perf: reuse buffer when hashing source files for checksums

internal/fingerprint/sources_checksum.go

@@ -88,6 +88,10 @@ func (*ChecksumChecker) Kind() string {
 	return "checksum"
 }
 
+// readerOnly hides any WriterTo/ReaderFrom implementation of the wrapped
+// reader, forcing io.CopyBuffer to use the caller-provided buffer.
+type readerOnly struct{ io.Reader }
+
 func (c *ChecksumChecker) checksum(t *ast.Task) (string, error) {
 	sources, err := Globs(t.Dir, t.Sources, t.ShouldUseGitignore())
 	if err != nil {
@@ -101,14 +105,18 @@ func (c *ChecksumChecker) checksum(t *ast.Task) (string, error) {
 		if _, err := io.CopyBuffer(h, strings.NewReader(filepath.Base(f)), buf); err != nil {
 			return "", err
 		}
-		f, err := os.Open(f)
+		file, err := os.Open(f)
 		if err != nil {
 			return "", err
 		}
-		if _, err = io.CopyBuffer(h, f, buf); err != nil {
+		// Wrap the file in a plain io.Reader so io.CopyBuffer cannot take the
+		// (*os.File).WriteTo fast path, which ignores buf and allocates a fresh
+		// 32KiB buffer for every file. Reusing buf keeps this loop allocation-free.
+		if _, err = io.CopyBuffer(h, readerOnly{file}, buf); err != nil {
+			file.Close()
 			return "", err
 		}
-		f.Close()
+		file.Close()
 	}
 
 	hash := h.Sum128()