Can't cancel long-running queries

[krlmlr]

Not in the RStudio IDE, not in the plain terminal. Hitting Ctrl + C when running this script will achieve nothing.

@hannes: I checked, we either need to call R_CheckUserInterrupt() periodically, or perhaps install our own signal handler and remove it for every single call to an R API. Is there a callback from duckdb that would be fired regularly from the main thread?

library(DBI)
con <- dbConnect(duckdb::duckdb())

dbExecute(con, "CREATE TABLE data AS SELECT unnest(generate_series(1, 100000)) AS a")
#> [1] 1e+05

system.time(dbGetQuery(con, "SELECT COUNT(*) FROM data JOIN data AS data2 ON data.a != data2.a"))
#>    user  system elapsed 
#>  10.027   0.060  10.514

dbDisconnect(con)

^{Created on 2024-10-19 with reprex v2.1.1}

[krlmlr]

We already use PendingQuery() and then ExecuteTask(), but for this particular query, cancellation works only "sometimes". Need to review timing of the checks, perhaps there's a long non-interruptible call to ExecuteTask() .

Draft branch: https://github.com/duckdb/duckdb-r/tree/f-514-cancel . Looks like we need to check R's internal R_interrupts_pending, seen this in other packages too.

Example that works flawlessly in the terminal and in RStudio: https://github.com/krlmlr/cancel.test .

Assuming that PendingQuery() can also be used for relational objects. We'll see.

[krlmlr]

The query is interesting: sometimes it issues only a handful of tasks, one of them very long-running, sometimes a lot of tiny tasks. This explains the "sometimes".

The PR contains a draft that should work on non-Windows. We no longer need PendingQuery(), instead it installs its own temporary signal handler, which is safe at this stage.

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