Implement "header hack"

internal/goofys.go

@@ -563,7 +563,9 @@ func (fs *Goofys) FreeSomeCleanBuffers(size int64) (int64, bool) {
 			if buf.dirtyID == 0 || buf.state == BUF_FLUSHED_FULL {
 				if buf.ptr != nil && !inode.IsRangeLocked(buf.offset, buf.length, false) &&
 					// Skip recent buffers when possible
-					(skipRecent == 0 || buf.recency <= skipRecent) {
+					(skipRecent == 0 || buf.recency <= skipRecent) &&
+					// Do not evict modified header
+					(buf.state != BUF_FLUSHED_FULL || buf.offset >= fs.flags.PartSizes[0].PartSize) {
 					if fs.flags.CachePath != "" && !buf.onDisk {
 						if toFs == -1 {
 							toFs = 0
@@ -605,6 +607,8 @@ func (fs *Goofys) FreeSomeCleanBuffers(size int64) (int64, bool) {
 						// A flushed buffer can be removed at a cost of finalizing multipart upload
 						// to read it back later. However it's likely not a problem if we're uploading
 						// a large file because we may never need to read it back.
+						// One exception is that we don't do it with the header because various
+						// software commonly modifies header after writing the whole large file
 						prev := del-1
 						if prev < 0 {
 							prev = i-1

internal/goofys_test.go

@@ -549,10 +549,10 @@ func (s *GoofysTest) TestWriteLargeTruncateMem20M(t *C) {
 	err = in.SetAttributes(PUInt64(100*1024*1024), nil, nil, nil, nil)
 	t.Assert(err, IsNil)
 
-	//// Modify the beginning of the file - will work after adding "header hack"
-	//buf[4095] = 1
-	//err = fh.WriteFile(0, buf[0:4096], true)
-	//t.Assert(err, IsNil)
+	// Modify the beginning of the file - will work after adding "header hack"
+	buf[4095] = 1
+	err = fh.WriteFile(0, buf[0:4096], true)
+	t.Assert(err, IsNil)
 
 	// Close
 	fh.Release()