builder: remove redundant ExecBackend.ContainerKill()

The `ExecBackend.ContainerKill()` function was called before removing a build-
container.

This function is backed by `daemon.ContainerKill()` which, if no signal is passed,
performed a `daemon.Kill()`, using `SIGKILL` as signal. However, the
`ExecBackend.ContainerRm()` (backed by `daemonContainerRm()`), which is called
after this, is executed with the `ForceRemove` option set, which calls
`daemon.cleanupContainer()` with `ForceRemove` set, which also results in
`daemon.Kill()` being called:
1a0c15abbb/daemon/delete.go (L84-L95)

This makes the `ExecBackend.ContainerKill()` redundant, so removing this from
the interface.

While looking at this code, one (possible) race-condition was found in
`daemon.cleanupContainer()`, where `daemon.Kill()` could return a `errdefs.Conflict`
if the container was already stopped. An extra check was added for this case to
prevent `daemon.cleanupContainer()` from terminating early.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/builder.go

@@ -62,8 +62,6 @@ type ExecBackend interface {
 	ContainerCreateIgnoreImagesArgsEscaped(ctx context.Context, config backend.ContainerCreateConfig) (container.CreateResponse, error)
 	// ContainerRm removes a container specified by `id`.
 	ContainerRm(name string, config *backend.ContainerRmConfig) error
-	// ContainerKill stops the container execution abruptly.
-	ContainerKill(containerID string, sig string) error
 	// ContainerStart starts a new container
 	ContainerStart(ctx context.Context, containerID string, hostConfig *container.HostConfig, checkpoint string, checkpointDir string) error
 	// ContainerWait stops processing until the given container is stopped.

builder/dockerfile/containerbackend.go

@@ -61,8 +61,7 @@ func (c *containerManager) Run(ctx context.Context, cID string, stdout, stderr i
 	go func() {
 		select {
 		case <-ctx.Done():
-			log.G(ctx).Debugln("Build cancelled, killing and removing container:", cID)
-			c.backend.ContainerKill(cID, "")
+			log.G(ctx).Debugln("Build cancelled, removing container:", cID)
 			err = c.backend.ContainerRm(cID, &backend.ContainerRmConfig{ForceRemove: true, RemoveVolume: true})
 			if err != nil {
 				_, _ = fmt.Fprintf(stdout, "Removing container %s: %v\n", stringid.TruncateID(cID), err)

builder/dockerfile/mockbackend_test.go

@@ -45,10 +45,6 @@ func (m *MockBackend) CommitBuildStep(ctx context.Context, c backend.CommitConfi
 	return "", nil
 }
 
-func (m *MockBackend) ContainerKill(containerID string, sig string) error {
-	return nil
-}
-
 func (m *MockBackend) ContainerStart(ctx context.Context, containerID string, hostConfig *container.HostConfig, checkpoint string, checkpointDir string) error {
 	return nil
 }