fusefrontend: rename "createsHole" to clearer "createsCiphertextHole"

...and add comments for what is happening.

2 files changed, 9 insertions, 3 deletions

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index 35c898d..9fb9f7e 100644
--- a/internal/fusefrontend/file.go
+++ b/internal/fusefrontend/file.go
@@ -306,7 +306,7 @@ func (f *file) Write(data []byte, off int64) (uint32, fuse.Status) {
 		return 0, fuse.ToStatus(err)
 	}
 	plainSize := f.contentEnc.CipherSizeToPlainSize(uint64(fi.Size()))
-	if f.createsHole(plainSize, off) {
+	if f.createsCiphertextHole(plainSize, off) {
 		status := f.zeroPad(plainSize)
 		if status != fuse.OK {
 			tlog.Warn.Printf("zeroPad returned error %v", status)
diff --git a/internal/fusefrontend/file_holes.go b/internal/fusefrontend/file_holes.go
index 34f702f..ff626d1 100644
--- a/internal/fusefrontend/file_holes.go
+++ b/internal/fusefrontend/file_holes.go
@@ -8,10 +8,16 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
-// Will a write to offset "off" create a file hole?
-func (f *file) createsHole(plainSize uint64, off int64) bool {
+// Will a write to plaintext offset "off" create a file hole in the ciphertext?
+func (f *file) createsCiphertextHole(plainSize uint64, off int64) bool {
+	// Appending a single byte to the file (equivalent to writing to
+	// offset=plainSize) would write to "nextBlock".
 	nextBlock := f.contentEnc.PlainOffToBlockNo(plainSize)
+	// targetBlock is the block the user wants to write to.
 	targetBlock := f.contentEnc.PlainOffToBlockNo(uint64(off))
+	// If the write goes past the next block, nextBlock will have
+	// to be zero-padded to the block boundary and at least nextBlock+1
+	// becomes a file hole in the ciphertext.
 	return targetBlock > nextBlock
 }