add example code. add sync.onlyOnce

1 files changed, 58 insertions, 0 deletions

diff --git a/cache.go b/cache.go
new file mode 100644
index 0000000..c139306
--- /dev/null
+++ b/cache.go
@@ -0,0 +1,58 @@
+// I put this here to look at and think about
+// from mod/sumdb/cache.go
+
+// Parallel cache.
+// This file is copied from cmd/go/internal/par.
+
+package forgepb
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// parCache runs an action once per key and caches the result.
+type parCache struct {
+	m sync.Map
+}
+
+type cacheEntry struct {
+	done   uint32
+	mu     sync.Mutex
+	result interface{}
+}
+
+// Do calls the function f if and only if Do is being called for the first time with this key.
+// No call to Do with a given key returns until the one call to f returns.
+// Do returns the value returned by the one call to f.
+func (c *parCache) Do(key interface{}, f func() interface{}) interface{} {
+	entryIface, ok := c.m.Load(key)
+	if !ok {
+		entryIface, _ = c.m.LoadOrStore(key, new(cacheEntry))
+	}
+	e := entryIface.(*cacheEntry)
+	if atomic.LoadUint32(&e.done) == 0 {
+		e.mu.Lock()
+		if atomic.LoadUint32(&e.done) == 0 {
+			e.result = f()
+			atomic.StoreUint32(&e.done, 1)
+		}
+		e.mu.Unlock()
+	}
+	return e.result
+}
+
+// Get returns the cached result associated with key.
+// It returns nil if there is no such result.
+// If the result for key is being computed, Get does not wait for the computation to finish.
+func (c *parCache) Get(key interface{}) interface{} {
+	entryIface, ok := c.m.Load(key)
+	if !ok {
+		return nil
+	}
+	e := entryIface.(*cacheEntry)
+	if atomic.LoadUint32(&e.done) == 0 {
+		return nil
+	}
+	return e.result
+}