fusefrontend: sharedstorage: use byte-range lock on file header creation

Multiple host writing to the same empty file at the same time
could have overwritten each other's newly created file header,
leading to data corruption.

Fix the race by placing a byte-range lock on the file when
creating the file header.

2 files changed, 8 insertions, 0 deletions

diff --git a/internal/syscallcompat/sys_darwin.go b/internal/syscallcompat/sys_darwin.go
index 52d4e0f..17e6a9e 100644
--- a/internal/syscallcompat/sys_darwin.go
+++ b/internal/syscallcompat/sys_darwin.go
@@ -33,6 +33,10 @@ const (
 	// On Darwin we use O_SYMLINK which allows opening a symlink itself.
 	// On Linux, we only have O_NOFOLLOW.
 	OpenatFlagNofollowSymlink = unix.O_SYMLINK
+
+	// F_OFD_SETLKW only exists on Linux. On Darwin, fall back to F_SETLKW as a
+	// flawed replacement.
+	F_OFD_SETLKW = unix.F_SETLKW
 )
 
 // Unfortunately fsetattrlist does not have a syscall wrapper yet.
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index a850ba1..51a2c09 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -35,6 +35,10 @@ const (
 
 	// Only defined on Linux
 	ENODATA = unix.ENODATA
+
+	// F_OFD_SETLKW only exists on Linux. On Darwin, fall back to F_SETLKW as a
+	// flawed replacement.
+	F_OFD_SETLKW = unix.F_OFD_SETLKW
 )
 
 var preallocWarn sync.Once