Migration executed but not required

[serluca]

I had this issue in my app and I tried to figure out what was happening, I realised that sometimes the migration happened also when it was already happened, or on a new app.

The problem was always happening when apps was called in background, in my case I'm using background fetch, investigating a bit I realised that NSUserDefaults uses NSFileProtectionCompleteUntilFirstUserAuthentication. This cause the problem.

If the background fetch is executed before the first user authentication, the migration happens.

[fortinmike]

Hi @serluca, thanks for the bug report.

Makes sense. So if I understood well, you run MFMigrationManager in your background fetch and it fails to read NSUserDefaults if the user didn't authenticate beforehand (by typing their iOS PIN), leading to incorrect results, e.g. MFMigrationManager thinking that migrations are required.

Have you tried breaking into the -[MFMigrationManager lastMigrationVersion] method to confirm this?

Quickly, I see a few possible solutions:

• Do not perform migrations in your background fetch, if that's possible at all.
• Fork MFMigrationManager and implement -lastMigrationVersion and -setLastMigrationVersion:(NSString *)version to use a background-fetch-accessible plist file as a backing store instead of NSUserDefaults, then use your fork with CocoaPods. A quick and easy fix for your use case.
• Fork MFMigrationManager and instead of reimplementing version accessors, implement an abstraction for version storage that defaults to NSUserDefaults unless specified, then send a pull request :)

[serluca]

Hi @fortinmike thank you for getting back to me.

You understood what i meant, I'm using MFMigrationManager in my application:didFinishLaunchingWithOptions: that is called also in background as apple describe in the documentation.

I'm thinking to go with the solution number three but there is maybe a problem: the apps that are already using MFMigrationManager will lose the lastMigrationVersion am I right?

[fortinmike]

They wouldn't if NSUserDefaults is used as the default storage and the proper key is fetched. I'm thinking of something like this (incomplete, untested):

@protocol MFMigrationManagerStorage

- (NSString *)lastMigrationVersion;
- (void)setLastMigrationVersion:(NSString *)version;

@end

@interface MFMigrationManager : NSObject
...
@property (nonatomic, strong) id<MFMigrationManagerStorage> versionStorage; // Set to an instance of  MFMigrationManagerUserDefaultsStorage (which would implement MFMigrationManagerStorage) by default
...
@end

@implementation MFMigrationManagerUserDefaultsStorage <MFMigrationManagerStorage>

- (NSString *)lastMigrationVersion
{
    // Get "MFMigrationManagerLastVersionKey" from user defaults
}

- (void)setLastMigrationVersion:(NSString *)version
{
    // Set "MFMigrationManagerLastVersionKey" in user defaults
}

@end

@implementation MFMigrationManagerPlistStorage <MFMigrationManagerStorage>

- (instancetype)initWithPath:(NSString *)path
{
    _path = path;
}

- (NSString *)lastMigrationVersion
{
    // Get "MFMigrationManagerLastVersionKey" from _path
}

- (void)setLastMigrationVersion:(NSString *)version
{
    // Set "MFMigrationManagerLastVersionKey" in the file at _path
}

@end

Then people could continue using MFMigrationManager as if nothing changed. When a different backing store is required, usage would be slightly different:

MFMigrationManager *manager = [MFMigrationManager migrationManager];
MFMigrationManagerPlistStorage *myFileStorage= [[MFMigrationManagerPlistStorage alloc] initWithFile:@"path/to/my/file.plist"];
[manager setVersionStorage: myFileStorage];

// Then use migrations as usual

[fortinmike]

Oh and named migration managers must be taken into account (key name logic is in - (id)initWithName:(NSString *)name currentVersion:(NSString *)currentVersion). The best approach might be to pass the MFMigrationManager-generated key to the storage when getting / setting values instead of hard-coding the "MFMigrationManagerLastVersionKey" key name:

@protocol MFMigrationManagerStorage

- (NSString *)lastMigrationVersionForKey:(NSString *)lastVersionKey;
- (void)setLastMigrationVersion:(NSString *)version forKey:(NSString *)lastVersionKey;

@end