Do not error when src has no functions

[mpadge]

Hi @jimhester - looks like you're the only one left maintaining this package, so sorry to both you 😏 I've got an issue with some src C code which contains no actual functions. This fails on the gcov call here:

covr/R/compiled.R

Lines 77 to 79 in caed186

	system_check(gcov_path,
	args = c(gcov_args, src, "-p", "-o", dirname(src)),
	quiet = quiet, echo = !quiet)

because gcov errors when there are no executable lines. Here's a reprex based on this src file:

library (covr)
d <- file.path (tempdir (), "typetracer")
path <- gert::git_clone ("https://github.com/mpadge/typetracer", path = d)
flags <- getOption("covr.flags")
covr:::clean_objects (path)
install_path <- covr:::temp_file("R_LIBS")
dir.create(install_path)
Sys.setenv ("R_LIBS_USER" = file.path (tempdir (), "Library"))
withr::with_makevars(flags, assignment = "+=",
    utils::install.packages(
        repos = NULL,
        lib = install_path,
        path,
        type = "source",
        INSTALL_opts = c("--example",
                         "--install-tests",
                         "--with-keep.source",
                         "--with-keep.parse.data",
                         "--no-staged-install",
                         "--no-multiarch"),
        quiet = TRUE
))

quiet <- FALSE
gcov_path <- getOption("covr.gcov", "")
gcov_args <- NULL
src_path <- covr:::normalize_path(file.path(path, "src"))
gcov_inputs <- list.files(path, pattern = rex::rex(".gcno", end), recursive = TRUE, full.names = TRUE)
print (gcov_inputs)
#> [1] "/tmp/RtmpT9LnNK/typetracer//src/utils.gcno"

# 'run_gcov_one' fn in 'run_gov' with 'src' param:
src <- gcov_inputs
    covr:::system_check(gcov_path,
        args = c(gcov_args, src, "-p", "-o", dirname(src)),
        quiet = quiet, echo = !quiet)
#> '/usr/bin/gcov' '/tmp/RtmpT9LnNK/typetracer//src/utils.gcno' '-p' '-o'  \
#>   '/tmp/RtmpT9LnNK/typetracer//src'
#> Error: Command ''/usr/bin/gcov' '/tmp/RtmpT9LnNK/typetracer//src/utils.gcno' '-p' '-o' '/tmp/RtmpT9LnNK/typetracer//src'' failed (3)

# Re-written to capture actual messages:
args <- c(gcov_args, src, "-p", "-o", dirname(src))
full <- paste(c(shQuote(gcov_path), lapply(args, shQuote)), collapse = " ")
s = system(full, intern = TRUE, ignore.stderr = FALSE, ignore.stdout = FALSE)
#> Warning in system(full, intern = TRUE, ignore.stderr = FALSE, ignore.stdout =
#> FALSE): running command ''/usr/bin/gcov'
#> '/tmp/RtmpT9LnNK/typetracer//src/utils.gcno' '-p' '-o'
#> '/tmp/RtmpT9LnNK/typetracer//src'' had status 3
print (s)
#> [1] "No executable lines"
#> attr(,"status")
#> [1] 3

^{Created on 2023-06-23 with reprex v2.0.2}

Plus the console output of gcov has the extra line

#> '/tmp/RtmpT9LnNK/typetracer//src/utils.gcno:no functions found

There is no way to circumvent this, and thus no way to get package_coverage to succeed on this package, so any help would be appreciated! Just some kind of modification of the system_check shown above to return empty yet still succeed when no functions found?