In r-prof/winch: Portable Native and Joint Stack Traces

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%"
)
pkgload::load_all()

winch

Winch provides stack traces for call chains that cross between R and C function calls. This is a useful tool for developers of R packages where a substantial portion of the code is C or C++.

Installation

Install the released version of winch from CRAN with:

install.packages("winch")

Install the development version from GitHub with:

devtools::install_github("r-lib/winch")

Example

Below is an example where an R function calls into C which calls back into R. Note the second-to-last entry in the trace:

library(winch)

foo <- function() {
  winch_call(bar)
}

bar <- function() {
  winch_trace_back()
}

foo()

rlang::entrace() checks if winch is installed, and adds a native backtrace. As this cannot be easily demonstrated in a knitr document, the output is copied from a GitHub Actions run.

options(
  error = rlang::entrace,
  rlang_backtrace_on_error = "full",
  rlang_trace_use_winch = TRUE
)

vctrs::vec_as_location(quote, 2)

Error: Must subset elements with a valid subscript vector. ✖ Subscript has the wrong type `function`. ℹ It must be logical, numeric, or character. Backtrace: █ 1. └─vctrs::vec_as_location(quote, 2) 2. └─`/vctrs.so`::vctrs_as_location() 3. └─`/vctrs.so`::vec_as_location_opts()

How does it work?

winch uses a very simple heuristic. R's traceback (and also profiling) infrastructure introduces the notion of a "context". Every call to an R function opens a new context and closes it when execution of the function ends. Unfortunately, no new context is established for native code called with .Call() or .External(). Establishing contexts expends precious run time, so this may be the reason for the omission.

To work around this limitation, the source code of all R functions along the call chain is scanned for instances of .Call and .External. The native call stack (obtained via libunwind or libbacktrace) is scanned for chunks of code outside of libR.so (R's main library) -- these are assumed to correspond to .Call() or .External(). The native traces are embedded as artificial calls into the R stack trace.

Limitations

The matching will not be perfect, but it may still lead to faster discovery of the cause of an error.
On Windows winch only works on x64, and there the traces can be obtained only for one shared library at a time. See winch_init_library() for details.