runtime: TestArenaCollision failures

[gopherbot]

#!watchflakes
default <- pkg == "runtime" && test == "TestArenaCollision"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestArenaCollision
    malloc_test.go:648: === RUN   TestArenaCollision
            malloc_test.go:665: reserved [0x3d4b34800000, 0x3d4b34c00000)
            malloc_test.go:665: reserved [0x3dcb34800000, 0x3dcb34c00000)
        --- FAIL: TestArenaCollision (0.00s)
        panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
        [signal 0xc0000005 code=0x0 addr=0x0 pc=0x7ff617b923c7]
        
        goroutine 8 gp=0x3d4b34773500 m=6 mp=0x3d4b34791008 [running]:
        panic({0x7ff617cfa800?, 0x7ff618173630?})
...
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1885 +0x2d9 fp=0x3dcb34713fc8 sp=0x3dcb34713f40 pc=0x7ff617811359
        runtime.gcBgMarkStartWorkers.gowrap1()
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1668 +0x17 fp=0x3dcb34713fe0 sp=0x3dcb34713fc8 pc=0x7ff617811057
        runtime.goexit({})
        	C:/b/s/w/ir/x/w/goroot/src/runtime/asm_amd64.s:1727 +0x1 fp=0x3dcb34713fe8 sp=0x3dcb34713fe0 pc=0x7ff61787c021
        created by runtime.gcBgMarkStartWorkers in goroutine 8
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1668 +0x105
        
        (exit status exit status 2)
--- FAIL: TestArenaCollision (0.03s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "runtime" && test == "TestArenaCollision"

2025-11-20 16:10 gotip-windows-amd64 go@a18aff80 runtime.TestArenaCollision (log)

=== RUN   TestArenaCollision
    malloc_test.go:648: === RUN   TestArenaCollision
            malloc_test.go:665: reserved [0x3d4b34800000, 0x3d4b34c00000)
            malloc_test.go:665: reserved [0x3dcb34800000, 0x3dcb34c00000)
        --- FAIL: TestArenaCollision (0.00s)
        panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
        [signal 0xc0000005 code=0x0 addr=0x0 pc=0x7ff617b923c7]
        
        goroutine 8 gp=0x3d4b34773500 m=6 mp=0x3d4b34791008 [running]:
        panic({0x7ff617cfa800?, 0x7ff618173630?})
...
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1885 +0x2d9 fp=0x3dcb34713fc8 sp=0x3dcb34713f40 pc=0x7ff617811359
        runtime.gcBgMarkStartWorkers.gowrap1()
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1668 +0x17 fp=0x3dcb34713fe0 sp=0x3dcb34713fc8 pc=0x7ff617811057
        runtime.goexit({})
        	C:/b/s/w/ir/x/w/goroot/src/runtime/asm_amd64.s:1727 +0x1 fp=0x3dcb34713fe8 sp=0x3dcb34713fe0 pc=0x7ff61787c021
        created by runtime.gcBgMarkStartWorkers in goroutine 8
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1668 +0x105
        
        (exit status exit status 2)
--- FAIL: TestArenaCollision (0.03s)

2025-11-26 19:56 gotip-windows-amd64-longtest go@b194f5d2 runtime.TestArenaCollision (log)

=== RUN   TestArenaCollision
    malloc_test.go:648: === RUN   TestArenaCollision
            malloc_test.go:665: reserved [0x15778cc00000, 0x15778d000000)
            malloc_test.go:665: reserved [0x15f78cc00000, 0x15f78d000000)
        --- FAIL: TestArenaCollision (0.00s)
        panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
        [signal 0xc0000005 code=0x0 addr=0x0 pc=0x7ff7add89187]
        
        goroutine 8 gp=0x15778ca75500 m=0 mp=0x7ff7ae3a0760 [running]:
        panic({0x7ff7adef7420?, 0x7ff7ae378660?})
...
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1764 +0xeb fp=0x15778cb95fc8 sp=0x15778cb95f40 pc=0x7ff7ada01a2b
        runtime.gcBgMarkStartWorkers.gowrap1()
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1668 +0x17 fp=0x15778cb95fe0 sp=0x15778cb95fc8 pc=0x7ff7ada01917
        runtime.goexit({})
        	C:/b/s/w/ir/x/w/goroot/src/runtime/asm_amd64.s:1730 +0x1 fp=0x15778cb95fe8 sp=0x15778cb95fe0 pc=0x7ff7ada6e4a1
        created by runtime.gcBgMarkStartWorkers in goroutine 8
        	C:/b/s/w/ir/x/w/goroot/src/runtime/mgc.go:1668 +0x105
        
        (exit status exit status 2)
--- FAIL: TestArenaCollision (0.04s)

— watchflakes

[gopherbot]

Change https://go.dev/cl/726527 mentions this issue: runtime: skip TestArenaCollision if we run out of hints