Enable OpenMP cancellation so in-loop errors exit early

Error paths inside parallel loops emit `!$omp cancel do`, but per the
OpenMP spec cancel constructs are no-ops unless the cancel-var ICV is
true, which requires OMP_CANCELLATION=true in the environment when the
OpenMP runtime first initializes. Nothing set it, so an error raised
inside a parallel loop recorded its message correctly (first-wins
critical section) but every remaining iteration still ran -- wasted
work, and statements after a failed in-loop check kept executing in
that iteration's thread.

Set OMP_CANCELLATION=true in .onLoad when unset (a pre-set value is
respected). Caveats documented in ?declare: no effect if another
package already initialized the OpenMP runtime, so early exit is
best-effort -- error messages are always correct either way.

Author: mns-nordicals

R/declare.R

@@ -18,6 +18,15 @@
 #' and `OMP_DYNAMIC` (disable/enable runtime adjustment). Set them before
 #' calling a compiled function, e.g. `Sys.setenv(OMP_NUM_THREADS = "4")`.
 #'
+#' When an error is raised inside a parallel loop, quickr cancels the
+#' remaining iterations via OpenMP cancellation, which the OpenMP runtime
+#' only honors when `OMP_CANCELLATION=true` is set before the runtime first
+#' initializes in the process. quickr sets it when the package loads (unless
+#' already set), but this has no effect if another package initialized the
+#' OpenMP runtime first. Early exit is best-effort either way: the error
+#' message is always recorded correctly; without cancellation the remaining
+#' iterations simply run to completion before the error is raised.
+#'
 #' @param ... Declarations, typically calls like `type(x = double(n))`.
 #' @returns `NULL`, invisibly.
 #' @rawNamespace if (getRversion() < "4.4.0") export(declare)

R/zzz.R

@@ -33,5 +33,14 @@
     S7::methods_register()
     asNamespace("dotty")$dotify()
   })
+  # Generated error paths inside OpenMP loops emit `!$omp cancel do`, which
+  # the OpenMP runtime treats as a no-op unless the cancel-var ICV is true.
+  # The ICV is read from OMP_CANCELLATION when the runtime first initializes
+  # in the process, so set it as early as quickr can; this has no effect if
+  # another package already started the OpenMP runtime. Error *messages* are
+  # recorded correctly either way -- cancellation only enables early exit.
+  if (!nzchar(Sys.getenv("OMP_CANCELLATION"))) {
+    Sys.setenv(OMP_CANCELLATION = "true")
+  }
   # on_load_register_.AtNames.default()
 }

man/declare.Rd

@@ -0,0 +1,14 @@
+# Unit test for package-load side effects
+
+skip_on_cran()
+
+test_that("OMP_CANCELLATION is set on load when unset, respected when preset", {
+  withr::with_envvar(c(OMP_CANCELLATION = NA), {
+    quickr:::.onLoad()
+    expect_identical(Sys.getenv("OMP_CANCELLATION"), "true")
+  })
+  withr::with_envvar(c(OMP_CANCELLATION = "false"), {
+    quickr:::.onLoad()
+    expect_identical(Sys.getenv("OMP_CANCELLATION"), "false")
+  })
+})