Improve cache context (#23330)

Related to: #22294 #23186 #23054

Replace: #23218

Some discussion is in the comments of #23218.

Highlights:
- Add Expiration for cache context. If a cache context has been used for
more than 10s, the cache data will be ignored, and warning logs will be
printed.
- Add `discard` field to `cacheContext`, a `cacheContext` with `discard`
true will drop all cached data and won't store any new one.
- Introduce `WithNoCacheContext`, if one wants to run long-life tasks,
but the parent context is a cache context,
`WithNoCacheContext(perentCtx)` will discard the cache data, so it will
be safe to keep the context for a long time.
- It will be fine to treat an original context as a cache context, like
`GetContextData(context.Backgraud())`, no warning logs will be printed.

Some cases about nesting:

When:
- *A*, *B* or *C* means a cache context.
- ~*A*~, ~*B*~ or ~*C*~ means a discard cache context.
- `ctx` means `context.Backgrand()`
- *A(ctx)* means a cache context with `ctx` as the parent context.
- *B(A(ctx))* means a cache context with `A(ctx)` as the parent context.
- `With` means `WithCacheContext`
- `WithNo` means `WithNoCacheContext`

So:
- `With(ctx)` -> *A(ctx)*
- `With(With(ctx))` -> *A(ctx)*, not *B(A(ctx))*
- `With(With(With(ctx)))` -> *A(ctx)*, not *C(B(A(ctx)))*
- `WithNo(ctx)` -> *ctx*, not *~A~(ctx)*
- `WithNo(With(ctx))` -> *~A~(ctx)*
- `WithNo(WithNo(With(ctx)))` -> *~A~(ctx)*, not *~B~(~A~(ctx))*
- `With(WithNo(With(ctx)))` -> *B(~A~(ctx))*
- `WithNo(With(WithNo(With(ctx))))` -> *~B~(~A~(ctx))*
- `With(WithNo(With(WithNo(With(ctx)))))` -> *C(~B~(~A~(ctx)))*

3 files changed, 141 insertions, 18 deletions

diff --git a/modules/cache/context.go b/modules/cache/context.go
index f741a8744..62bbf5dcb 100644
--- a/modules/cache/context.go
+++ b/modules/cache/context.go
@@ -6,6 +6,7 @@ package cache
 import (
 	"context"
 	"sync"
+	"time"
 
 	"code.gitea.io/gitea/modules/log"
 )
@@ -14,65 +15,151 @@ import (
 // This is useful for caching data that is expensive to calculate and is likely to be
 // used multiple times in a request.
 type cacheContext struct {
-	ctx  context.Context
-	data map[any]map[any]any
-	lock sync.RWMutex
+	data    map[any]map[any]any
+	lock    sync.RWMutex
+	created time.Time
+	discard bool
 }
 
 func (cc *cacheContext) Get(tp, key any) any {
 	cc.lock.RLock()
 	defer cc.lock.RUnlock()
-	if cc.data[tp] == nil {
-		return nil
-	}
 	return cc.data[tp][key]
 }
 
 func (cc *cacheContext) Put(tp, key, value any) {
 	cc.lock.Lock()
 	defer cc.lock.Unlock()
-	if cc.data[tp] == nil {
-		cc.data[tp] = make(map[any]any)
+
+	if cc.discard {
+		return
+	}
+
+	d := cc.data[tp]
+	if d == nil {
+		d = make(map[any]any)
+		cc.data[tp] = d
 	}
-	cc.data[tp][key] = value
+	d[key] = value
 }
 
 func (cc *cacheContext) Delete(tp, key any) {
 	cc.lock.Lock()
 	defer cc.lock.Unlock()
-	if cc.data[tp] == nil {
-		return
-	}
 	delete(cc.data[tp], key)
 }
 
+func (cc *cacheContext) Discard() {
+	cc.lock.Lock()
+	defer cc.lock.Unlock()
+	cc.data = nil
+	cc.discard = true
+}
+
+func (cc *cacheContext) isDiscard() bool {
+	cc.lock.RLock()
+	defer cc.lock.RUnlock()
+	return cc.discard
+}
+
+// cacheContextLifetime is the max lifetime of cacheContext.
+// Since cacheContext is used to cache data in a request level context, 10s is enough.
+// If a cacheContext is used more than 10s, it's probably misuse.
+const cacheContextLifetime = 10 * time.Second
+
+var timeNow = time.Now
+
+func (cc *cacheContext) Expired() bool {
+	return timeNow().Sub(cc.created) > cacheContextLifetime
+}
+
 var cacheContextKey = struct{}{}
 
+/*
+Since there are both WithCacheContext and WithNoCacheContext,
+it may be confusing when there is nesting.
+
+Some cases to explain the design:
+
+When:
+- A, B or C means a cache context.
+- A', B' or C' means a discard cache context.
+- ctx means context.Backgrand().
+- A(ctx) means a cache context with ctx as the parent context.
+- B(A(ctx)) means a cache context with A(ctx) as the parent context.
+- With is alias of WithCacheContext.
+- WithNo is alias of WithNoCacheContext.
+
+So:
+- With(ctx) -> A(ctx)
+- With(With(ctx)) -> A(ctx), not B(A(ctx)), always reuse parent cache context if possible.
+- With(With(With(ctx))) -> A(ctx), not C(B(A(ctx))), ditto.
+- WithNo(ctx) -> ctx, not A'(ctx), don't create new cache context if we don't have to.
+- WithNo(With(ctx)) -> A'(ctx)
+- WithNo(WithNo(With(ctx))) -> A'(ctx), not B'(A'(ctx)), don't create new cache context if we don't have to.
+- With(WithNo(With(ctx))) -> B(A'(ctx)), not A(ctx), never reuse a discard cache context.
+- WithNo(With(WithNo(With(ctx)))) -> B'(A'(ctx))
+- With(WithNo(With(WithNo(With(ctx))))) -> C(B'(A'(ctx))), so there's always only one not-discard cache context.
+*/
+
 func WithCacheContext(ctx context.Context) context.Context {
+	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		if !c.isDiscard() {
+			// reuse parent context
+			return ctx
+		}
+	}
 	return context.WithValue(ctx, cacheContextKey, &cacheContext{
-		ctx:  ctx,
-		data: make(map[any]map[any]any),
+		data:    make(map[any]map[any]any),
+		created: timeNow(),
 	})
 }
 
