Add a Restore() method to ValueStore and make StorageAPI use it

chrome/browser/value_store/value_store.h

Index: chrome/browser/value_store/value_store.h
diff --git a/chrome/browser/value_store/value_store.h b/chrome/browser/value_store/value_store.h
index 2b203ea57b779297a647e991b0825b64e64de9ae..3eaa1c91bba457daf601bf2b13cd86ecee102684 100644
--- a/chrome/browser/value_store/value_store.h
+++ b/chrome/browser/value_store/value_store.h
@@ -191,6 +191,27 @@ class ValueStore {
 
   // Clears the storage.
   virtual WriteResult Clear() = 0;
+
+  // In the event of corruption, the ValueStore should be able to restore
+  // itself. This means deleting local corrupted files. If only a few keys are
+  // corrupted, then some of the database may be saved. If the full database is
+  // corrupted, this will erase it in its entirety.
+  // Returns true on success, false on failure. The only way this will fail is
+  // if we also cannot delete the database file.
+  // Note: This method needs to read (and possibly delete) the whole database,

[not at google - send to devlin, 2014/02/14 19:35:56]

why does it need to read the whole database?

[Devlin, 2014/02/18 23:55:22]

Because it relies on Get() to ensure that there's no corruption, which reads
everything in the database.  I put in RestoreKey() as a more fine-grained
approach, but sometimes there's no specific key at fault, and we have to blow it
all away.  The only way to make sure the restore succeeded, though, is to Get()
it. :/

[not at google - send to devlin, 2014/02/19 21:25:29]

Ok. That's an implementation detail though. It may need to Get(), or leveldb may
add some cool method down the line which sanity checks state.

I think a comment like "Note: this method may be expensive, for example, some
implementations may need to read the entire database in order to restore. Use
sparing."

along those lines.

[Devlin, 2014/02/19 23:12:28]

Done.

+  // so it is slow and should be used sparingly.
+  // Note: This method (and the following RestoreKey()) are rude, and do not
+  // make any logs, track changes, or other generally polite things. Please do
+  // not use these as substitutes for Clear() and Remove().
+  virtual bool Restore() = 0;
+
+  // Similar to Restore(), but for only a particular key. If the key is corrupt,
+  // this will forcefully remove the key. It does not look at the database on
+  // the whole, which makes it faster, but does not guarantee there is no
+  // additional corruption.
+  // Returns true on success, and false on failure. If false, the next step is
+  // probably to Restore() the whole database.
+  virtual bool RestoreKey(const std::string& key) = 0;
 };
 
 #endif  // CHROME_BROWSER_VALUE_STORE_VALUE_STORE_H_