X-Git-Url: https://git.arvados.org/arvados.git/blobdiff_plain/04b9d6e93ca8bd18dca697e56689820516c8c572..a7631a1ccb6e2a6925d00a06562e171c4ce4ea2f:/sdk/go/arvados/fs_collection.go

diff --git a/sdk/go/arvados/fs_collection.go b/sdk/go/arvados/fs_collection.go
index f6afadba5b..060b57b493 100644
--- a/sdk/go/arvados/fs_collection.go
+++ b/sdk/go/arvados/fs_collection.go
@@ -5,10 +5,10 @@
 package arvados
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"io"
-	"log"
 	"os"
 	"path"
 	"regexp"
@@ -16,10 +16,14 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
-var maxBlockSize = 1 << 26
+var (
+	maxBlockSize      = 1 << 26
+	concurrentWriters = 4 // max goroutines writing to Keep in background and during flush()
+)
 
 // A CollectionFileSystem is a FileSystem that can be serialized as a
 // manifest and stored as a collection.
@@ -34,6 +38,9 @@ type CollectionFileSystem interface {
 
 	// Total data bytes in all files.
 	Size() int64
+
+	// Memory consumed by buffered file data.
+	memorySize() int64
 }
 
 type collectionFileSystem struct {
@@ -43,16 +50,15 @@ type collectionFileSystem struct {
 
 // FileSystem returns a CollectionFileSystem for the collection.
 func (c *Collection) FileSystem(client apiClient, kc keepClient) (CollectionFileSystem, error) {
-	var modTime time.Time
-	if c.ModifiedAt == nil {
+	modTime := c.ModifiedAt
+	if modTime.IsZero() {
 		modTime = time.Now()
-	} else {
-		modTime = *c.ModifiedAt
 	}
 	fs := &collectionFileSystem{
 		uuid: c.UUID,
 		fileSystem: fileSystem{
 			fsBackend: keepBackend{apiClient: client, keepClient: kc},
+			thr:       newThrottle(concurrentWriters),
 		},
 	}
 	root := &dirnode{
@@ -115,31 +121,128 @@ func (fs *collectionFileSystem) newNode(name string, perm os.FileMode, modTime t
 	}
 }
 
