sync: add Map.Clear

Fixes #61696

Change-Id: I0a31afd3bc433fc84280d56f2798bda10da61eba
GitHub-Last-Rev: 17bedc864f1685178a42b59f7083677a6124f831
GitHub-Pull-Request: golang/go#61702
Reviewed-on: https://go-review.googlesource.com/c/go/+/515015
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/sync/map.go b/src/sync/map.go
index 7a9eebdce3..1f26cdd8bb 100644
--- a/src/sync/map.go
+++ b/src/sync/map.go
@@ -155,6 +155,26 @@ func (m *Map) Store(key, value any) {
 	_, _ = m.Swap(key, value)
 }
 
+// Clear deletes all the keys.
+func (m *Map) Clear() {
+	read := m.loadReadOnly()
+	if len(read.m) == 0 && !read.amended {
+		// Avoid allocating a new readOnly when the map is already clear.
+		return
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	read = m.loadReadOnly()
+	if len(read.m) > 0 || read.amended {
+		m.read.Store(&readOnly{})
+	}
+
+	clear(m.dirty)
+	m.misses = 0 // Don't immediately promote the newly-cleared dirty map on the next operation
+}
+
 // tryCompareAndSwap compare the entry with the given old value and swaps
 // it with a new value if the entry is equal to the old value, and the entry
 // has not been expunged.