fs: add Options.Logger, and use throughout

Change-Id: I4fabf222a306e5d3abdfda28422b046cd75c9a8c

fs/api.go

@@ -168,6 +168,7 @@ package fs
 
 import (
 	"context"
+	"log"
 	"syscall"
 	"time"
 
@@ -603,4 +604,10 @@ type Options struct {
 	// ServerCallbacks can be provided to stub out notification
 	// functions for testing a filesystem without mounting it.
 	ServerCallbacks ServerCallbacks
+
+	// Logger is a sink for diagnostic messages. Diagnostic
+	// messages are printed under conditions where we cannot
+	// return error, but want to signal something seems off
+	// anyway. If unset, no messages are printed.
+	Logger *log.Logger
 }

fs/bridge.go

@@ -119,7 +119,7 @@ func (b *rawBridge) newInodeUnlocked(ops InodeEmbedder, id StableAttr, persisten
 
 		t = expSleep(t)
 		if i%5000 == 0 {
-			log.Printf("blocked for %.0f seconds waiting for FORGET on i%d", time.Since(t0).Seconds(), id.Ino)
+			b.logf("blocked for %.0f seconds waiting for FORGET on i%d", time.Since(t0).Seconds(), id.Ino)
 		}
 		b.mu.Lock()
 	}
@@ -129,6 +129,12 @@ func (b *rawBridge) newInodeUnlocked(ops InodeEmbedder, id StableAttr, persisten
 	return ops.embed()
 }
 
+func (b *rawBridge) logf(format string, args ...interface{}) {
+	if b.options.Logger != nil {
+		b.options.Logger.Printf(format, args...)
+	}
+}
+
 // expSleep sleeps for time `t` and returns an exponentially increasing value
 // for the next sleep time, capped at 1 ms.
 func expSleep(t time.Duration) time.Duration {