Add parallel execution code for container operations

This code will run container operations in parallel, up to a
given maximum number of threads. Currently, it has only been
enabled for local `podman rm` as a proof of concept.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

pkg/parallel/parallel.go

@@ -0,0 +1,44 @@
+package parallel
+
+import (
+	"sync"
+
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/sync/semaphore"
+)
+
+var (
+	// Maximum number of jobs that will be used.
+	// Set a low, but non-zero, default. We'll be overriding it by default
+	// anyways.
+	numThreads uint = 8
+	// Semaphore to control thread creation and ensure numThreads is
+	// respected.
+	jobControl *semaphore.Weighted
+	// Lock to control changing the semaphore - we don't want to do it
+	// while anyone is using it.
+	jobControlLock sync.RWMutex
+)
+
+// SetMaxThreads sets the number of threads that will be used for parallel jobs.
+func SetMaxThreads(threads uint) error {
+	if threads == 0 {
+		return errors.New("must give a non-zero number of threads to execute with")
+	}
+
+	jobControlLock.Lock()
+	defer jobControlLock.Unlock()
+
+	numThreads = threads
+	jobControl = semaphore.NewWeighted(int64(threads))
+	logrus.Infof("Setting parallel job count to %d", threads)
+
+	return nil
+}
+
+// GetMaxThreads returns the current number of threads that will be used for
+// parallel jobs.
+func GetMaxThreads() uint {
+	return numThreads
+}