+func WithNoCacheContext(ctx context.Context) context.Context {
+	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		// The caller want to run long-life tasks, but the parent context is a cache context.
+		// So we should disable and clean the cache data, or it will be kept in memory for a long time.
+		c.Discard()
+		return ctx
+	}
+
+	return ctx
+}
+
 func GetContextData(ctx context.Context, tp, key any) any {
 	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		if c.Expired() {
+			// The warning means that the cache context is misused for long-life task,
+			// it can be resolved with WithNoCacheContext(ctx).
+			log.Warn("cache context is expired, may be misused for long-life tasks: %v", c)
+			return nil
+		}
 		return c.Get(tp, key)
 	}
-	log.Warn("cannot get cache context when getting data: %v", ctx)
 	return nil
 }
 
 func SetContextData(ctx context.Context, tp, key, value any) {
 	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		if c.Expired() {
+			// The warning means that the cache context is misused for long-life task,
+			// it can be resolved with WithNoCacheContext(ctx).
+			log.Warn("cache context is expired, may be misused for long-life tasks: %v", c)
+			return
+		}
 		c.Put(tp, key, value)
 		return
 	}
-	log.Warn("cannot get cache context when setting data: %v", ctx)
 }
 
 func RemoveContextData(ctx context.Context, tp, key any) {
 	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		if c.Expired() {
+			// The warning means that the cache context is misused for long-life task,
+			// it can be resolved with WithNoCacheContext(ctx).
+			log.Warn("cache context is expired, may be misused for long-life tasks: %v", c)
+			return
+		}
 		c.Delete(tp, key)
 	}
 }
diff --git a/modules/cache/context_test.go b/modules/cache/context_test.go
index 77e3ecad2..531554786 100644
--- a/modules/cache/context_test.go
+++ b/modules/cache/context_test.go
@@ -6,6 +6,7 @@ package cache
 import (
 	"context"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -25,7 +26,7 @@ func TestWithCacheContext(t *testing.T) {
 	assert.EqualValues(t, 1, v.(int))
 
 	RemoveContextData(ctx, field, "my_config1")
-	RemoveContextData(ctx, field, "my_config2") // remove an non-exist key
+	RemoveContextData(ctx, field, "my_config2") // remove a non-exist key
 
 	v = GetContextData(ctx, field, "my_config1")
 	assert.Nil(t, v)
@@ -38,4 +39,40 @@ func TestWithCacheContext(t *testing.T) {
 
 	v = GetContextData(ctx, field, "my_config1")
 	assert.EqualValues(t, 1, v)
+
+	now := timeNow
+	defer func() {
+		timeNow = now
+	}()
+	timeNow = func() time.Time {
+		return now().Add(10 * time.Second)
+	}
+	v = GetContextData(ctx, field, "my_config1")
+	assert.Nil(t, v)
+}
+
+func TestWithNoCacheContext(t *testing.T) {
+	ctx := context.Background()
+
+	const field = "system_setting"
+
+	v := GetContextData(ctx, field, "my_config1")
+	assert.Nil(t, v)
+	SetContextData(ctx, field, "my_config1", 1)
+	v = GetContextData(ctx, field, "my_config1")
+	assert.Nil(t, v) // still no cache
+
+	ctx = WithCacheContext(ctx)
+	v = GetContextData(ctx, field, "my_config1")
+	assert.Nil(t, v)
+	SetContextData(ctx, field, "my_config1", 1)
+	v = GetContextData(ctx, field, "my_config1")
+	assert.NotNil(t, v)
+
+	ctx = WithNoCacheContext(ctx)
+	v = GetContextData(ctx, field, "my_config1")
+	assert.Nil(t, v)
+	SetContextData(ctx, field, "my_config1", 1)
+	v = GetContextData(ctx, field, "my_config1")
+	assert.Nil(t, v) // still no cache
 }
diff --git a/services/repository/push.go b/services/repository/push.go
index 355c28781..cdf030396 100644
--- a/services/repository/push.go
+++ b/services/repository/push.go
@@ -80,7 +80,6 @@ func pushUpdates(optsList []*repo_module.PushUpdateOptions) error {
 
 	ctx, _, finished := process.GetManager().AddContext(graceful.GetManager().HammerContext(), fmt.Sprintf("PushUpdates: %s/%s", optsList[0].RepoUserName, optsList[0].RepoName))
 	defer finished()
-	ctx = cache.WithCacheContext(ctx)
 
 	repo, err := repo_model.GetRepositoryByOwnerAndName(ctx, optsList[0].RepoUserName, optsList[0].RepoName)
 	if err != nil {