Try harder to find build logs

Due to the fixed-output derivation hashing scheme, there can be
multiple derivations of the same output path.  But build logs are
indexed by derivation path.  Thus, we may not be able to find the
log of a build or build step using its derivation.  So as a fallback,
Hydra now looks for other derivations with the same output paths.

src/lib/Hydra/Helper/Nix.pm

@@ -16,7 +16,7 @@ our @EXPORT = qw(
     getPrimaryBuildsForView
     getPrimaryBuildTotal
     getViewResult getLatestSuccessfulViewResult
-    jobsetOverview removeAsciiEscapes getDrvLogPath logContents
+    jobsetOverview removeAsciiEscapes getDrvLogPath findLog logContents
     getMainOutput
     getEvals getMachines
     pathIsInsidePrefix
@@ -264,10 +264,36 @@ sub getDrvLogPath {
 }
 
 
+# Find the log of the derivation denoted by $drvPath.  It it doesn't
+# exist, try other derivations that produced its outputs (@outPaths).
+sub findLog {
+    my ($c, $drvPath, @outPaths) = @_;
+
+    if (defined $drvPath) {
+	my $logPath = getDrvLogPath($drvPath);
+	return $logPath if defined $logPath;
+    }
+    
+    return undef if scalar @outPaths == 0;
+
+    my @steps = $c->model('DB::BuildSteps')->search(
+	{ path => { -in => [@outPaths] } },
+	{ select => ["drvpath"]
+        , distinct => 1
+	, join => "buildstepoutputs"
+	});
+
+    foreach my $step (@steps) {
+	my $logPath = getDrvLogPath($step->drvpath);
+	return $logPath if defined $logPath;
+    }
+
+    return undef;
+}
+
+
 sub logContents {
-    my ($drvPath, $tail) = @_;
-    my $logPath = getDrvLogPath($drvPath);
-    die unless defined $logPath;
+    my ($logPath, $tail) = @_;
     my $cmd;
     if ($logPath =~ /.bz2$/) {
         $cmd = "bzip2 -d < $logPath";