From 2da1eef317464036e3a122368508a5f3e4d21f6d Mon Sep 17 00:00:00 2001
From: Lasse Blaauwbroek
Date: Sun, 22 Nov 2020 11:25:04 +0100
Subject: Properly support nested timeouts

---
 doc/changelog/12-misc/13586-nested-timeout.rst |  7 +++++
 lib/control.ml                                 | 36 ++++++++++++++++----------
 test-suite/bugs/closed/bug_13586.v             |  6 +++++
 3 files changed, 36 insertions(+), 13 deletions(-)
 create mode 100644 doc/changelog/12-misc/13586-nested-timeout.rst
 create mode 100644 test-suite/bugs/closed/bug_13586.v

diff --git a/doc/changelog/12-misc/13586-nested-timeout.rst b/doc/changelog/12-misc/13586-nested-timeout.rst
new file mode 100644
index 0000000000..2c31dc210a
--- /dev/null
+++ b/doc/changelog/12-misc/13586-nested-timeout.rst
@@ -0,0 +1,7 @@
+- **Fixed:**
+  Fix the timeout facility on Unix to allow for nested timeouts.
+  Previous behavior on nested timeouts was that an "inner" timeout would replace an "outer"
+  timeout, so that the outer timeout would no longer fire. With the new behavior, Unix and Windows
+  implementations should be (approximately) equivalent.
+  (`#13586 <https://github.com/coq/coq/pull/13586>`_,
+  by Lasse Blaauwbroek).
diff --git a/lib/control.ml b/lib/control.ml
index ea94bda064..5a38022291 100644
--- a/lib/control.ml
+++ b/lib/control.ml
@@ -29,22 +29,32 @@ let check_for_interrupt () =
   end
 
 (** This function does not work on windows, sigh... *)
+(* This function assumes it is the only function calling [setitimer] *)
 let unix_timeout n f x =
   let open Unix in
   let timeout_handler _ = raise Timeout in
-  let psh = Sys.signal Sys.sigalrm (Sys.Signal_handle timeout_handler) in
-  let _ = setitimer ITIMER_REAL {it_interval = 0.; it_value = n} in
-  let restore_timeout () =
-    let _ = setitimer ITIMER_REAL { it_interval = 0.; it_value = 0. } in
-    Sys.set_signal Sys.sigalrm psh
-  in
-  try
-    let res = f x in
-    restore_timeout ();
-    Some res
-  with Timeout ->
-    restore_timeout ();
-    None
+  let old_timer = getitimer ITIMER_REAL in
+  (* Here we assume that the existing timer will also interrupt us. *)
+  if old_timer.it_value > 0. && old_timer.it_value <= n then Some (f x) else
+    let psh = Sys.signal Sys.sigalrm (Sys.Signal_handle timeout_handler) in
+    let old_timer = setitimer ITIMER_REAL {it_interval = 0.; it_value = n} in
+    let restore_timeout () =
+      let timer_status = getitimer ITIMER_REAL in
+      let old_timer_value = old_timer.it_value -. n +. timer_status.it_value in
+      (* We want to make sure that the parent timer triggers, even if somehow the parent timeout
+         has already passed. This should not happen, but due to timer imprecision it can happen in practice *)
+      let old_timer_value = if old_timer.it_value <= 0. then 0. else
+          (if old_timer_value <= 0. then epsilon_float else old_timer_value) in
+      let _ = setitimer ITIMER_REAL { old_timer with it_value = old_timer_value } in
+      Sys.set_signal Sys.sigalrm psh
+    in
+    try
+      let res = f x in
+      restore_timeout ();
+      Some res
+    with Timeout ->
+      restore_timeout ();
+      None
 
 
 let windows_timeout n f x =
diff --git a/test-suite/bugs/closed/bug_13586.v b/test-suite/bugs/closed/bug_13586.v
new file mode 100644
index 0000000000..6a739c364a
--- /dev/null
+++ b/test-suite/bugs/closed/bug_13586.v
@@ -0,0 +1,6 @@
+Goal True.
+Fail timeout 2 ((timeout 1 repeat cut True) || (repeat cut True)).
+Fail Timeout 2 ((timeout 1 repeat cut True) || (repeat cut True)).
+Fail timeout 1 ((timeout 2 repeat cut True) || idtac "fail").
+auto.
+Qed.
-- 
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