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Blame SOURCES/0754-pid1-fix-collection-of-cycles-of-units-which-referen.patch

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From 703cc4991049cdf3ad3506e432cda982b3b3b007 Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
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Date: Tue, 13 Feb 2018 14:37:11 +0100
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Subject: [PATCH] pid1: fix collection of cycles of units which reference one
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 another
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A .socket will reference a .service unit, by registering a UnitRef with the
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.service unit. If this .service unit has the .socket unit listed in Wants or
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Sockets or such, a cycle will be created. We would not free this cycle
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properly, because we treated any unit with non-empty refs as uncollectable. To
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solve this issue, treats refs with UnitRef in u->refs_by_target similarly to
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the refs in u->dependencies, and check if the "other" unit is known to be
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needed. If it is not needed, do not treat the reference from it as preventing
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the unit we are looking at from being freed.
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(cherry picked from commit 2641f02e23ac7d5385db7f932aff221a063f245e)
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Resolves: #1718953
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---
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 src/core/manager.c | 14 ++++++++++++++
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 src/core/unit.c    |  9 +++++----
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 2 files changed, 19 insertions(+), 4 deletions(-)
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diff --git a/src/core/manager.c b/src/core/manager.c
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index 9dfdd67860..fdbb3c0fd9 100644
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--- a/src/core/manager.c
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+++ b/src/core/manager.c
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@@ -888,6 +888,20 @@ static void unit_gc_sweep(Unit *u, unsigned gc_marker) {
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                         is_bad = false;
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         }
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+        if (u->refs_by_target) {
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+                const UnitRef *ref;
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+
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+                LIST_FOREACH(refs_by_target, ref, u->refs_by_target) {
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+                        unit_gc_sweep(ref->source, gc_marker);
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+
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+                        if (ref->source->gc_marker == gc_marker + GC_OFFSET_GOOD)
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+                                goto good;
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+
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+                        if (ref->source->gc_marker != gc_marker + GC_OFFSET_BAD)
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+                                is_bad = false;
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+                }
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+        }
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+
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         if (is_bad)
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                 goto bad;
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diff --git a/src/core/unit.c b/src/core/unit.c
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index 5376ef862f..2204be26d2 100644
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--- a/src/core/unit.c
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+++ b/src/core/unit.c
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@@ -288,7 +288,11 @@ bool unit_may_gc(Unit *u) {
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         /* Checks whether the unit is ready to be unloaded for garbage collection.
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          * Returns true when the unit may be collected, and false if there's some
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-         * reason to keep it loaded. */
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+         * reason to keep it loaded.
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+         *
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+         * References from other units are *not* checked here. Instead, this is done
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+         * in unit_gc_sweep(), but using markers to properly collect dependency loops.
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+         */
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         if (u->job)
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                 return false;
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@@ -315,9 +319,6 @@ bool unit_may_gc(Unit *u) {
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         if (u->no_gc)
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                 return false;
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-        if (u->refs_by_target)
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-                return false;
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-
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         if (UNIT_VTABLE(u)->may_gc && !UNIT_VTABLE(u)->may_gc(u))
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                 return false;
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