+func (fs *collectionFileSystem) Child(name string, replace func(inode) (inode, error)) (inode, error) {
+	return fs.rootnode().Child(name, replace)
+}
+
+func (fs *collectionFileSystem) FS() FileSystem {
+	return fs
+}
+
+func (fs *collectionFileSystem) FileInfo() os.FileInfo {
+	return fs.rootnode().FileInfo()
+}
+
+func (fs *collectionFileSystem) IsDir() bool {
+	return true
+}
+
+func (fs *collectionFileSystem) Lock() {
+	fs.rootnode().Lock()
+}
+
+func (fs *collectionFileSystem) Unlock() {
+	fs.rootnode().Unlock()
+}
+
+func (fs *collectionFileSystem) RLock() {
+	fs.rootnode().RLock()
+}
+
+func (fs *collectionFileSystem) RUnlock() {
+	fs.rootnode().RUnlock()
+}
+
+func (fs *collectionFileSystem) Parent() inode {
+	return fs.rootnode().Parent()
+}
+
+func (fs *collectionFileSystem) Read(_ []byte, ptr filenodePtr) (int, filenodePtr, error) {
+	return 0, ptr, ErrInvalidOperation
+}
+
+func (fs *collectionFileSystem) Write(_ []byte, ptr filenodePtr) (int, filenodePtr, error) {
+	return 0, ptr, ErrInvalidOperation
+}
+
+func (fs *collectionFileSystem) Readdir() ([]os.FileInfo, error) {
+	return fs.rootnode().Readdir()
+}
+
+func (fs *collectionFileSystem) SetParent(parent inode, name string) {
+	fs.rootnode().SetParent(parent, name)
+}
+
+func (fs *collectionFileSystem) Truncate(int64) error {
+	return ErrInvalidOperation
+}
+
 func (fs *collectionFileSystem) Sync() error {
-	log.Printf("cfs.Sync()")
 	if fs.uuid == "" {
 		return nil
 	}
 	txt, err := fs.MarshalManifest(".")
 	if err != nil {
-		log.Printf("WARNING: (collectionFileSystem)Sync() failed: %s", err)
-		return err
+		return fmt.Errorf("sync failed: %s", err)
 	}
 	coll := &Collection{
 		UUID:         fs.uuid,
 		ManifestText: txt,
 	}
-	err = fs.RequestAndDecode(nil, "PUT", "arvados/v1/collections/"+fs.uuid, fs.UpdateBody(coll), map[string]interface{}{"select": []string{"uuid"}})
+	err = fs.RequestAndDecode(nil, "PUT", "arvados/v1/collections/"+fs.uuid, nil, map[string]interface{}{
+		"collection": map[string]string{
+			"manifest_text": coll.ManifestText,
+		},
+		"select": []string{"uuid"},
+	})
 	if err != nil {
-		log.Printf("WARNING: (collectionFileSystem)Sync() failed: %s", err)
+		return fmt.Errorf("sync failed: update %s: %s", fs.uuid, err)
+	}
+	return nil
+}
+
+func (fs *collectionFileSystem) Flush(path string, shortBlocks bool) error {
+	node, err := rlookup(fs.fileSystem.root, path)
+	if err != nil {
+		return err
+	}
+	dn, ok := node.(*dirnode)
+	if !ok {
+		return ErrNotADirectory
+	}
+	dn.Lock()
+	defer dn.Unlock()
+	names := dn.sortedNames()
+	if path != "" {
+		// Caller only wants to flush the specified dir,
+		// non-recursively.  Drop subdirs from the list of
+		// names.
+		var filenames []string
+		for _, name := range names {
+			if _, ok := dn.inodes[name].(*filenode); ok {
+				filenames = append(filenames, name)
+			}
+		}
+		names = filenames
+	}
+	for _, name := range names {
+		child := dn.inodes[name]
+		child.Lock()
+		defer child.Unlock()
 	}
-	return err
+	return dn.flush(context.TODO(), names, flushOpts{sync: false, shortBlocks: shortBlocks})
+}
+
+func (fs *collectionFileSystem) memorySize() int64 {
+	fs.fileSystem.root.Lock()
+	defer fs.fileSystem.root.Unlock()
+	return fs.fileSystem.root.(*dirnode).memorySize()
 }
 
 func (fs *collectionFileSystem) MarshalManifest(prefix string) (string, error) {
 	fs.fileSystem.root.Lock()
 	defer fs.fileSystem.root.Unlock()
-	return fs.fileSystem.root.(*dirnode).marshalManifest(prefix)
+	return fs.fileSystem.root.(*dirnode).marshalManifest(context.TODO(), prefix)
 }
 
 func (fs *collectionFileSystem) Size() int64 {
@@ -465,8 +568,6 @@ func (fn *filenode) Write(p []byte, startPtr filenodePtr) (n int, ptr filenodePt
 				seg.Truncate(len(cando))
 				fn.memsize += int64(len(cando))
 				fn.segments[cur] = seg
-				cur++
-				prev++
 			}
 		}
 
