1 files changed, 76 insertions, 0 deletions
diff --git a/internal/iolimit/expect_length_reader.go b/internal/iolimit/expect_length_reader.go
new file mode 100644
index 00000000..477c207f
--- /dev/null
+++ b/internal/iolimit/expect_length_reader.go
@@ -0,0 +1,76 @@
+// Package iolimit provides small internal reader wrappers for length-constrained
+// stream I/O.
+package iolimit
+
+import (
+	"errors"
+	"io"
+)
+
+// ErrExpectedLengthExceeded reports that a stream produced bytes beyond the
+// expected length.
+var ErrExpectedLengthExceeded = errors.New("iolimit: stream exceeded expected length")
+
+// ExpectLengthReader wraps src and enforces an expected byte length.
+//
+// It returns io.ErrUnexpectedEOF if src ends before expected bytes are read.
+// It returns ErrExpectedLengthExceeded if reads continue beyond the expected
+// boundary and src still produces bytes.
+//
+// This reader does not drain src on close or at the expected boundary. As a
+// result, overlength streams are detected only when a caller reads at or past
+// the boundary.
+func ExpectLengthReader(src io.Reader, expected int64) io.Reader {
+	return &expectLengthReader{
+		src:       src,
+		remaining: expected,
+	}
+}
+
+type expectLengthReader struct {
+	src       io.Reader
+	remaining int64
+}
+
+func (reader *expectLengthReader) Read(dst []byte) (int, error) {
+	if len(dst) == 0 {
+		return 0, nil
+	}
+
+	if reader.remaining == 0 {
+		var probe [1]byte
+		n, err := reader.src.Read(probe[:])
+		if n > 0 {
+			return 0, ErrExpectedLengthExceeded
+		}
+		if err == nil {
+			return 0, nil
+		}
+		return 0, err
+	}
+
+	if reader.remaining < 0 {
+		return 0, ErrExpectedLengthExceeded
+	}
+
+	if int64(len(dst)) > reader.remaining {
+		dst = dst[:reader.remaining]
+	}
+
+	n, err := reader.src.Read(dst)
+	if n > 0 {
+		reader.remaining -= int64(n)
+	}
+
+	if err == io.EOF {
+		if reader.remaining > 0 {
+			return n, io.ErrUnexpectedEOF
+		}
+		if n > 0 {
+			return n, nil
+		}
+		return 0, io.EOF
+	}
+
+	return n, err
+}