From 291254414b2adcea8e36374c6dbcdb3d9ede06c5 Mon Sep 17 00:00:00 2001 From: Joe Tsai Date: Wed, 7 Dec 2016 22:25:41 -0800 Subject: [PATCH] io: fix MultiReader panic on nil reader. The combination of two prior CLs can cause panics: * CL/17873: make chained multiReader Read more efficient * CL/28533: make MultiReader nil exhausted Readers for earlier GC The first CL allows MultiReader to "inherit" another MultiReader's list of Readers for efficiency reasons. This is problematic when combined with the later CL since that can set prior Readers in that list to nil for GC reasons. This causes panics when two MultiReaders are used together (even synchronously). To fix this, rather than setting consumed Readers as nil, we set them with a special eofReader that always returns EOF. Fixes #18232 Change-Id: I2a9357ab217e3d54d38ea9022d18e4d14f4182d3 Reviewed-on: https://go-review.googlesource.com/34140 Reviewed-by: Brad Fitzpatrick Run-TryBot: Brad Fitzpatrick TryBot-Result: Gobot Gobot --- src/io/multi.go | 12 ++++++++++-- src/io/multi_test.go | 24 ++++++++++++++++++++++++ 2 files changed, 34 insertions(+), 2 deletions(-) diff --git a/src/io/multi.go b/src/io/multi.go index 46e45a60e8..d784846862 100644 --- a/src/io/multi.go +++ b/src/io/multi.go @@ -4,13 +4,19 @@ package io +type eofReader struct{} + +func (eofReader) Read([]byte) (int, error) { + return 0, EOF +} + type multiReader struct { readers []Reader } func (mr *multiReader) Read(p []byte) (n int, err error) { for len(mr.readers) > 0 { - // Optimization to flatten nested multiReaders (Issue 13558) + // Optimization to flatten nested multiReaders (Issue 13558). if len(mr.readers) == 1 { if r, ok := mr.readers[0].(*multiReader); ok { mr.readers = r.readers @@ -19,7 +25,9 @@ func (mr *multiReader) Read(p []byte) (n int, err error) { } n, err = mr.readers[0].Read(p) if err == EOF { - mr.readers[0] = nil // permit earlier GC + // Use eofReader instead of nil to avoid nil panic + // after performing flatten (Issue 18232). + mr.readers[0] = eofReader{} // permit earlier GC mr.readers = mr.readers[1:] } if n > 0 || err != EOF { diff --git a/src/io/multi_test.go b/src/io/multi_test.go index 16e351a879..1a6292fa8a 100644 --- a/src/io/multi_test.go +++ b/src/io/multi_test.go @@ -264,3 +264,27 @@ func TestMultiReaderFreesExhaustedReaders(t *testing.T) { t.Fatalf(`ReadFull = %d (%q), %v; want 2, "ar", nil`, n, buf[:n], err) } } + +func TestInterleavedMultiReader(t *testing.T) { + r1 := strings.NewReader("123") + r2 := strings.NewReader("45678") + + mr1 := MultiReader(r1, r2) + mr2 := MultiReader(mr1) + + buf := make([]byte, 4) + + // Have mr2 use mr1's []Readers. + // Consume r1 (and clear it for GC to handle) and consume part of r2. + n, err := ReadFull(mr2, buf) + if got := string(buf[:n]); got != "1234" || err != nil { + t.Errorf(`ReadFull(mr2) = (%q, %v), want ("1234", nil)`, got, err) + } + + // Consume the rest of r2 via mr1. + // This should not panic even though mr2 cleared r1. + n, err = ReadFull(mr1, buf) + if got := string(buf[:n]); got != "5678" || err != nil { + t.Errorf(`ReadFull(mr1) = (%q, %v), want ("5678", nil)`, got, err) + } +} -- 2.50.0