Reset user data directory and disk cache directory after downgrade.

chrome/browser/user_profiles_cleanup.h

Index: chrome/browser/user_profiles_cleanup.h
diff --git a/chrome/browser/user_profiles_cleanup.h b/chrome/browser/user_profiles_cleanup.h
new file mode 100644
index 0000000000000000000000000000000000000000..5f03bcb783d68bcd0a6f10f3305dfe8464d08af7
--- /dev/null
+++ b/chrome/browser/user_profiles_cleanup.h
@@ -0,0 +1,16 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_USER_PROFILES_CLEANUP_H_
+#define CHROME_BROWSER_USER_PROFILES_CLEANUP_H_
+
+// Rename user data directory and disk cache directory during first laucnh of
+// Chrome after downgrade. This function does not support root directory.
+void ResetUserProfilesAfterDowngrade();

[grt (UTC plus 2), 2016/05/19 03:24:18]

(pre-comment: make function/method doc comments descriptive rather than
imperative as per
https://google.github.io/styleguide/cppguide.html#Function_Comments: "Renames
the ...")

"profile" is the wrong word since it's too easily confused with multi-profile.
the user data directory is what we're talking about in practice. in the
abstract, though, what we're doing here is removing all locally stored data
because a downgrade has happened and we can't rely on an older version of Chrome
being able to read data from a newer version.

"AfterDowngrade" relates more to why this would be called rather than what it
does. i think it's good to separate these two things: name this function for
what it does, and document at the callsite why it is being called.

in that light, what do you think about "MoveUserDataForFirstRun" or something
like that to convey that this function moves all user data directories out of
the way so that a first-run will commence? one thing that this leaves out is
that a subsequent call to the Remove function below is needed. i think it's safe
to cover this in the doc comment. as such, please explain in the doc comment
that, while this function does involve blocking I/O, it does only preliminary
work that must be followed up with a call to Remove... at some point after
startup.

maybe:

// Moves aside all directories containing user data to prevent the browser from
// reading them. When called early in startup, this will result in the browser
// performing first-run for the user. To avoid long-running I/O operations, this
// function merely moves directories aside. A subsequent call to
// RemoveMovedUserData (below) must be made later in startup to free up disk
// space.
void MoveUserDataForFirstRun();

[grt (UTC plus 2), 2016/05/19 14:08:05]

I wrote that before reading the code and seeing that this function does the
downgrade detection. Please ignore this part. I think it is important to put
"AfterDowngrade" or "OnDowngrade" or something in the name and document it.

[zmin, 2016/05/19 23:08:03]

Change the name to MoveUserDataForFirstRunAfterDowngrade(). Also mentioned
downgrade in the comments. 

+
+// Delete the renamed user data directory and disk cache directory after Chrome

[grt (UTC plus 2), 2016/05/19 03:24:18]

suggestion:
// Deletes any user data directories left behind after a previous
Reset...AfterDowngrade.

[zmin, 2016/05/19 23:08:03]

Done.

+// is lauched.
+void RemoveUnusedUserProfilesInBlockingPool();

[grt (UTC plus 2), 2016/05/19 03:24:18]

this poses an interesting design decision: should the caller be responsible for
picking the thread, or should the impl take care of it? in my world view, most
of the browser lives on the main (UI) thread, except for all of the stuff that
doesn't. :-)  with that in mind, it's nice for the expectation to be that this
function will be called from the main thread and that it will magically do the
right thing. the fact that it may use the blocking pool is an implementation
detail that doesn't need to bleed into the interface, so i'd leave that out of
the function name.

here's my suggestion based on the comment above (and inspired by
SequencedTaskRunner::DeleteSoon):
// Schedules a search for the removal of any directories moved aside by
// MoveUserDataForFirstRun. This operation is idempotent, and may be safely
// called when no such directories exist.
void DeleteMovedUserDataSoon();

[zmin, 2016/05/19 23:08:03]

Done.

+
+#endif  // CHROME_BROWSER_USER_PROFILES_CLEANUP_H_