@@ -493,30 +594,70 @@ func (fn *filenode) Write(p []byte, startPtr filenodePtr) (n int, ptr filenodePt
 // Write some data out to disk to reduce memory use. Caller must have
 // write lock.
 func (fn *filenode) pruneMemSegments() {
-	// TODO: async (don't hold Lock() while waiting for Keep)
-	// TODO: share code with (*dirnode)sync()
+	// TODO: share code with (*dirnode)flush()
 	// TODO: pack/flush small blocks too, when fragmented
 	for idx, seg := range fn.segments {
 		seg, ok := seg.(*memSegment)
-		if !ok || seg.Len() < maxBlockSize {
-			continue
-		}
-		locator, _, err := fn.FS().PutB(seg.buf)
-		if err != nil {
-			// TODO: stall (or return errors from)
-			// subsequent writes until flushing
-			// starts to succeed
+		if !ok || seg.Len() < maxBlockSize || seg.flushing != nil {
 			continue
 		}
-		fn.memsize -= int64(seg.Len())
-		fn.segments[idx] = storedSegment{
-			kc:      fn.FS(),
-			locator: locator,
-			size:    seg.Len(),
-			offset:  0,
-			length:  seg.Len(),
+		// Setting seg.flushing guarantees seg.buf will not be
+		// modified in place: WriteAt and Truncate will
+		// allocate a new buf instead, if necessary.
+		idx, buf := idx, seg.buf
+		done := make(chan struct{})
+		seg.flushing = done
+		// If lots of background writes are already in
+		// progress, block here until one finishes, rather
+		// than pile up an unlimited number of buffered writes
+		// and network flush operations.
+		fn.fs.throttle().Acquire()
+		go func() {
+			defer close(done)
+			locator, _, err := fn.FS().PutB(buf)
+			fn.fs.throttle().Release()
+			fn.Lock()
+			defer fn.Unlock()
+			if seg.flushing != done {
+				// A new seg.buf has been allocated.
+				return
+			}
+			if err != nil {
+				// TODO: stall (or return errors from)
+				// subsequent writes until flushing
+				// starts to succeed.
+				return
+			}
+			if len(fn.segments) <= idx || fn.segments[idx] != seg || len(seg.buf) != len(buf) {
+				// Segment has been dropped/moved/resized.
+				return
+			}
+			fn.memsize -= int64(len(buf))
+			fn.segments[idx] = storedSegment{
+				kc:      fn.FS(),
+				locator: locator,
+				size:    len(buf),
+				offset:  0,
+				length:  len(buf),
+			}
+		}()
+	}
+}
+
+// Block until all pending pruneMemSegments/flush work is
+// finished. Caller must NOT have lock.
+func (fn *filenode) waitPrune() {
+	var pending []<-chan struct{}
+	fn.Lock()
+	for _, seg := range fn.segments {
+		if seg, ok := seg.(*memSegment); ok && seg.flushing != nil {
+			pending = append(pending, seg.flushing)
 		}
 	}
+	fn.Unlock()
+	for _, p := range pending {
+		<-p
+	}
 }
 
 type dirnode struct {
@@ -549,112 +690,300 @@ func (dn *dirnode) Child(name string, replace func(inode) (inode, error)) (inode
 	return dn.treenode.Child(name, replace)
 }
 
-// sync flushes in-memory data (for the children with the given names,
-// which must be children of dn) to persistent storage. Caller must
-// have write lock on dn and the named children.
-func (dn *dirnode) sync(names []string) error {
-	type shortBlock struct {
-		fn  *filenode
-		idx int
-	}
-	var pending []shortBlock
-	var pendingLen int
+type fnSegmentRef struct {
+	fn  *filenode
+	idx int
+}
 
