fusefrontend: doWrite: delete file header if first write fails

xfstests generic/083 fills the filesystem almost completely while
running fsstress in parallel. In fsck, these would show up:

  readFileID 2580: incomplete file, got 18 instead of 19 bytes

This could happen when writing the file header works, but writing
the actual data fails.

Now we kill the header again by truncating the file to zero.

1 files changed, 21 insertions, 0 deletions

diff --git a/internal/syscallcompat/helpers.go b/internal/syscallcompat/helpers.go
new file mode 100644
index 0000000..e2a2215
--- /dev/null
+++ b/internal/syscallcompat/helpers.go
@@ -0,0 +1,21 @@
+package syscallcompat
+
+import (
+	"os"
+	"syscall"
+)
+
+// IsENOSPC tries to find out if "err" is a (potentially wrapped) ENOSPC error.
+func IsENOSPC(err error) bool {
+	// syscallcompat.EnospcPrealloc returns the naked syscall error
+	if err == syscall.ENOSPC {
+		return true
+	}
+	// os.File.WriteAt returns &PathError
+	if err2, ok := err.(*os.PathError); ok {
+		if err2.Err == syscall.ENOSPC {
+			return true
+		}
+	}
+	return false
+}