fix(cache): keep the in-place truncate on windows

Unlinking before re-creating fixes the premature-EOF spin on unix, but Windows
refuses to remove a file another handle still has open and returns a sharing
violation. Because Create surfaces that error, every cache miss on a path with a
live reader would have failed outright - worse than the spin it was meant to fix.

Split the create behind a build tag: unix unlinks for a fresh inode, Windows keeps
truncating in place and stays exposed to the spin, which is the behaviour it
already had. The unix-only spec is skipped there.

utils/cache/spread_fs.go

@@ -128,12 +128,7 @@ func (sfs *spreadFS) Create(name string) (stream.File, error) {
 	if err != nil {
 		return nil, err
 	}
-	// Unlink instead of truncating: an older stream may still be serving readers from
-	// this path, and shrinking the file under them spins them at a premature EOF.
-	if err := os.Remove(name); err != nil && !os.IsNotExist(err) {
-		return nil, err
-	}
-	return os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
+	return createDataFile(name)
 }
 
 func (sfs *spreadFS) Open(name string) (stream.File, error) {

utils/cache/spread_fs_test.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 
 	"github.com/djherbis/stream"
@@ -45,6 +46,9 @@ var _ = Describe("Spread FS", func() {
 		It("leaves an already-open reader's bytes intact", func() {
 			// A re-created entry must not shrink the file an older stream is still
 			// serving: its reader would spin forever at the premature EOF.
+			if runtime.GOOS == "windows" {
+				Skip("Windows cannot unlink a file with open handles, so Create reuses the inode")
+			}
 			name := filepath.Join(rootDir, "aa", "bb", "data")
 			s, err := stream.NewStream(name, fs)
 			Expect(err).To(BeNil())

utils/cache/spread_fs_unix.go

@@ -0,0 +1,18 @@
+//go:build !windows
+
+package cache
+
+import (
+	"os"
+
+	"github.com/djherbis/stream"
+)
+
+// createDataFile unlinks instead of truncating, so a re-created entry gets a fresh
+// inode and readers still holding the old one see its full contents.
+func createDataFile(name string) (stream.File, error) {
+	if err := os.Remove(name); err != nil && !os.IsNotExist(err) {
+		return nil, err
+	}
+	return os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
+}

utils/cache/spread_fs_windows.go

@@ -0,0 +1,15 @@
+//go:build windows
+
+package cache
+
+import (
+	"os"
+
+	"github.com/djherbis/stream"
+)
+
+// createDataFile truncates in place: Windows refuses to unlink a file another handle
+// has open, and failing the create would break every miss on a path with a live reader.
+func createDataFile(name string) (stream.File, error) {
+	return os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
+}