-	flush := func(sbs []shortBlock) error {
-		if len(sbs) == 0 {
+// commitBlock concatenates the data from the given filenode segments
+// (which must be *memSegments), writes the data out to Keep as a
+// single block, and replaces the filenodes' *memSegments with
+// storedSegments that reference the relevant portions of the new
+// block.
+//
+// bufsize is the total data size in refs. It is used to preallocate
+// the correct amount of memory when len(refs)>1.
+//
+// If sync is false, commitBlock returns right away, after starting a
+// goroutine to do the writes, reacquire the filenodes' locks, and
+// swap out the *memSegments. Some filenodes' segments might get
+// modified/rearranged in the meantime, in which case commitBlock
+// won't replace them.
+//
+// Caller must have write lock.
+func (dn *dirnode) commitBlock(ctx context.Context, refs []fnSegmentRef, bufsize int, sync bool) error {
+	if len(refs) == 0 {
+		return nil
+	}
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	done := make(chan struct{})
+	var block []byte
+	segs := make([]*memSegment, 0, len(refs))
+	offsets := make([]int, 0, len(refs)) // location of segment's data within block
+	for _, ref := range refs {
+		seg := ref.fn.segments[ref.idx].(*memSegment)
+		if !sync && seg.flushingUnfinished() {
+			// Let the other flushing goroutine finish. If
+			// it fails, we'll try again next time.
+			close(done)
 			return nil
+		} else {
+			// In sync mode, we proceed regardless of
+			// whether another flush is in progress: It
+			// can't finish before we do, because we hold
+			// fn's lock until we finish our own writes.
 		}
-		block := make([]byte, 0, maxBlockSize)
-		for _, sb := range sbs {
-			block = append(block, sb.fn.segments[sb.idx].(*memSegment).buf...)
+		seg.flushing = done
+		offsets = append(offsets, len(block))
+		if len(refs) == 1 {
+			block = seg.buf
+		} else if block == nil {
+			block = append(make([]byte, 0, bufsize), seg.buf...)
+		} else {
+			block = append(block, seg.buf...)
 		}
+		segs = append(segs, seg)
+	}
+	blocksize := len(block)
+	dn.fs.throttle().Acquire()
+	errs := make(chan error, 1)
+	go func() {
+		defer close(done)
+		defer close(errs)
 		locator, _, err := dn.fs.PutB(block)
+		dn.fs.throttle().Release()
 		if err != nil {
-			return err
+			errs <- err
+			return
 		}
-		off := 0
-		for _, sb := range sbs {
-			data := sb.fn.segments[sb.idx].(*memSegment).buf
-			sb.fn.segments[sb.idx] = storedSegment{
+		for idx, ref := range refs {
+			if !sync {
+				ref.fn.Lock()
+				// In async mode, fn's lock was
+				// released while we were waiting for
+				// PutB(); lots of things might have
+				// changed.
+				if len(ref.fn.segments) <= ref.idx {
+					// file segments have
+					// rearranged or changed in
+					// some way
+					ref.fn.Unlock()
+					continue
+				} else if seg, ok := ref.fn.segments[ref.idx].(*memSegment); !ok || seg != segs[idx] {
+					// segment has been replaced
+					ref.fn.Unlock()
+					continue
+				} else if seg.flushing != done {
+					// seg.buf has been replaced
+					ref.fn.Unlock()
+					continue
+				}
+			}
+			data := ref.fn.segments[ref.idx].(*memSegment).buf
+			ref.fn.segments[ref.idx] = storedSegment{
 				kc:      dn.fs,
 				locator: locator,
-				size:    len(block),
-				offset:  off,
+				size:    blocksize,
+				offset:  offsets[idx],
 				length:  len(data),
 			}
-			off += len(data)
-			sb.fn.memsize -= int64(len(data))
+			// atomic is needed here despite caller having
+			// lock: caller might be running concurrent
+			// commitBlock() goroutines using the same
+			// lock, writing different segments from the
+			// same file.
+			atomic.AddInt64(&ref.fn.memsize, -int64(len(data)))
+			if !sync {
+				ref.fn.Unlock()
+			}
 		}
+	}()
+	if sync {
+		return <-errs
+	} else {
 		return nil
 	}
+}
+
+type flushOpts struct {
+	sync        bool
+	shortBlocks bool
+}
 
