Fix bufferpool puts (ref #4976) (#6125)

* Fix bufferpool puts (ref #4976)

There was a logic error in Put() which made us put all large blocks into
segment zero, where we subsequently did not look for them.

I also added a lowest threshold, as we otherwise allocate a 128KiB
buffer when we need 24 bytes for a header and such.

* wip

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* smaller stress

* cap/len

* wip

* wip

lib/protocol/bufferpool_test.go

@@ -0,0 +1,132 @@
+// Copyright (C) 2019 The Protocol Authors.
+
+package protocol
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/rand"
+)
+
+func TestGetBucketNumbers(t *testing.T) {
+	cases := []struct {
+		size   int
+		bkt    int
+		panics bool
+	}{
+		{size: 1024, bkt: 0},
+		{size: MinBlockSize, bkt: 0},
+		{size: MinBlockSize + 1, bkt: 1},
+		{size: 2*MinBlockSize - 1, bkt: 1},
+		{size: 2 * MinBlockSize, bkt: 1},
+		{size: 2*MinBlockSize + 1, bkt: 2},
+		{size: MaxBlockSize, bkt: len(BlockSizes) - 1},
+		{size: MaxBlockSize + 1, panics: true},
+	}
+
+	for _, tc := range cases {
+		if tc.panics {
+			shouldPanic(t, func() { getBucketForLen(tc.size) })
+		} else {
+			res := getBucketForLen(tc.size)
+			if res != tc.bkt {
+				t.Errorf("block of size %d should get from bucket %d, not %d", tc.size, tc.bkt, res)
+			}
+		}
+	}
+}
+
+func TestPutBucketNumbers(t *testing.T) {
+	cases := []struct {
+		size   int
+		bkt    int
+		panics bool
+	}{
+		{size: 1024, panics: true},
+		{size: MinBlockSize, bkt: 0},
+		{size: MinBlockSize + 1, panics: true},
+		{size: 2 * MinBlockSize, bkt: 1},
+		{size: MaxBlockSize, bkt: len(BlockSizes) - 1},
+		{size: MaxBlockSize + 1, panics: true},
+	}
+
+	for _, tc := range cases {
+		if tc.panics {
+			shouldPanic(t, func() { putBucketForCap(tc.size) })
+		} else {
+			res := putBucketForCap(tc.size)
+			if res != tc.bkt {
+				t.Errorf("block of size %d should put into bucket %d, not %d", tc.size, tc.bkt, res)
+			}
+		}
+	}
+}
+
+func TestStressBufferPool(t *testing.T) {
+	if testing.Short() {
+		t.Skip()
+	}
+
+	const routines = 10
+	const runtime = 2 * time.Second
+
+	bp := newBufferPool()
+	t0 := time.Now()
+
+	var wg sync.WaitGroup
+	fail := make(chan struct{}, routines)
+	for i := 0; i < routines; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for time.Since(t0) < runtime {
+				blocks := make([][]byte, 10)
+				for i := range blocks {
+					// Request a block of random size with the range
+					// covering smaller-than-min to larger-than-max and
+					// everything in between.
+					want := rand.Intn(1.5 * MaxBlockSize)
+					blocks[i] = bp.Get(want)
+					if len(blocks[i]) != want {
+						fail <- struct{}{}
+						return
+					}
+				}
+				for i := range blocks {
+					bp.Put(blocks[i])
+				}
+			}
+		}()
+	}
+
+	wg.Wait()
+	select {
+	case <-fail:
+		t.Fatal("a block was bad size")
+	default:
+	}
+
+	t.Log(bp.puts, bp.skips, bp.misses, bp.hits)
+	if bp.puts == 0 || bp.skips == 0 || bp.misses == 0 {
+		t.Error("didn't exercise some paths")
+	}
+	var hits int64
+	for _, h := range bp.hits {
+		hits += h
+	}
+	if hits == 0 {
+		t.Error("didn't exercise some paths")
+	}
+}
+
+func shouldPanic(t *testing.T, fn func()) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Errorf("did not panic")
+		}
+	}()
+
+	fn()
+}