disk_block_cache: only do explicit compaction on desktop

Issue: HOTPOT-2508

go/kbfs/libkbfs/disk_block_cache.go

@@ -282,8 +282,8 @@ func newDiskBlockCacheLocalFromStorage(
 		},
 		tlfSizes:      map[tlf.ID]uint64{},
 		tlfLastUnrefs: map[tlf.ID]kbfsmd.Revision{},
-		compactCh:     make(chan struct{}),
-		useCh:         make(chan struct{}),
+		compactCh:     make(chan struct{}, 1),
+		useCh:         make(chan struct{}, 1),
 		startedCh:     startedCh,
 		startErrCh:    startErrCh,
 		shutdownCh:    make(chan struct{}),
@@ -313,7 +313,12 @@ func newDiskBlockCacheLocalFromStorage(
 		close(startedCh)
 	}()
 
-	go cache.compactLoop()
+	// Only do background compaction on desktop for now, because on
+	// mobile we'd probably cause issues if we try to do it while
+	// backgrounded.
+	if mode.DiskCacheCompactionEnabled() {
+		go cache.compactLoop()
+	}
 
 	return cache, nil
 }

go/kbfs/libkbfs/interfaces.go

@@ -2074,6 +2074,9 @@ type InitMode interface {
 	// DelayInitialConnect indicates whether the initial connection to KBFS
 	// servers should be delayed.
 	DelayInitialConnect() bool
+	// DiskCacheCompactionEnabled indicates whether the local disk
+	// block cache should trigger compaction automatically.
+	DiskCacheCompactionEnabled() bool
 
 	ldbutils.DbWriteBufferSizeGetter
 }

go/kbfs/libkbfs/modes.go

@@ -207,6 +207,10 @@ func (md modeDefault) DbWriteBufferSize() int {
 	return 10 * opt.MiB // 10 MB
 }
 
+func (md modeDefault) DiskCacheCompactionEnabled() bool {
+	return true
+}
+
 // Minimal mode:
 
 type modeMinimal struct {
@@ -389,6 +393,10 @@ func (mm modeMinimal) DbWriteBufferSize() int {
 	return 1 * opt.KiB // 1 KB
 }
 
+func (mm modeMinimal) DiskCacheCompactionEnabled() bool {
+	return false
+}
+
 // Single op mode:
 
 type modeSingleOp struct {
@@ -483,6 +491,10 @@ func (mso modeSingleOp) IsSingleOp() bool {
 	return true
 }
 
+func (mso modeSingleOp) DiskCacheCompactionEnabled() bool {
+	return false
+}
+
 // Single-op mode with QR:
 
 type modeSingleOpWithQR struct {
@@ -623,6 +635,10 @@ func (mc modeConstrained) DbWriteBufferSize() int {
 	return 100 * opt.KiB // 100 KB
 }
 
+func (mc modeConstrained) DiskCacheCompactionEnabled() bool {
+	return false
+}
+
 // Memory limited mode
 
 type modeMemoryLimited struct {