PRW2:Add bound checking for symbol referencs

[SungJin1212]

I have found the Distributor gets stuck in panic when UnitRef exceeds the symbols array length in
https://github.com/cortexproject/cortex/blob/master/pkg/util/push/push.go#L220
This PR adds bound checking logic on the server side.

Panic log

Prometheus version: v3.9.1

2026/02/23 00:00:46 http: panic serving 192.168.119.93:60442: runtime error: index out of range [2484] with length 2160
goroutine 10056052 [running]:
net/http.(*conn).serve.func1()
	/usr/local/go/src/net/http/server.go:1943 +0xd3
panic({0x3844be0?, 0xc00daff5a8?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
github.com/opentracing-contrib/go-stdlib/nethttp.MiddlewareFunc.func5.1()
	/go/src/github.com/cortexproject/cortex/vendor/github.com/opentracing-contrib/go-stdlib/nethttp/server.go:159 +0x169
panic({0x3844be0?, 0xc00daff5a8?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
github.com/cortexproject/cortex/pkg/util/push.convertV2RequestToV1(0xc00c9000c0, 0x0)
	/go/src/github.com/cortexproject/cortex/pkg/util/push/push.go:220 +0xc10
github.com/cortexproject/cortex/pkg/api.(*API).RegisterDistributor.Handler.func1.2()
	/go/src/github.com/cortexproject/cortex/pkg/util/push/push.go:114 +0x1e8
github.com/cortexproject/cortex/pkg/api.(*API).RegisterDistributor.Handler.func1({0x4139850, 0xc001e8aa80}, 0xc00044a000)
	/go/src/github.com/cortexproject/cortex/pkg/util/push/push.go:192 +0x6f0
net/http.HandlerFunc.ServeHTTP(0xc0006afe00?, {0x4139850?, 0xc001e8aa80?}, 0x413f2c8?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/weaveworks/common/middleware.init.func1.1({0x4139850, 0xc001e8aa80}, 0xc0006afe00)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/http_auth.go:17 +0x108
net/http.HandlerFunc.ServeHTTP(0x0?, {0x4139850?, 0xc001e8aa80?}, 0x0?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/cortexproject/cortex/pkg/api.(*API).RegisterRoute.HTTPHeaderMiddleware.Wrap.func1({0x4139850, 0xc001e8aa80}, 0xc0166bf680?)
	/go/src/github.com/cortexproject/cortex/pkg/api/middlewares.go:50 +0x66
net/http.HandlerFunc.ServeHTTP(0xc0006afb80?, {0x4139850?, 0xc001e8aa80?}, 0x2?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/gorilla/mux.(*Router).ServeHTTP(0xc0002bb5c0, {0x4139850, 0xc001e8aa80}, 0xc0006af7c0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/gorilla/mux/mux.go:212 +0x1e2
github.com/weaveworks/common/middleware.(*Instrument).Wrap.Instrument.Wrap.func1.2({0x4139850?, 0xc001e8aa80?})
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/instrument.go:70 +0x33
github.com/felixge/httpsnoop.(*Metrics).CaptureMetrics(0xc001ba8570, {0x7f081fe2a9b0, 0xc00c900060}, 0xc00098f7f0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/felixge/httpsnoop/capture_metrics.go:84 +0x1e5
github.com/felixge/httpsnoop.CaptureMetricsFn({0x7f081fe2a9b0, 0xc00c900060}, 0xc00098f7f0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/felixge/httpsnoop/capture_metrics.go:39 +0x4e
github.com/weaveworks/common/middleware.(*Instrument).Wrap.Instrument.Wrap.func1({0x7f081fe2a9b0, 0xc00c900060}, 0xc0006af7c0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/instrument.go:69 +0x28b
net/http.HandlerFunc.ServeHTTP(0x41349b0?, {0x7f081fe2a9b0?, 0xc00c900060?}, 0xc0166bf560?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/weaveworks/common/middleware.(*Log).Wrap.Log.Wrap.func1({0x41349b0, 0xc000476e60}, 0xc0006af7c0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/logging.go:81 +0x209
net/http.HandlerFunc.ServeHTTP(0x41e045?, {0x41349b0?, 0xc000476e60?}, 0x1?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/opentracing-contrib/go-stdlib/nethttp.MiddlewareFunc.func5({0x41326e0, 0xc00544a5a0}, 0xc0006af540)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/opentracing-contrib/go-stdlib/nethttp/server.go:163 +0x49d
net/http.HandlerFunc.ServeHTTP(0x41e045?, {0x41326e0?, 0xc00544a5a0?}, 0xc00544a501?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
net/http.serverHandler.ServeHTTP({0x41250c0?}, {0x41326e0?, 0xc00544a5a0?}, 0x1?)
	/usr/local/go/src/net/http/server.go:3340 +0x8e
net/http.(*conn).serve(0xc00425d0e0, {0x413f2c8, 0xc000a033b0})
	/usr/local/go/src/net/http/server.go:2109 +0x665
