Concurrent map access panic in `ToEntry()` if `ClearEntryCache()` is used

[sengleung]

The following panics occurred due to entryCache being cleared with ClearEntryCache() and ToEntry() being called concurrently.

fatal error: concurrent map read and map write

goroutine 334 [running]:
runtime.fatal({0x1ac4ce1?, 0xc00035cf00?})
        runtime/panic.go:1096 +0x5c fp=0xc01c599b80 sp=0xc01c599b50 pc=0x43d61c
runtime.mapaccess1(0x17fd740, 0xc01c91def0, 0xc0008e5683?)
        runtime/map.go:416 +0x50 fp=0xc01c599bc0 sp=0xc01c599b80 pc=0x4141b0
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59380, 0xc0006d7c20})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:562 +0x7e fp=0xc01c59a430 sp=0xc01c599bc0 pc=0xa4d35e
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59340, 0xc00094a000})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:879 +0x4c13 fp=0xc01c59aca0 sp=0xc01c59a430 pc=0xa51ef3
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d592c0, 0xc0008f2200})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:762 +0x3f4e fp=0xc01c59b510 sp=0xc01c59aca0 pc=0xa5122e
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59340, 0xc000857c00})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:826 +0x5059 fp=0xc01c59bd80 sp=0xc01c59b510 pc=0xa52339
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59d40, 0xc000941380})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:762 +0x3f4e fp=0xc01c59c5f0 sp=0xc01c59bd80 pc=0xa5122e
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59380, 0xc00094c140})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:662 +0xc46 fp=0xc01c59ce60 sp=0xc01c59c5f0 pc=0xa4df26
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d592c0, 0xc0008f2400})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:879 +0x4c13 fp=0xc01c59d6d0 sp=0xc01c59ce60 pc=0xa51ef3
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59340, 0xc00094a1c0})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:826 +0x5059 fp=0xc01c59df40 sp=0xc01c59d6d0 pc=0xa52339
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59d40, 0xc000941500})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:762 +0x3f4e fp=0xc01c59e7b0 sp=0xc01c59df40 pc=0xa5122e
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59380, 0xc00094c1e0})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:662 +0xc46 fp=0xc01c59f020 sp=0xc01c59e7b0 pc=0xa4df26
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d577c0, 0xc00035cf00})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:879 +0x4c13 fp=0xc01c59f890 sp=0xc01c59f020 pc=0xa51ef3
github.com/openconfig/goyang/pkg/yang.(*Entry).Find(0xc002482c00, {0xc000d26de0?, 0x20?})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:1337 +0x469 fp=0xc01c59f9a0 sp=0xc01c59f890 pc=0xa55aa9

fatal error: concurrent map writes

goroutine 398 [running]:
runtime.fatal({0x1aa0280?, 0x17038af?})
        runtime/panic.go:1096 +0x5c fp=0xc01ffa7e50 sp=0xc01ffa7e20 pc=0x43d61c
runtime.mapassign(0x17fd740, 0xc018cd5e30, 0x1d59340?)
        runtime/map.go:596 +0x4b fp=0xc01ffa7ed8 sp=0xc01ffa7e50 pc=0x4147cb
github.com/openconfig/goyang/pkg/yang.ToEntry.func1()
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:566 +0x2f fp=0xc01ffa7f08 sp=0xc01ffa7ed8 pc=0xa529cf
runtime.deferreturn()
        runtime/panic.go:477 +0x31 fp=0xc01ffa7f40 sp=0xc01ffa7f08 pc=0x43c251
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59340, 0xc001139500})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:1045 +0x3b17 fp=0xc01ffa87b0 sp=0xc01ffa7f40 pc=0xa50df7
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d59d40, 0xc00113f080})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:762 +0x3f4e fp=0xc01ffa9020 sp=0xc01ffa87b0 pc=0xa5122e
github.com/openconfig/goyang/pkg/yang.ToEntry({0x1d577c0, 0xc000f80f00})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:768 +0x3fc6 fp=0xc01ffa9890 sp=0xc01ffa9020 pc=0xa512a6
github.com/openconfig/goyang/pkg/yang.(*Entry).Find(0xc001d64180, {0xc001108870?, 0x20?})
        github.com/openconfig/goyang@v1.4.4/pkg/yang/entry.go:1337 +0x469 fp=0xc01ffa99a0 sp=0xc01ffa9890 pc=0xa55aa9

• ClearEntryCache() is called after processing YANG modules (added recently in https://github.com/openconfig/goyang/pull/260).

https://github.com/openconfig/goyang/blob/5ad0d2feb9ce655fb39e414bd4e3696356780cdb/pkg/yang/modules.go#L448-L450

• entryCache is accessed in ToEntry() (by calling Find()) concurrently.

https://github.com/openconfig/goyang/blob/5ad0d2feb9ce655fb39e414bd4e3696356780cdb/pkg/yang/entry.go#L562-L567

• Because entryCache has been cleared, calling ToEntry() will write to entryCache.
• If another goroutine tries read or write to entryCache at the same time, a panic can occur.
• This would not happen if ClearEntryCache() is not called because entryCache would be fully populated after processing YANG modules and the cache can be safely read concurrently as no writes would occur.