+// flush in-memory data and remote-cluster block references (for the
+// children with the given names, which must be children of dn) to
+// local-cluster persistent storage.
+//
+// Caller must have write lock on dn and the named children.
+//
+// If any children are dirs, they will be flushed recursively.
+func (dn *dirnode) flush(ctx context.Context, names []string, opts flushOpts) error {
+	cg := newContextGroup(ctx)
+	defer cg.Cancel()
+
+	goCommit := func(refs []fnSegmentRef, bufsize int) {
+		cg.Go(func() error {
+			return dn.commitBlock(cg.Context(), refs, bufsize, opts.sync)
+		})
+	}
+
+	var pending []fnSegmentRef
+	var pendingLen int = 0
+	localLocator := map[string]string{}
 	for _, name := range names {
-		fn, ok := dn.inodes[name].(*filenode)
-		if !ok {
-			continue
-		}
-		for idx, seg := range fn.segments {
-			seg, ok := seg.(*memSegment)
-			if !ok {
-				continue
-			}
-			if seg.Len() > maxBlockSize/2 {
-				if err := flush([]shortBlock{{fn, idx}}); err != nil {
-					return err
-				}
-				continue
+		switch node := dn.inodes[name].(type) {
+		case *dirnode:
+			grandchildNames := node.sortedNames()
+			for _, grandchildName := range grandchildNames {
+				grandchild := node.inodes[grandchildName]
+				grandchild.Lock()
+				defer grandchild.Unlock()
 			}
-			if pendingLen+seg.Len() > maxBlockSize {
-				if err := flush(pending); err != nil {
-					return err
+			cg.Go(func() error { return node.flush(cg.Context(), grandchildNames, opts) })
+		case *filenode:
+			for idx, seg := range node.segments {
+				switch seg := seg.(type) {
+				case storedSegment:
+					loc, ok := localLocator[seg.locator]
+					if !ok {
+						var err error
+						loc, err = dn.fs.LocalLocator(seg.locator)
+						if err != nil {
+							return err
+						}
+						localLocator[seg.locator] = loc
+					}
+					seg.locator = loc
+					node.segments[idx] = seg
+				case *memSegment:
+					if seg.Len() > maxBlockSize/2 {
+						goCommit([]fnSegmentRef{{node, idx}}, seg.Len())
+						continue
+					}
+					if pendingLen+seg.Len() > maxBlockSize {
+						goCommit(pending, pendingLen)
+						pending = nil
+						pendingLen = 0
+					}
+					pending = append(pending, fnSegmentRef{node, idx})
+					pendingLen += seg.Len()
+				default:
+					panic(fmt.Sprintf("can't sync segment type %T", seg))
 				}
-				pending = nil
-				pendingLen = 0
 			}
-			pending = append(pending, shortBlock{fn, idx})
-			pendingLen += seg.Len()
 		}
 	}
-	return flush(pending)
+	if opts.shortBlocks {
+		goCommit(pending, pendingLen)
+	}
+	return cg.Wait()
 }
 
-// caller must have read lock.
-func (dn *dirnode) marshalManifest(prefix string) (string, error) {
-	var streamLen int64
-	type filepart struct {
-		name   string
-		offset int64
-		length int64
+// caller must have write lock.
+func (dn *dirnode) memorySize() (size int64) {
+	for _, name := range dn.sortedNames() {
+		node := dn.inodes[name]
+		node.Lock()
+		defer node.Unlock()
+		switch node := node.(type) {
+		case *dirnode:
+			size += node.memorySize()
+		case *filenode:
+			for _, seg := range node.segments {
+				switch seg := seg.(type) {
+				case *memSegment:
+					size += int64(seg.Len())
+				}
+			}
+		}
 	}
-	var fileparts []filepart
-	var subdirs string
-	var blocks []string
+	return
+}
 
