[bug#56728] [PATCH] guile-fibers: Backport the ‘epoll instance is dead’ bugfix.

[Maxime Devos]

This fixes some indeterministic test failures for gnunet-scheme observed while
developing gnunet-scheme.  The following checks were made:

All dependents compile for x86_64:

$ ./pre-inst-env guix build shepherd gnunet-scheme guix-build-coordinator 
nar-herder guix-data-service guile-fibers --system=x86_64-linux

Everything except guix-build-coordinator (which depends on the
non-cross-compilable sqitch) cross-compile from x86_64 to aarch64-linux-gnu:

$ ./pre-inst-env guix build shepherd gnunet-scheme nar-herder  guile-fibers 
--system=x86_64-linux --target=aarch64-linux-gnu

I tried testing for reproducibility but the check phase took to long.
However, you can compare the hash you get against the hash I got:

$ ./pre-inst-env guix hash --serializer=nar $(./pre-inst-env guix build 
guile-fibers --no-grafts --system=x86_64-linux) 
0qsaqx27b6q2s22h7y93417ijpkwl6gc1fhhck8aha6bvmawk5h0

I tried an emulated build for another architecture, but there the tests took
overly long.

There were no lint warnings for guile-fibers.

* gnu/packages/patches/guile-fibers-epoll-instance-is-dead.patch: New patch.
* gnu/local.mk (dist_patch_DATA): Register the patch.
* gnu/packages/guile-xyz.scm (guile-fibers)[source]{patches}: Use the patch.
---
 gnu/local.mk                                  |  1 +
 gnu/packages/guile-xyz.scm                    |  3 +-
 .../guile-fibers-epoll-instance-is-dead.patch | 99 +++++++++++++++++++
 3 files changed, 102 insertions(+), 1 deletion(-)
 create mode 100644 
gnu/packages/patches/guile-fibers-epoll-instance-is-dead.patch

