darwin: workaround PROC_PIDLISTFDS on processes with no fds

This has been causing various seemingly spurious CI failures as well as
some failures on people running tests on beta builds.

lix> ++(nix-collect-garbage-dry-run.sh:20) nix-store --gc --print-dead
lix> ++(nix-collect-garbage-dry-run.sh:20) wc -l
lix> finding garbage collector roots...
lix> error: Listing pid 87261 file descriptors: Undefined error: 0

There is no real way to write a proper test for this, other than to
start a process like the following:

int main(void) {
    for (int i = 0; i < 1000; ++i) {
        close(i);
    }
    sleep(10000);
}

and then let Lix's gc look at it.

I have a relatively high confidence this *will* fix the problem since I
have manually confirmed the behaviour of the libproc call is
as-unexpected, and it would perfectly explain the observed symptom.

Fixes: https://git.lix.systems/lix-project/lix/issues/446
Change-Id: I67669b98377af17895644b3bafdf42fc33abd076

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/manual/rl-next/haunted-gc-macos.md b/doc/manual/rl-next/haunted-gc-macos.md
new file mode 100644
index 000000000..3ce912b2d
--- /dev/null
+++ b/doc/manual/rl-next/haunted-gc-macos.md
@@ -0,0 +1,15 @@
+---
+synopsis: "Fix unexpectedly-successful GC failures on macOS"
+cls: 1723
+issues: fj#446
+credits: jade
+category: Fixes
+---
+
+Has the following happened to you on macOS? This failure has been successfully eliminated, thanks to our successful deployment of advanced successful-failure detection technology (it's just `if (failed && errno == 0)`. Patent pending<sup>not really</sup>):
+
+```
+$ nix-store --gc --print-dead
+finding garbage collector roots...
+error: Listing pid 87261 file descriptors: Undefined error: 0
+```