demo: crash since Config.bigOrigin is nil

[arl]

Reproducer:

) demo
# This is a demo of ivy. Type a newline to advance to each new step. Type one now.
? 2 ** 32
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x49ace4]

goroutine 6 [running]:
robpike.io/ivy/parse.(*Parser).runFromReader.func1()
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/parse/special.go:358 +0x1ea
panic({0x4fa4e0, 0x5fbc80})
        /home/aurelien/sdk/go1.17.4/src/runtime/panic.go:1038 +0x215
robpike.io/ivy/parse.(*Parser).runUntilError.func1()
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/parse/special.go:384 +0x1ea
panic({0x4fa4e0, 0x5fbc80})
        /home/aurelien/sdk/go1.17.4/src/runtime/panic.go:1038 +0x215
math/big.(*Int).Add(0xc0000a8080, 0x49a93f, 0x0)
        /home/aurelien/sdk/go1.17.4/src/math/big/int.go:118 +0x24
robpike.io/ivy/value.bigIntRand({0x544d70, 0xc0000753e0}, 0xffffffffffffffff, 0x4108b7)
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/value/unary.go:56 +0x77
robpike.io/ivy/value.unaryBigIntOp({0x544d70, 0xc0000753e0}, 0x5201e0, {0x544eb0, 0xc0000a8060})
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/value/unary.go:21 +0x8d
robpike.io/ivy/value.init.1.func2({0x544d70, 0xc0000753e0}, {0x544eb0, 0xc0000a8060})
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/value/unary.go:137 +0x71
robpike.io/ivy/value.(*unaryOp).EvalUnary(0xc00007e050, {0x544d70, 0xc0000753e0}, {0x544eb0, 0xc0000a8060})
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/value/eval.go:52 +0x119
robpike.io/ivy/exec.(*Context).EvalUnary(0xc0000753e0, {0xc0000a2020, 0x1}, {0x544eb0, 0xc0000a8060})
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/exec/context.go:138 +0x128
robpike.io/ivy/parse.(*unary).Eval(0xc0000a8000, {0x544d70, 0xc0000753e0})
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/parse/parse.go:193 +0x62
robpike.io/ivy/parse.(*Parser).runUntilError(0xc00009c000, {0x50f24a, 0x0})
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/parse/special.go:389 +0x113
robpike.io/ivy/parse.(*Parser).runFromReader(0xc000154d80, {0x544d70, 0xc0000753e0}, {0x50f24a, 0x0}, {0x541ca0, 0xc000176d80}, 0x0)
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/parse/special.go:362 +0x3ca
created by robpike.io/ivy/parse.DemoRunner
        /home/aurelien/godev/pkg/mod/robpike.io/ivy@v0.1.7/parse/special.go:434 +0x1ef

The fix is easy but since this is relatively minor (only happens during demo) and since there are different ways to fix it I'm not opening a PR, preferring to check if you're interested to fix this, and how.

When the demo starts, DemoRunner is provided a zero-valued default config.Config. Config.bigOrigin is a nil big.Int. Also, Config.BigOrigin() doesn't call Config.init(). Finally, Config.init doesn't initialize Config.bigOrigin.

One possible fix:

diff --git a/config/config.go b/config/config.go
index 74b2a8e..4c210d3 100644
--- a/config/config.go
+++ b/config/config.go
@@ -57,6 +57,7 @@ func (c *Config) init() {
 		c.output = os.Stdout
 		c.errOutput = os.Stderr
 		c.origin = 1
+		c.bigOrigin = big.NewInt(1)
 		c.seed = time.Now().UnixNano()
 		c.source = rand.NewSource(c.seed)
 		c.random = rand.New(c.source)
@@ -171,6 +172,7 @@ func (c *Config) Origin() int {
 
 // BigOrigin returns the index origin as a *big.Int.
 func (c *Config) BigOrigin() *big.Int {
+	c.init()
 	return c.bigOrigin
 }

[robpike]

What is the platform you are on? And what version of ivy are you (what is the head commit)? I can't reproduce it. I'd like to do that so I can write a test to verify the fix.

But I'm sure the fix looks right.

[robpike]

Ok, I've been able to reproduce it on my nth try. Thanks for the bug report.