created by net/http.(*Server).Serve in goroutine 196
	/usr/local/go/src/net/http/server.go:3493 +0x485
2026/02/23 00:00:46 http: panic serving 192.168.119.93:60438: runtime error: index out of range [2512] with length 2127
goroutine 10056051 [running]:
net/http.(*conn).serve.func1()
	/usr/local/go/src/net/http/server.go:1943 +0xd3
panic({0x3844be0?, 0xc00dafe0c0?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
github.com/opentracing-contrib/go-stdlib/nethttp.MiddlewareFunc.func5.1()
	/go/src/github.com/cortexproject/cortex/vendor/github.com/opentracing-contrib/go-stdlib/nethttp/server.go:159 +0x169
panic({0x3844be0?, 0xc00dafe0c0?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
github.com/cortexproject/cortex/pkg/util/push.convertV2RequestToV1(0xc0085befc0, 0x0)
	/go/src/github.com/cortexproject/cortex/pkg/util/push/push.go:220 +0xc10
github.com/cortexproject/cortex/pkg/api.(*API).RegisterDistributor.Handler.func1.2()
	/go/src/github.com/cortexproject/cortex/pkg/util/push/push.go:114 +0x1e8
github.com/cortexproject/cortex/pkg/api.(*API).RegisterDistributor.Handler.func1({0x4139850, 0xc000ef5840}, 0xc00e0f4a00)
	/go/src/github.com/cortexproject/cortex/pkg/util/push/push.go:192 +0x6f0
net/http.HandlerFunc.ServeHTTP(0xc00e0f48c0?, {0x4139850?, 0xc000ef5840?}, 0x413f2c8?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/weaveworks/common/middleware.init.func1.1({0x4139850, 0xc000ef5840}, 0xc00e0f48c0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/http_auth.go:17 +0x108
net/http.HandlerFunc.ServeHTTP(0x0?, {0x4139850?, 0xc000ef5840?}, 0x0?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/cortexproject/cortex/pkg/api.(*API).RegisterRoute.HTTPHeaderMiddleware.Wrap.func1({0x4139850, 0xc000ef5840}, 0xc000919d40?)
	/go/src/github.com/cortexproject/cortex/pkg/api/middlewares.go:50 +0x66
net/http.HandlerFunc.ServeHTTP(0xc00e0f4640?, {0x4139850?, 0xc000ef5840?}, 0x2?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/gorilla/mux.(*Router).ServeHTTP(0xc0002bb5c0, {0x4139850, 0xc000ef5840}, 0xc00e0f4500)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/gorilla/mux/mux.go:212 +0x1e2
github.com/weaveworks/common/middleware.(*Instrument).Wrap.Instrument.Wrap.func1.2({0x4139850?, 0xc000ef5840?})
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/instrument.go:70 +0x33
github.com/felixge/httpsnoop.(*Metrics).CaptureMetrics(0xc0004dbe00, {0x7f081fe2a9b0, 0xc0085bef60}, 0xc0019e77f0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/felixge/httpsnoop/capture_metrics.go:84 +0x1e5
github.com/felixge/httpsnoop.CaptureMetricsFn({0x7f081fe2a9b0, 0xc0085bef60}, 0xc0019e77f0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/felixge/httpsnoop/capture_metrics.go:39 +0x4e
github.com/weaveworks/common/middleware.(*Instrument).Wrap.Instrument.Wrap.func1({0x7f081fe2a9b0, 0xc0085bef60}, 0xc00e0f4500)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/instrument.go:69 +0x28b
net/http.HandlerFunc.ServeHTTP(0x41349b0?, {0x7f081fe2a9b0?, 0xc0085bef60?}, 0xc000918d20?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/weaveworks/common/middleware.(*Log).Wrap.Log.Wrap.func1({0x41349b0, 0xc000966820}, 0xc00e0f4500)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/weaveworks/common/middleware/logging.go:81 +0x209
net/http.HandlerFunc.ServeHTTP(0x41e045?, {0x41349b0?, 0xc000966820?}, 0x1?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
github.com/opentracing-contrib/go-stdlib/nethttp.MiddlewareFunc.func5({0x41326e0, 0xc0053f25a0}, 0xc00e0f43c0)
	/go/src/github.com/cortexproject/cortex/vendor/github.com/opentracing-contrib/go-stdlib/nethttp/server.go:163 +0x49d
net/http.HandlerFunc.ServeHTTP(0x41e045?, {0x41326e0?, 0xc0053f25a0?}, 0xc0053f2501?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
net/http.serverHandler.ServeHTTP({0x41250c0?}, {0x41326e0?, 0xc0053f25a0?}, 0x1?)
	/usr/local/go/src/net/http/server.go:3340 +0x8e
net/http.(*conn).serve(0xc00425d050, {0x413f2c8, 0xc000a033b0})
	/usr/local/go/src/net/http/server.go:2109 +0x665
created by net/http.(*Server).Serve in goroutine 196
	/usr/local/go/src/net/http/server.go:3493 +0x485

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[dosubot]

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