reject multiple rescan requests

[raedah]

Using dcrctl, its currently possible to run rescanwallet multiple times concurrently. Expected behavior is to reject the second request saying that rescan is already in progress.

[jrick]

The RPC API is permissioned. Why would this not be expected behavior, when clients could serialize or combine the rescan requests themselves? Combining rescans would be better anyways, e.g. call importscript multiple times without rescanning, and then rescan once.

[JoeGruffins]

One might interpret this comment to mean it is blocking future calls.

dcrwallet/rpc/jsonrpc/methods.go

Lines 2429 to 2430 in 746bab5

	// rescanWallet initiates a rescan of the block chain for wallet data, blocking
	// until the rescan completes or exits with an error.

[matheusd]

"blocking" in this context means that the rescan operation is blocking the caller from continuing to process stuff until it concludes (not that it's blocking other callers from executing another rescan).