diff --git a/src/script/hydra-build b/src/script/hydra-build
index bb067ea8..cbbad07f 100755
--- a/src/script/hydra-build
+++ b/src/script/hydra-build
@@ -215,6 +215,57 @@ sub addBuildStepOutputs {
 }
 
 
+sub nextFreeStepNr {
+    my ($build) = @_;
+    my $max = $build->buildsteps->find(
+        {}, {select => {max => 'stepnr + 1'}, as => ['max']});
+    return (defined $max && defined $max->get_column('max')) ? $max->get_column('max') : 1;
+}
+
+
+sub failDependents {
+    my ($drvPath, $errorMsg) = @_;
+
+    # Get the referrer closure of $drvPath.
+    my @dependentDrvs = computeFSClosure(1, 0, $drvPath);
+
+    my $time = time();
+
+    txn_do($db, sub {
+
+        my @dependentBuilds = $db->resultset('Builds')->search(
+            { drvpath => [ @dependentDrvs ], finished => 0, busy => 0 });
+
+        for my $d (@dependentBuilds) {
+            print STDERR "failing dependent build ", $d->id, " of ", $d->project->name, ":", $d->jobset->name, ":", $d->job->name, "\n";
+            $d->update(
+                { finished => 1
+                , logfile => ''
+                , timestamp => time # !!! Why change the timestamp?
+                , iscachedbuild => 0
+                , buildstatus => $drvPath eq $d->drvpath ? 1 : 2
+                , starttime => $time
+                , stoptime => $time
+                , errormsg => undef
+                });
+
+            my $step = $d->buildsteps->create(
+                { stepnr => nextFreeStepNr($d)
+                , type => 0 # = build
+                , drvpath => $drvPath
+                , busy => 0
+                , status => 1
+                , starttime => time
+                , stoptime => time
+                , errormsg => $errorMsg
+                });
+            addBuildStepOutputs($step);
+        }
+
+    });
+}
+
+
 sub doBuild {
     my ($build) = @_;
 
@@ -252,10 +303,7 @@ sub doBuild {
             "--no-build-output --log-type flat --print-build-trace " .
             "--add-root " . gcRootFor($outputs{out} // $outputs{(sort keys %outputs)[0]}) . " 2>&1";
 
-        my $max = $build->buildsteps->find(
-            {}, {select => {max => 'stepnr + 1'}, as => ['max']});
-        my $buildStepNr = (defined $max && defined $max->get_column('max')) ? $max->get_column('max') : 1;
-
+        my $buildStepNr = nextFreeStepNr($build);
         my %buildSteps;
 
         open OUT, "$cmd |" or die;
@@ -328,6 +376,9 @@ sub doBuild {
                         addBuildStepOutputs($step);
                     }
                 });
+
+                # Immediately fail all builds that depend on this derivation.
+                failDependents($drvPathStep, $errorMsg);
             }
 
             elsif (/^@\s+substituter-started\s+(\S+)\s+(\S+)$/) {