server: buildfile: ensure that ONBUILD triggers aren't committed

This patch fixes the bug where ONBUILD triggers are committed each build
step created during the ONBUILD trigger execution, since the triggers are
only wiped *after* all ONBUILD trigger steps have been committed. This was
fixed by simply copying the ONBUILD triggers and wiping the config
*before* committing anything.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)

server/buildfile.go

@@ -123,7 +123,12 @@ func (b *buildFile) CmdFrom(name string) error {
 	if nTriggers := len(b.config.OnBuild); nTriggers != 0 {
 		fmt.Fprintf(b.errStream, "# Executing %d build triggers\n", nTriggers)
 	}
-	for n, step := range b.config.OnBuild {
+
+	// Copy the ONBUILD triggers, and remove them from the config, since the config will be commited.
+	onBuildTriggers := b.config.OnBuild
+	b.config.OnBuild = []string{}
+
+	for n, step := range onBuildTriggers {
 		splitStep := strings.Split(step, " ")
 		stepInstruction := strings.ToUpper(strings.Trim(splitStep[0], " "))
 		switch stepInstruction {
@@ -136,7 +141,6 @@ func (b *buildFile) CmdFrom(name string) error {
 			return err
 		}
 	}
-	b.config.OnBuild = []string{}
 	return nil
 }