x/tools/gopls: panic related to go.mod

[muirdm]

On master (7534955) I saw the below panic when I was messing around with my go.mod. I don't remember exactly what I was doing.

panic: runtime error: index out of range [0] with length 0

goroutine 2387481 [running]:
golang.org/x/tools/internal/imports.(*ModuleResolver).init(0xc07d75c000, 0x0, 0x0)
	/Users/muir/projects/tools/internal/imports/mod.go:91 +0x12bf
golang.org/x/tools/internal/imports.(*ModuleResolver).ClearForNewMod(0xc07d75c000)
	/Users/muir/projects/tools/internal/imports/mod.go:201 +0xb1
golang.org/x/tools/internal/lsp/cache.(*importsState).runProcessEnvFunc(0xc0008483c0, 0x1b17180, 0xc0552bdb30, 0xc0d2405ee0, 0xc07fce3980, 0x0, 0x0)
	/Users/muir/projects/tools/internal/lsp/cache/imports.go:77 +0x725
golang.org/x/tools/internal/lsp/cache.(*snapshot).RunProcessEnvFunc(0xc0d2405ee0, 0x1b17180, 0xc0552bdb30, 0xc07fce3980, 0xc0f0ce8000, 0x2)
	/Users/muir/projects/tools/internal/lsp/cache/view.go:315 +0x56
golang.org/x/tools/internal/lsp/source.AllImportsFixes(0x1b17180, 0xc0552bdb30, 0x1b28900, 0xc0d2405ee0, 0x1b1abc0, 0xc0f0ce8000, 0x1b28900, 0xc0d2405ee0, 0x1b1fbe0, 0xc0f0ce8000, ...)
	/Users/muir/projects/tools/internal/lsp/source/format.go:97 +0x34f
golang.org/x/tools/internal/lsp.(*Server).codeAction(0xc0001a2460, 0x1b170c0, 0xc037df8a00, 0xc0d5815780, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/muir/projects/tools/internal/lsp/code_action.go:84 +0x1c88
golang.org/x/tools/internal/lsp.(*Server).CodeAction(0xc0001a2460, 0x1b170c0, 0xc037df8a00, 0xc0d5815780, 0xc0d5815780, 0x0, 0x0, 0x0, 0x1b045a0)
	/Users/muir/projects/tools/internal/lsp/server_gen.go:12 +0x4d
golang.org/x/tools/internal/lsp/protocol.serverDispatch(0x1b170c0, 0xc037df8a00, 0x1b2b3e0, 0xc0001a2460, 0xc0552bd800, 0x1b17300, 0xc037df8940, 0x0, 0x1b04480, 0xc0f10836b0)
	/Users/muir/projects/tools/internal/lsp/protocol/tsserver.go:341 +0x20af
golang.org/x/tools/internal/lsp/protocol.ServerHandler.func1(0x1b170c0, 0xc037df8a00, 0xc0552bd800, 0x1b17300, 0xc037df8940, 0x40b64d043b8f, 0x1f83780)
	/Users/muir/projects/tools/internal/lsp/protocol/protocol.go:63 +0xc5
golang.org/x/tools/internal/lsp/lsprpc.handshaker.func1(0x1b170c0, 0xc037df8a00, 0xc0552bd800, 0x1b17300, 0xc037df8940, 0x0, 0x0)
	/Users/muir/projects/tools/internal/lsp/lsprpc/lsprpc.go:559 +0x452
golang.org/x/tools/internal/jsonrpc2.MustReplyHandler.func1(0x1b170c0, 0xc037df8a00, 0xc072622e40, 0x1b17300, 0xc037df8940, 0xc0c91eab60, 0x10426d6)
	/Users/muir/projects/tools/internal/jsonrpc2/handler.go:35 +0xcf
golang.org/x/tools/internal/jsonrpc2.AsyncHandler.func1.2(0xc0a7342900, 0xc139b555f0, 0xc0001fa690, 0x1b170c0, 0xc037df8a00, 0xc072622e40, 0x1b17300, 0xc037df8940)
	/Users/muir/projects/tools/internal/jsonrpc2/handler.go:103 +0x86
created by golang.org/x/tools/internal/jsonrpc2.AsyncHandler.func1
	/Users/muir/projects/tools/internal/jsonrpc2/handler.go:100 +0x173

[heschi]

It appears that the imports code got an environment with an empty GOPATH, which should be impossible unless you managed to completely break go env somehow. I don't think I'm going to be able to figure out the root cause, but we can certainly return an error here rather than crashing.

[gopherbot]

Change https://golang.org/cl/275451 mentions this issue: internal/imports: prevent panic in imports GOMODCACHE logic