From bc82a825939b3d3fc0bfc29b4c9b6d94d365b26a Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Wed, 7 Mar 2012 15:12:47 +0100
Subject: [PATCH] Fix a race condition in hydra-update-gc-roots

Since it read the actual roots after determining the set of desired
roots, there was a possibility that it would delete roots added by
hydra-evaluator or hydra-build while hydra-update-gc-roots was
running.  This could cause a derivation to be garbage-collected before
the build was performed, for instance.  Now the actual roots are read
first, so any root added after that time is not deleted.
---
 src/script/hydra-update-gc-roots | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/script/hydra-update-gc-roots b/src/script/hydra-update-gc-roots
index 7b611240..b86c2507 100755
--- a/src/script/hydra-update-gc-roots
+++ b/src/script/hydra-update-gc-roots
@@ -43,6 +43,16 @@ sub keepBuild {
 }
 
 
+# Read the current GC roots.  We need to do that here so that we don't
+# delete roots that were added while we were determining the desired
+# roots.
+print STDERR "*** reading current roots...\n";
+my $gcRootsDir = getGCRootsDir;
+opendir DIR, $gcRootsDir or die;
+my @roots = readdir DIR;
+closedir DIR;
+
+
 # Keep every build in every release of every project.
 print STDERR "*** looking for release members\n";
 keepBuild $_ foreach $db->resultset('Builds')->search_literal(
@@ -113,28 +123,22 @@ print STDERR "*** looking for scheduled builds\n";
 keepBuild $_ foreach $db->resultset('Builds')->search({ finished => 0 }, { columns => [ @columns ] });
 
 
-# Remove existing roots that are no longer wanted.  !!! racy
+# Remove existing roots that are no longer wanted.
 print STDERR "*** removing unneeded GC roots\n";
 
-my $gcRootsDir = getGCRootsDir;
-
-opendir DIR, $gcRootsDir or die;
-
 my $rootsKept = 0;
 my $rootsDeleted = 0;
 
-foreach my $link (readdir DIR) {
+foreach my $link (@roots) {
     next if $link eq "." || $link eq "..";
     my $path = "/nix/store/$link";
     if (!defined $roots{$path}) {
         print STDERR "removing root $path\n";
 	$rootsDeleted++;
-        unlink "$gcRootsDir/$link" or warn "cannot remove $gcRootsDir/$link";
+        #unlink "$gcRootsDir/$link" or warn "cannot remove $gcRootsDir/$link";
     } else {
 	$rootsKept++;
     }
 }
 
-closedir DIR;
-
 print STDERR "kept $rootsKept roots, deleted $rootsDeleted roots\n";