Issue with defers in elses

[dgryski]

From Slack:

~/go/src/github.com/u-root/u-root/cmds/core/hexdump $ tinygo test -run='Hex/from_file$' -v
=== RUN   TestHexdump
=== RUN   TestHexdump/hexdump_from_file
opening /var/folders/5b/_hr1d1fd3qn4t9vtfx5p61wm0000gp/T/TestHexdump228065111/000/testfile
returning nil
starting defer chain
defer w
starting defer chain
    --- PASS: TestHexdump/hexdump_from_file (0.00s)
--- PASS: TestHexdump (0.00s)
PASS
ok  	github.com/u-root/u-root/cmds/core/hexdump	0.553s
~/go/src/github.com/u-root/u-root/cmds/core/hexdump $ git diff .
diff --git a/cmds/core/hexdump/hexdump.go b/cmds/core/hexdump/hexdump.go
index d7018ea7..cf2c7156 100644
--- a/cmds/core/hexdump/hexdump.go
+++ b/cmds/core/hexdump/hexdump.go
@@ -31,22 +31,27 @@ func hexdump(filenames []string, reader io.Reader, writer io.Writer) error {
                readers = make([]io.Reader, 0, len(filenames))

                for _, filename := range filenames {
+                       println("opening", filename)
                        f, err := os.Open(filename)
                        if err != nil {
                                return err
                        }
-                       defer f.Close()
+                       defer func() { println("defer file"); f.Close() }()
                        readers = append(readers, f)
                }
        }

        r := io.MultiReader(readers...)
        w := hex.Dumper(writer)
-       defer w.Close()
+       defer func() { println("defer w"); w.Close() }()

        if _, err := io.Copy(w, r); err != nil {
+               println("returning err")
                return err
        }
+       _ = w
+       println("returning nil")
+       defer func() { println("starting defer chain") }()
        return nil
 }

We see starting defer chain twice but no defer file.
The second starting defer chain should be the defer file entry; it's like it gets overwritten.

[dgryski]

Reproducer:

package main

//go:noinline
func deferbug(b bool) {
        if !b {
                defer println("defer else func")
        }
}

func main() {
	deferbug(false)
}

Running:

~/go/src/github.com/dgryski/bug/defer $ go run main.go
defer else func
~/go/src/github.com/dgryski/bug/defer $ tinygo run main.go
error: failed to run compiled binary /var/folders/5b/_hr1d1fd3qn4t9vtfx5p61wm0000gp/T/tinygo1067563306/main: signal: segmentation fault

[dgryski]

Looks like createRunDefers() is called twice, and depending on exact order of the if blocks in the SSA the appropriate entries might not have been created yet. So .. we need to delay createRunDefers handling until the all the basic blocks have been translated and all the potential defers have been seen.

Here's the SSA for the buggy function:

func deferbug(b bool):
0:                                                                entry P:0 S:2
	if b goto 2 else 1
1:                                                              if.then P:1 S:1
	defer println("defer else func":string)
	jump 2
2:                                                              if.done P:2 S:0
	rundefers
	return
3:                                                              recover P:0 S:0
	return

We first translate the then branch which is 2 which includes the call to rundefers, but the defers haven't been filled in yet. They won't be filled in until we process label 1 in the else block.