migration - How to downgrade from Chrome 35 to 34 without losing data?

After upgrading Chromium from 34 to 35, the usability of Chromium went downhill. So I decided to downgrade Chromium. Fortunately, the previous version of Chromium was still somewhere on my disk, so downgrading (in ArchLinux) was as simple as:

sudo pacman -U /var/cache/pacman/pkg/chromium-34.0.1847.137-1-x86_64.pkg.tar.xz

After running the previous command, Chromium did start, but unfortunately, I kept getting bugged by the following message on start-up:

Your profile can not be used because it is from a newer version of Google Chrome. Some features may be unavailable. Please specify a different profile directory or use a newer version of Chrome.

One way to get rid of this message is to delete ~/.config/chromium/Default/Web Data, but this also caused my search engine keywords to disappear.

How can I downgrade Chromium without losing any data?

^{Note: I'm using Linux, but this problem is platform-independent. Feel free to answer from the perspective of a non-Linux operating system such as Windows.}

Answer

Note: Close Chrome and back up your profile before messing with it!

Web Data is a SQLite database, so I used the sqlite3 program to open the database.

1. 
   

   What is the current version of the "Web Data" database? (run SELECT * from meta;)

   
   

   sqlite> SELECT * FROM meta;
   Default Search Provider ID|34
   last_compatible_version|55
   version|55
   Builtin Keyword Version|70



2. 
   

   The emphasized rows in the previous output refer to internal database version numbers in Chromium. For every version, you can find the relevant migration codes in Chromium's source code (autofill_table.cc). Specifically, take a look at the AutofillTable::MigrateToVersion method. Since my database version is 55, I looked at "case 55"

   
   

   case 55:
         *update_compatible_version = true;
         return MigrateToVersion55MergeAutofillDatesTable();

   
   

   After finding out this method, I was almost ready to write a SQLite query to reverse the migration.

   
   


3. 
   

   Using the knowledge from the previous step (and the original schema from a different table on another computer), I constructed the following query (copy-paste the query to sqlite3):
   Note: This query is specific to downgrading Chrome 35 to 34!

   
   

   CREATE TABLE autofill_v54 (
       name VARCHAR,
       value VARCHAR,
       value_lower VARCHAR,
       pair_id INTEGER PRIMARY KEY,
       count INTEGER DEFAULT 1);
   CREATE TABLE autofill_dates (
       pair_id INTEGER DEFAULT 0,
       date_created INTEGER DEFAULT 0);        
   
   INSERT INTO autofill_v54 (
       name,
       value,
       value_lower,
       count
   ) SELECT name, value, value_lower, count FROM autofill;
   
   INSERT INTO autofill_dates (
       pair_id,
       date_created
   ) SELECT pair_id, date_created
     FROM autofill_v54 a54 JOIN autofill a USING (name, value);
   
   DROP TABLE autofill;
   ALTER TABLE autofill_v54 RENAME TO autofill;
   
   CREATE INDEX autofill_name ON autofill (name);
   CREATE INDEX autofill_name_value_lower ON autofill (name, value_lower);
   



4. 
   

   After reverting the migration, you can safely lower the database version:

   
   

   UPDATE meta SET value='54' WHERE key='version' OR
                                    key = 'last_compatible_version';
   



5. 
   

   Close the database using .quit

   
   

After completing all of these steps, I started Chromium, there were no annoying popups and all profile data was still intact!