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Joe Tsai d145c594ad util/deephash: improve cycle detection (#2470) ... The previous algorithm used a map of all visited pointers. The strength of this approach is that it quickly prunes any nodes that we have ever visited before. The detriment of the approach is that pruning is heavily dependent on the order that pointers were visited. This is especially relevant for hashing a map where map entries are visited in a non-deterministic manner, which would cause the map hash to be non-deterministic (which defeats the point of a hash). This new algorithm uses a stack of all visited pointers, similar to how github.com/google/go-cmp performs cycle detection. When we visit a pointer, we push it onto the stack, and when we leave a pointer, we pop it from the stack. Before visiting a pointer, we first check whether the pointer exists anywhere in the stack. If yes, then we prune the node. The detriment of this approach is that we may hash a node more often than before since we do not prune as aggressively. The set of visited pointers up until any node is only the path of nodes up to that node and not any other pointers that may have been visited elsewhere. This provides us deterministic hashing regardless of visit order. We can now delete hashMapFallback and associated complexity, which only exists because the previous approach was non-deterministic in the presence of cycles. This fixes a failure of the old algorithm where obviously different values are treated as equal because the pruning was too aggresive. See https://github.com/tailscale/tailscale/issues/2443#issuecomment-883653534 The new algorithm is slightly slower since it prunes less aggresively: name old time/op new time/op delta Hash-8 66.1µs ± 1% 68.8µs ± 1% +4.09% (p=0.000 n=19+19) HashMapAcyclic-8 63.0µs ± 1% 62.5µs ± 1% -0.76% (p=0.000 n=18+19) TailcfgNode-8 9.79µs ± 2% 9.88µs ± 1% +0.95% (p=0.000 n=19+17) HashArray-8 643ns ± 1% 653ns ± 1% +1.64% (p=0.000 n=19+19) However, a slower but more correct algorithm seems more favorable than a faster but incorrect algorithm. Signed-off-by: Joe Tsai <joetsai@digital-static.net>		3 years ago
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addlicense	cmd/addlicense: add command to add licenseheaders to generated code	3 years ago
cloner	cmd/cloner: support maps with clone ptrs	3 years ago
derper	cmd/derper: remove default for non-root users	3 years ago
derpprobe	cmd/derpprobe: add in a delay to wait for mesh info to sync	3 years ago
hello	all: adapt to opaque netaddr types	4 years ago
microproxy	tsweb: replace NewMux with a more flexible DebugHandler.	4 years ago
mkpkg	cmd/mkpkg: allow zero files in a package.	4 years ago
speedtest	Implemented Commandline Download Speedtest (#2064)	3 years ago
tailscale	cmd/tailscale/cli: allow effective GOOS to be changed for integration tests	3 years ago
tailscaled	util/deephash: improve cycle detection (#2470)	3 years ago
tsshd	net/interfaces: return all Tailscale addresses from Tailscale().	4 years ago