diff --git a/gnu/local.mk b/gnu/local.mk
index 412d512775..64776792b2 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -1241,6 +1241,7 @@ dist_patch_DATA =                                         
\
   %D%/packages/patches/guile-ac-d-bus-fix-tests.patch          \
   %D%/packages/patches/guile-cross-compilation.patch           \
   %D%/packages/patches/guile-fibers-destroy-peer-schedulers.patch \
+  %D%/packages/patches/guile-fibers-epoll-instance-is-dead.patch \
   %D%/packages/patches/guile-fibers-wait-for-io-readiness.patch \
   %D%/packages/patches/guile-gdbm-ffi-support-gdbm-1.14.patch  \
   %D%/packages/patches/guile-git-adjust-for-libgit2-1.2.0.patch \
diff --git a/gnu/packages/guile-xyz.scm b/gnu/packages/guile-xyz.scm
index 2456509b97..59cd8d0096 100644
--- a/gnu/packages/guile-xyz.scm
+++ b/gnu/packages/guile-xyz.scm
@@ -619,7 +619,8 @@ (define-public guile-fibers-1.1
                (base32
                 "0ll63d7202clapg1k4bilbnlmfa4qvpjnsd7chbkka4kxf5klilc"))
               (patches
-               (search-patches "guile-fibers-wait-for-io-readiness.patch"))))
+               (search-patches "guile-fibers-wait-for-io-readiness.patch"
+                               "guile-fibers-epoll-instance-is-dead.patch"))))
     (build-system gnu-build-system)
     (native-inputs
      (list texinfo pkg-config autoconf automake libtool
diff --git a/gnu/packages/patches/guile-fibers-epoll-instance-is-dead.patch 
b/gnu/packages/patches/guile-fibers-epoll-instance-is-dead.patch
new file mode 100644
index 0000000000..ba191f765d
--- /dev/null
+++ b/gnu/packages/patches/guile-fibers-epoll-instance-is-dead.patch
@@ -0,0 +1,99 @@
+From 5db4077e9f5166033637d2af9532ec6144b85646 Mon Sep 17 00:00:00 2001
+From: Maxime Devos <maximedevos@telenet.be>
+Date: Thu, 30 Jun 2022 14:21:47 +0000
+Subject: [PATCH 1/2] Fix behaviour of 'epoll-wake!' after 'run-fibers'.
+
+This avoids the "epoll instance is dead" error noticed in
+GNUnet-Scheme's test suite, as reported at
+<https://github.com/wingo/fibers/issues/61>.
+A test is added in the next commit.
+
+This patch has been applied upstream, but there hasn't been
+a new release yet at time of writing.
+
+* fibers/epoll.scm (epoll-wake!)[dead]: Instead of throwing an error,
+just return #t.
+---
+ fibers/epoll.scm | 8 +++++++-
+ 1 file changed, 7 insertions(+), 1 deletion(-)
+
+diff --git a/fibers/epoll.scm b/fibers/epoll.scm
+index d26db4d..eb63242 100644
+--- a/fibers/epoll.scm
++++ b/fibers/epoll.scm
+@@ -1,6 +1,7 @@
+ ;; epoll
+ 
+ ;;;; Copyright (C) 2016 Andy Wingo <wingo@pobox.com>
++;;;; Copyright (C) 2022 Maxime Devos <maximedevos@telenet.be>
+ ;;;; 
+ ;;;; This library is free software; you can redistribute it and/or
+ ;;;; modify it under the terms of the GNU Lesser General Public
+@@ -135,7 +136,12 @@ epoll wait (if appropriate)."
+     ('waiting
+      (primitive-epoll-wake (fileno (epoll-wake-write-pipe epoll))))
+     ('not-waiting #t)
+-    ('dead (error "epoll instance is dead"))))
++    ;; This can happen if a fiber was waiting on a condition and
++    ;; run-fibers completes before the fiber completes and afterwards
++    ;; the condition is signalled.  In that case, we don't have to
++    ;; resurrect the fiber or something, we can just do nothing.
++    ;; (Bug report: https://github.com/wingo/fibers/issues/61)
++    ('dead #t)))
+ 
+ (define (epoll-default-folder fd events seed)
+   (acons fd events seed))
+
+From c01d3853eb56ea4adacc31f51f6e917f8c0abe1c Mon Sep 17 00:00:00 2001
+From: Maxime Devos <maximedevos@telenet.be>
+Date: Thu, 30 Jun 2022 14:18:36 +0000
+Subject: [PATCH 2/2] Test for issue #61.
+
+* tests/conditions.scm: Add a test.
+---
+ tests/conditions.scm | 20 ++++++++++++++++++++
+ 1 file changed, 20 insertions(+)
+
+diff --git a/tests/conditions.scm b/tests/conditions.scm
+index 505c42a..179605a 100644
+--- a/tests/conditions.scm
++++ b/tests/conditions.scm
+@@ -1,6 +1,7 @@
+ ;; Fibers: cooperative, event-driven user-space threads.
+ 
+ ;;;; Copyright (C) 2016 Free Software Foundation, Inc.
++;;;; Copyright (C) 2022 Maxime Devos <maximedevos@telenet.be>
+ ;;;;
+ ;;;; This library is free software; you can redistribute it and/or
+ ;;;; modify it under the terms of the GNU Lesser General Public
+@@ -21,6 +22,7 @@
+   #:use-module (fibers)
+   #:use-module (fibers conditions)
+   #:use-module (fibers operations)
++  #:use-module (fibers scheduler)
+   #:use-module (fibers timers))
+ 
+ (define failed? #f)
+@@ -78,4 +80,22 @@
+                              (wait cv)
+                              #t))
+ 
++;; Make a condition, wait for it inside a fiber, let the fiber abruptly
++;; terminate and signal the condition afterwards.  This tests for the bug
++;; noticed at <https://github.com/wingo/fibers/issues/61>.
++(assert-equal #t
++            (let ((cv (make-condition)))
++              (run-fibers
++               (lambda ()
++                 (spawn-fiber (lambda () (wait cv)))
++                 (yield-current-task)) ; let the other fiber wait forever
++               ;; This test relies on not draining -- this is the default,
++               ;; but let's make this explicit.
++               #:drain? #false ;
++               ;; For simplicity, disable concurrency and preemption.
++               ;; That way, we can use 'yield-current-task' instead of an
++               ;; arbitrary sleep time.
++               #:hz 0 #:parallelism 1)
++              (signal-condition! cv)))
++
+ (exit (if failed? 1 0))

base-commit: d519305d83d08058e4def2c4d72fe62102d9599d
-- 
2.37.0