Auto-refresh help topics

[renkun-ken]

Currently, it looks like help topics could be properly shown if getAliases.R is run with the latest installed packages.

As reported in #661, #853, if a new package is installed but getAliases.R is not run, the cache is out of date and thus both "Open help for selection" and "hover" will not work with the newly installed packages. Manually clicking "Clear Cached Index Files & Restart Help Server" could refresh the aliases but it seems possible to make it more automatic.

@ManuelHentschel what if we make some changes to helpServer.R to periodically detect changes of installed packages and write the aliases json once there are changes, and getAliases.R will be no longer needed.

# get values from extension-set env values
lim <- Sys.getenv("VSCR_LIM")

cat(
    lim,
    tools::startDynamicHelp(),
    lim,
    sep = ""
)

pkgs <- character()
while (TRUE) {
    cur_pkgs <- .packages(all.available = TRUE)
    if (!identical(pkgs, cur_pkgs)) {
        ip <- installed.packages()

        ord <- order(ip[, "Priority"])
        ip <- ip[ord, c("LibPath", "Package")]

        ret <- lapply(rownames(ip), function(row) {
            libPath <- ip[row, "LibPath"]
            pkg <- ip[row, "Package"]
            filename <- file.path(libPath, pkg, "help", "aliases.rds")
            info <- list(
                package = pkg,
                libPath = libPath,
                aliasFile = filename,
                aliases = NULL
            )
            if (file.exists(filename)) {
                info[["aliases"]] <- as.list(readRDS(filename))
            }
            info
        })
        names(ret) <- rownames(ip)
        json <- jsonlite::toJSON(ret, auto_unbox = TRUE)
        cat(lim, json, lim, "\n", sep = "")
        pkgs <- cur_pkgs
    }
    Sys.sleep(1)
}

Whenever a new package installed, the latest aliases will be written to stdout, or otherwise it is checking every second, which should not be expensive.

And the help manager should handle both output accordingly. One is help server port message, and the other is aliases json message.

[ManuelHentschel]

I agree we should implement sth. like this. Your proposition looks sensible, I'll try to take a closer look in the coming days!