+// caller must have write lock.
+func (dn *dirnode) sortedNames() []string {
 	names := make([]string, 0, len(dn.inodes))
 	for name := range dn.inodes {
 		names = append(names, name)
 	}
 	sort.Strings(names)
+	return names
+}
+
+// caller must have write lock.
+func (dn *dirnode) marshalManifest(ctx context.Context, prefix string) (string, error) {
+	cg := newContextGroup(ctx)
+	defer cg.Cancel()
+
+	if len(dn.inodes) == 0 {
+		if prefix == "." {
+			return "", nil
+		}
+		// Express the existence of an empty directory by
+		// adding an empty file named `\056`, which (unlike
+		// the more obvious spelling `.`) is accepted by the
+		// API's manifest validator.
+		return manifestEscape(prefix) + " d41d8cd98f00b204e9800998ecf8427e+0 0:0:\\056\n", nil
+	}
+
+	names := dn.sortedNames()
+
+	// Wait for children to finish any pending write operations
+	// before locking them.
 	for _, name := range names {
 		node := dn.inodes[name]
-		node.Lock()
-		defer node.Unlock()
-	}
-	if err := dn.sync(names); err != nil {
-		return "", err
+		if fn, ok := node.(*filenode); ok {
+			fn.waitPrune()
+		}
 	}
+
+	var dirnames []string
+	var filenames []string
 	for _, name := range names {
-		switch node := dn.inodes[name].(type) {
+		node := dn.inodes[name]
+		node.Lock()
+		defer node.Unlock()
+		switch node := node.(type) {
 		case *dirnode:
-			subdir, err := node.marshalManifest(prefix + "/" + name)
-			if err != nil {
-				return "", err
-			}
-			subdirs = subdirs + subdir
+			dirnames = append(dirnames, name)
 		case *filenode:
+			filenames = append(filenames, name)
+		default:
+			panic(fmt.Sprintf("can't marshal inode type %T", node))
+		}
+	}
+
+	subdirs := make([]string, len(dirnames))
+	rootdir := ""
+	for i, name := range dirnames {
+		i, name := i, name
+		cg.Go(func() error {
+			txt, err := dn.inodes[name].(*dirnode).marshalManifest(cg.Context(), prefix+"/"+name)
+			subdirs[i] = txt
+			return err
+		})
+	}
+
+	cg.Go(func() error {
+		var streamLen int64
+		type filepart struct {
+			name   string
+			offset int64
+			length int64
+		}
+
+		var fileparts []filepart
+		var blocks []string
+		if err := dn.flush(cg.Context(), filenames, flushOpts{sync: true, shortBlocks: true}); err != nil {
+			return err
+		}
+		for _, name := range filenames {
+			node := dn.inodes[name].(*filenode)
 			if len(node.segments) == 0 {
 				fileparts = append(fileparts, filepart{name: name})
-				break
+				continue
 			}
 			for _, seg := range node.segments {
 				switch seg := seg.(type) {
@@ -680,24 +1009,25 @@ func (dn *dirnode) marshalManifest(prefix string) (string, error) {
 				default:
 					// This can't happen: we
 					// haven't unlocked since
-					// calling sync().
+					// calling flush(sync=true).
 					panic(fmt.Sprintf("can't marshal segment type %T", seg))
 				}
 			}
-		default:
-			panic(fmt.Sprintf("can't marshal inode type %T", node))
 		}
-	}
-	var filetokens []string
-	for _, s := range fileparts {
-		filetokens = append(filetokens, fmt.Sprintf("%d:%d:%s", s.offset, s.length, manifestEscape(s.name)))
-	}
-	if len(filetokens) == 0 {
-		return subdirs, nil
-	} else if len(blocks) == 0 {
-		blocks = []string{"d41d8cd98f00b204e9800998ecf8427e+0"}
-	}
-	return manifestEscape(prefix) + " " + strings.Join(blocks, " ") + " " + strings.Join(filetokens, " ") + "\n" + subdirs, nil
+		var filetokens []string
+		for _, s := range fileparts {
+			filetokens = append(filetokens, fmt.Sprintf("%d:%d:%s", s.offset, s.length, manifestEscape(s.name)))
+		}
+		if len(filetokens) == 0 {
+			return nil
+		} else if len(blocks) == 0 {
+			blocks = []string{"d41d8cd98f00b204e9800998ecf8427e+0"}
+		}
+		rootdir = manifestEscape(prefix) + " " + strings.Join(blocks, " ") + " " + strings.Join(filetokens, " ") + "\n"
+		return nil
+	})
+	err := cg.Wait()
+	return rootdir + strings.Join(subdirs, ""), err
 }
 
