`EqualExportedValues` overflow the stack on recursive data structures

[iwahbe]

Description

assert.EqualExportedValues recurses through a value's exported fields via copyExportedFields. When the value contains a pointer cycle (a struct that, directly or indirectly, points back to itself), copyExportedFields follows the cycle forever and never terminates. The result is a fatal error: stack overflow that aborts the whole test binary. Tested with testify v1.11.1 and Go 1.26.4.

Step To Reproduce

Run this test file:

package repro

import (
	"testing"

	"github.com/stretchr/testify/assert"
)

type Node struct {
	Self *Node
}

func TestRecursive(t *testing.T) {
	a := &Node{}
	a.Self = a
	b := &Node{}
	b.Self = b
	assert.EqualExportedValues(t, a, b)
}

Expected behavior

The assertion should detect the cycle and report the two values as equal.

assert.EqualExportedValues already compares by structural equality, recursive structures are no different:

Details

func TestChain(t *testing.T) {
	a := &Node{Self: &Node{Self: &Node{}}}
	b := &Node{Self: &Node{Self: &Node{}}}
	assert.EqualExportedValues(t, a, b)
}

Actual behavior

copyExportedFields recurses on the self-referential pointer until the goroutine stack is exhausted, killing the entire test run:

runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0x... stack=[...]
fatal error: stack overflow

goroutine 5 [running]:
internal/abi.Name.Name(...)
	.../internal/abi/type.go:659 +0xc0 ...
reflect.(*structType).Field(...)
	.../reflect/type.go:1189 +0x90 ...
reflect.(*rtype).Field(...)
	.../reflect/type.go:777 +0x50 ...
github.com/stretchr/testify/assert.copyExportedFields(...)
	.../testify@v1.11.1/assert/assertions.go:99 +0x42c ...
github.com/stretchr/testify/assert.copyExportedFields(...)
	.../testify@v1.11.1/assert/assertions.go:114 +0x310 ...
github.com/stretchr/testify/assert.copyExportedFields(...)
	.../testify@v1.11.1/assert/assertions.go:106 +0x538 ...
[copyExportedFields repeats until the stack is exhausted]

---

I'd be happy to take a stab at addressing this if your willing to review.

[molloyzak13]

reply_target: #1915
The cycle is in the Ptr case at assertions.go:111-115. A *Node enters on line 111, dereferences on line 113, recurses into the struct, which hits the Self pointer field and calls back on line 105, landing in the Ptr case again with the same pointer. The nil guard on line 102 catches nil fields but doesn't detect already-visited pointers.

A map[uintptr]bool tracked alongside the recursion would fix it. In the Ptr case, check expectedValue.Pointer() against the set before the recursive call on line 113. If seen, return a zero-value pointer to break the cycle.

[tamish560]

Opened #1922 to fix this. copyExportedFields now tracks visited pointer addresses and breaks cycles by returning a zero value instead of recursing indefinitely.