fix(file): protect Zone.Expired with mutex#7940
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Copy() and CopyWithoutApex() read z.Expired without holding the mutex while TransferIn() writes it under Lock, causing a data race during concurrent zone transfers. Add RLock in both copy functions and wrap the bare write in Update() with Lock/Unlock. Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
yongtang
approved these changes
Mar 16, 2026
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1. Why is this pull request needed and what does it do?
Copy()andCopyWithoutApex()readz.Expiredwithout holding the mutex whileTransferIn()writes it underLock, causing a data race during concurrent zone transfers.Add
RLockin both copy functions and wrap the bare write inUpdate()with Lock/Unlock.2. Which issues (if any) are related?
See this for a recent data race that was caused by this issue in CI. For posterity:
Details
================== WARNING: DATA RACE Write at 0x00c000d6e4e0 by goroutine 126027: github.com/coredns/coredns/plugin/file.(*Zone).TransferIn() /home/runner/work/coredns/coredns/plugin/file/secondary.go:59 +0x651 github.com/coredns/coredns/plugin/secondary.setup.func2.1.1() /home/runner/work/coredns/coredns/plugin/secondary/setup.go:51 +0x9c Previous read at 0x00c000d6e4e0 by goroutine 126032: github.com/coredns/coredns/plugin/file.(*Zone).CopyWithoutApex() /home/runner/work/coredns/coredns/plugin/file/zone.go:69 +0x1e4 github.com/coredns/coredns/plugin/file.(*Zone).TransferIn() /home/runner/work/coredns/coredns/plugin/file/secondary.go:20 +0xbe github.com/coredns/coredns/plugin/file.File.ServeDNS() /home/runner/work/coredns/coredns/plugin/file/file.go:73 +0x90d github.com/coredns/coredns/plugin/secondary.(*Secondary).ServeDNS() :1 +0x10a github.com/coredns/coredns/core/dnsserver.(*Server).ServeDNS() /home/runner/work/coredns/coredns/core/dnsserver/server.go:315 +0x1399 github.com/coredns/coredns/core/dnsserver.(*Server).ServePacket.func1() /home/runner/work/coredns/coredns/core/dnsserver/server.go:178 +0x9c github.com/miekg/dns.HandlerFunc.ServeDNS() /home/runner/go/pkg/mod/github.com/miekg/dns@v1.1.72/server.go:37 +0x47 github.com/miekg/dns.(*Server).serveDNS() /home/runner/go/pkg/mod/github.com/miekg/dns@v1.1.72/server.go:683 +0x826 github.com/miekg/dns.(*Server).serveUDPPacket() /home/runner/go/pkg/mod/github.com/miekg/dns@v1.1.72/server.go:624 +0x37d github.com/miekg/dns.(*Server).serveUDP.gowrap2() /home/runner/go/pkg/mod/github.com/miekg/dns@v1.1.72/server.go:554 +0xd8 Goroutine 126027 (running) created at: github.com/coredns/coredns/plugin/secondary.setup.func2.1() /home/runner/work/coredns/coredns/plugin/secondary/setup.go:47 +0x168 sync.(*Once).doSlow() /opt/hostedtoolcache/go/1.26.1/x64/src/sync/once.go:78 +0xd1 sync.(*Once).Do() /opt/hostedtoolcache/go/1.26.1/x64/src/sync/once.go:69 +0x44 github.com/coredns/coredns/plugin/secondary.setup.func2() /home/runner/work/coredns/coredns/plugin/secondary/setup.go:46 +0xe5 github.com/coredns/caddy.startWithListenerFds() /home/runner/go/pkg/mod/github.com/coredns/caddy@v1.1.4-0.20250930002214-15135a999495/caddy.go:538 +0x52f github.com/coredns/caddy.Start() /home/runner/go/pkg/mod/github.com/coredns/caddy@v1.1.4-0.20250930002214-15135a999495/caddy.go:474 +0x1b4 github.com/coredns/coredns/test.CoreDNSServer() /home/runner/work/coredns/coredns/test/server.go:21 +0x164 github.com/coredns/coredns/test.CoreDNSServerAndPorts() /home/runner/work/coredns/coredns/test/server.go:49 +0x34 github.com/coredns/coredns/test.TestSecondaryZoneNotify() /home/runner/work/coredns/coredns/test/secondary_test.go:277 +0x364 testing.tRunner() /opt/hostedtoolcache/go/1.26.1/x64/src/testing/testing.go:2036 +0x21c testing.(*T).Run.gowrap1() /opt/hostedtoolcache/go/1.26.1/x64/src/testing/testing.go:2101 +0x38 Goroutine 126032 (running) created at: github.com/miekg/dns.(*Server).serveUDP() /home/runner/go/pkg/mod/github.com/miekg/dns@v1.1.72/server.go:554 +0x7a9 github.com/miekg/dns.(*Server).ActivateAndServe() /home/runner/go/pkg/mod/github.com/miekg/dns@v1.1.72/server.go:390 +0x6d9 github.com/coredns/coredns/core/dnsserver.(*Server).ServePacket() /home/runner/work/coredns/coredns/core/dnsserver/server.go:182 +0x2a4 github.com/coredns/caddy.startServers.func1.2() /home/runner/go/pkg/mod/github.com/coredns/caddy@v1.1.4-0.20250930002214-15135a999495/caddy.go:816 +0xfe ================== --- FAIL: TestSecondaryZoneNotify (0.01s) testing.go:1712: race detected during execution of testI think this is a latent bug that was surfaced by #7901 (March 2026), where two Go routines can call
TransferInfunction concurrently on the same zone. There's PRs #3024, #3056 and #3079 which tried to address the file locking issues earlier (in 2019). This latest change just introduced a new path that triggers the condition easily.3. Which documentation changes (if any) need to be made?
None.
4. Does this introduce a backward incompatible change or deprecation?
No.