 func (dn *dirnode) loadManifest(txt string) error {
@@ -758,8 +1088,14 @@ func (dn *dirnode) loadManifest(txt string) error {
 			}
 			name := dirname + "/" + manifestUnescape(toks[2])
 			fnode, err := dn.createFileAndParents(name)
-			if err != nil {
-				return fmt.Errorf("line %d: cannot use path %q: %s", lineno, name, err)
+			if fnode == nil && err == nil && length == 0 {
+				// Special case: an empty file used as
+				// a marker to preserve an otherwise
+				// empty directory in a manifest.
+				continue
+			}
+			if err != nil || (fnode == nil && length != 0) {
+				return fmt.Errorf("line %d: cannot use path %q with length %d: %s", lineno, name, length, err)
 			}
 			// Map the stream offset/range coordinates to
 			// block/offset/range coordinates and add
@@ -771,9 +1107,9 @@ func (dn *dirnode) loadManifest(txt string) error {
 				// situation might be rare anyway)
 				segIdx, pos = 0, 0
 			}
-			for next := int64(0); segIdx < len(segments); segIdx++ {
+			for ; segIdx < len(segments); segIdx++ {
 				seg := segments[segIdx]
-				next = pos + int64(seg.Len())
+				next := pos + int64(seg.Len())
 				if next <= offset || seg.Len() == 0 {
 					pos = next
 					continue
@@ -817,15 +1153,14 @@ func (dn *dirnode) loadManifest(txt string) error {
 	return nil
 }
 
-// only safe to call from loadManifest -- no locking
+// only safe to call from loadManifest -- no locking.
+//
+// If path is a "parent directory exists" marker (the last path
+// component is "."), the returned values are both nil.
 func (dn *dirnode) createFileAndParents(path string) (fn *filenode, err error) {
 	var node inode = dn
 	names := strings.Split(path, "/")
 	basename := names[len(names)-1]
-	if !permittedName(basename) {
-		err = fmt.Errorf("invalid file part %q in path %q", basename, path)
-		return
-	}
 	for _, name := range names[:len(names)-1] {
 		switch name {
 		case "", ".":
@@ -856,6 +1191,12 @@ func (dn *dirnode) createFileAndParents(path string) (fn *filenode, err error) {
 			return
 		}
 	}
+	if basename == "." {
+		return
+	} else if !permittedName(basename) {
+		err = fmt.Errorf("invalid file part %q in path %q", basename, path)
+		return
+	}
 	_, err = node.Child(basename, func(child inode) (inode, error) {
 		switch child := child.(type) {
 		case nil:
@@ -902,6 +1243,24 @@ type segment interface {
 
 type memSegment struct {
 	buf []byte
+	// If flushing is not nil and not ready/closed, then a) buf is
+	// being shared by a pruneMemSegments goroutine, and must be
+	// copied on write; and b) the flushing channel will close
+	// when the goroutine finishes, whether it succeeds or not.
+	flushing <-chan struct{}
+}
+
+func (me *memSegment) flushingUnfinished() bool {
+	if me.flushing == nil {
+		return false
+	}
+	select {
+	case <-me.flushing:
+		me.flushing = nil
+		return false
+	default:
+		return true
+	}
 }
 
 func (me *memSegment) Len() int {
@@ -918,28 +1277,31 @@ func (me *memSegment) Slice(off, length int) segment {
 }
 
 func (me *memSegment) Truncate(n int) {
-	if n > cap(me.buf) {
+	if n > cap(me.buf) || (me.flushing != nil && n > len(me.buf)) {
 		newsize := 1024
 		for newsize < n {
 			newsize = newsize << 2
 		}
 		newbuf := make([]byte, n, newsize)
 		copy(newbuf, me.buf)
-		me.buf = newbuf
+		me.buf, me.flushing = newbuf, nil
 	} else {
-		// Zero unused part when shrinking, in case we grow
-		// and start using it again later.
-		for i := n; i < len(me.buf); i++ {
+		// reclaim existing capacity, and zero reclaimed part
+		oldlen := len(me.buf)
+		me.buf = me.buf[:n]
+		for i := oldlen; i < n; i++ {
 			me.buf[i] = 0
 		}
 	}
-	me.buf = me.buf[:n]
 }
 
 func (me *memSegment) WriteAt(p []byte, off int) {
 	if off+len(p) > len(me.buf) {
 		panic("overflowed segment")
 	}
+	if me.flushing != nil {
+		me.buf, me.flushing = append([]byte(nil), me.buf...), nil
+	}
 	copy(me.buf[off:], p)
 }