[Ethereum] How to tell whether the Mist/Wallet is “fast” or “full” syncing

Summary

I downloaded the geth source, modified the source code to specify the fast sync pivot block, compiled the code, removed the old chaindata and started the fast syncing. Once this is complete, I'll be back to running the regular geth binaries.

UPDATE This experiment failed. There were a few different errors with my hack that prevented the blockchain to fast sync to the specified block and then normal sync after the specified block. Back to full archive node sync.

Anyone has any suggestions?

Details

I downloaded the source code for geth and modified the source code for section that calculates the fast sync pivot point eth/downloader/downloader.go, lines 419-441:

case FastSync:
    // Calculate the new fast/slow sync pivot point
    if d.fsPivotLock == nil {
        pivotOffset, err := rand.Int(rand.Reader, big.NewInt(int64(fsPivotInterval)))
        if err != nil {
            panic(fmt.Sprintf("Failed to access crypto random source: %v", err))
        }
        if height > uint64(fsMinFullBlocks)+pivotOffset.Uint64() {
            pivot = height - uint64(fsMinFullBlocks) - pivotOffset.Uint64()
        }
    } else {
        // Pivot point locked in, use this and do not pick a new one!
        pivot = d.fsPivotLock.Number.Uint64()
    }
    // If the point is below the origin, move origin back to ensure state download
    if pivot < origin {
        if pivot > 0 {
            origin = pivot - 1
        } else {
            origin = 0
        }
    }
    glog.V(logger.Debug).Infof("Fast syncing until pivot block #%d", pivot)

I modified the last line above to change the Debug into Info and added the following two lines below the code above:

    glog.V(logger.Info).Infof("Fast syncing until pivot block #%d", pivot)
    if (pivot >= 2394190) {
      pivot = 2394190;
    }
    glog.V(logger.Info).Infof("Fast syncing until modified pivot block #%d", pivot)

I recompiled and started off the fast sync process using the modified binaries:

Iota:go-ethereum user$ make geth
...
Done building.
Run "build/bin/geth" to launch geth.

I checked the version of the modified geth:

Iota:go-ethereum user$ build/bin/geth version
Geth
Version: 1.5.3-unstable

I removed the old damaged chaindata:

Iota:go-ethereum user$ build/bin/geth removedb
/Users/bok/Library/Ethereum/chaindata
Remove this database? [y/N] y

Removing...
Removed in 35.242291ms

I started the fast sync:

Iota:go-ethereum user$ build/bin/geth --fast --cache=1024 console
I1120 23:44:44.870142 ethdb/database.go:83] Allotted 1024MB cache and 1024 file handles to /Users/user/Library/Ethereum/geth/chaindata
I1120 23:44:44.878926 ethdb/database.go:176] closed db:/Users/user/Library/Ethereum/geth/chaindata
...
I1121 08:33:51.340811 eth/downloader/downloader.go:441] Fast syncing until pivot block #2664150
I1121 08:33:51.340847 eth/downloader/downloader.go:445] Fast syncing until modified pivot block #2394190

After the fast syncing is complete, I'll go back to using the regular geth binaries.

geth --fast has an interesting effect: geth cannot provide any information about accounts or contracts until the sync is fully complete.

Try querying the balance again after eth.syncing returns false.

Why?

The "fast" sync looks nearly complete because the reported current block is based on the best header you have. But that doesn't give you any information about how much of the account state data you have downloaded.

geth can sync the blocks much faster than it can download the state. An excerpt from @karalabe's explanation why:

This trie data structure is an intricate interlink of hundreds of millions of tiny cryptographic proofs (trie nodes). To truly have a synchronized node, you need to download all the account data, as well as all the tiny cryptographic proofs to verify that noone in the network is trying to cheat you. This itself is already a crazy number of data items. The part where it gets even messier is that this data is constantly morphing: at every block (15s), about 1000 nodes are deleted from this trie and about 2000 new ones are added. This means your node needs to synchronize a dataset that is changing 200 times per second. The worst part is that while you are synchronizing, the network is moving forward, and state that you begun to download might disappear while you're downloading, so your node needs to constantly follow the network while trying to gather all the recent data. But until you actually do gather all the data, your local node is not usable since it cannot cryptographically prove anything about any accounts.