac4e9ef88d437d476a586d22cda15952/client/tests/named_cache_test.py
[modify] https://crrev.com/d1ab255eac4e9ef88d437d476a586d22cda15952/client/tests/run_isolated_test.py

The following revision refers to this bug:
  https://chromium.googlesource.com/external/github.com/luci/luci-py.git/+/62d13e1c00497d006173c4d9cb496098284caf9a

commit 62d13e1c00497d006173c4d9cb496098284caf9a
Author: maruel <maruel@chromium.org>
Date: Thu May 11 17:28:22 2017

Revert "named caches: move instead of symlinking"

This reverts commit d1ab255eac4e9ef88d437d476a586d22cda15952.

Caused failures in task_runner_test.py and local_smoke_test.py. This passed the
presubmit because the CL only touches client/, so the Swarming server side tests
were bypassed.

TBR=nodir@chromium.org
BUG= 715326 

Review-Url: https://codereview.chromium.org/2875113002

[modify] https://crrev.com/62d13e1c00497d006173c4d9cb496098284caf9a/client/named_cache.py
[modify] https://crrev.com/62d13e1c00497d006173c4d9cb496098284caf9a/client/run_isolated.py
[modify] https://crrev.com/62d13e1c00497d006173c4d9cb496098284caf9a/client/tests/named_cache_test.py
[modify] https://crrev.com/62d13e1c00497d006173c4d9cb496098284caf9a/client/tests/run_isolated_test.py

The following revision refers to this bug:
  https://chromium.googlesource.com/external/github.com/luci/luci-py.git/+/b551d1cc05919318aa1d643cfa174a2e2c451ae8

commit b551d1cc05919318aa1d643cfa174a2e2c451ae8
Author: nodir <nodir@chromium.org>
Date: Thu May 11 20:21:53 2017

Reland "named caches: move instead of symlinking"

This relands d1ab255eac4e9ef88d437d476a586d22cda15952
with fixed tests.

BUG= 715326 

Review-Url: https://codereview.chromium.org/2877483004

[modify] https://crrev.com/b551d1cc05919318aa1d643cfa174a2e2c451ae8/appengine/swarming/local_smoke_test.py
[modify] https://crrev.com/b551d1cc05919318aa1d643cfa174a2e2c451ae8/appengine/swarming/swarming_bot/bot_code/task_runner_test.py
[modify] https://crrev.com/b551d1cc05919318aa1d643cfa174a2e2c451ae8/client/named_cache.py
[modify] https://crrev.com/b551d1cc05919318aa1d643cfa174a2e2c451ae8/client/run_isolated.py
[modify] https://crrev.com/b551d1cc05919318aa1d643cfa174a2e2c451ae8/client/tests/named_cache_test.py
[modify] https://crrev.com/b551d1cc05919318aa1d643cfa174a2e2c451ae8/client/tests/run_isolated_test.py

phosek@, fyi this was deployed. cache directories are now real dirs, not symlinks