It's not possible to run RPC commands (not even by bitcoin-cli) if the initial rescan hasn't completed. A long term alternative is to move this rescan after the initialization? /cc @ryanofsky

A long term alternative is to move this rescan after the initialization? /cc @ryanofsky

Since the rescan loop doesn't hold onto cs_main anymore, I'd expect that RPC commands could work during a rescan, and don't understand exactly what problem causes this crash, and what problem causes bitcoin-cli not to work right now.

Longer term, I think the main thing that needs to happen is for the rescan loop to move out of the wallet into the node (src/node/rescan.cpp or someplace like that) so when multiple wallets are loaded they are all rescanned in parallel instead of sequentially one after another other. I expect the way that would work is that during loading, each wallet would asynchronously request a rescan from the node, passing it a locator. Then after all the wallets are loaded, the node would do a single rescan from the earliest point, sending wallets the normal notifications.

In terms of implementation I would build this on top of #15719 by adding a CBlockLocator argument to Chain::requestNotifications() Chain::handleNotifications(). There would also need to be explicit requestRescan/abortRescan Chain methods to trigger rescans after wallets are loaded, for when users import keys or explicity request rescans.

and what problem causes bitcoin-cli not to work right now.

Because the server gives RPC_IN_WARMUP:

src/bitcoin-cli -regtest help
error code: -28
error message:
Rescanning...

don't understand exactly what problem causes this crash

Because autoCompleter is null here:
https://github.com/bitcoin/bitcoin/blob/727143b65cdb801a0b755cb8b3aff97da5fe8f27/src/qt/rpcconsole.cpp#L535

I think the main thing that needs to happen is for the rescan loop to move out of the wallet into the node

This would go away right?

tACK 2d8ad2f on macOS.

Using master (c7cfd20) it's possible to cause a crash during startup by opening Window -> Console typing anything into the console and hitting enter.

lldb Bitcoin-Qt.app
(lldb) target create "Bitcoin-Qt.app"
Current executable set to 'Bitcoin-Qt.app' (x86_64).
(lldb) run
Process 59937 launched: '/Users/michael/github/bitcoin/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt' (x86_64)
2019-05-31 09:53:29.580881-0400 Bitcoin-Qt[59937:3647765] MessageTracer: Falling back to default whitelist
Process 59937 stopped
* thread #1, name = 'bitcoin-main', queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x8)
    frame #0: 0x00000001011a78ba QtWidgets`QCompleter::popup() const + 10
QtWidgets`QCompleter::popup:
->  0x1011a78ba <+10>: movq   0x8(%rdi), %rbx
    0x1011a78be <+14>: movq   0x88(%rbx), %rax
    0x1011a78c5 <+21>: testq  %rax, %rax
    0x1011a78c8 <+24>: jne    0x1011a7942               ; <+146>
Target 0: (Bitcoin-Qt) stopped.

Using this PR (2d8ad2f), it's still possible to get to the console during startup, but it's impossible to enter or execute any commands. The console is enabled when startup completes.