x/tools/gopls/internal/lsp/template: panic - slice bounds out of range (crash)

[kleinnic74]

gopls version: v0.11.0 (go1.19.3)
gopls flags:
update flags: proxy
extension version: 0.37.0
go version: 1.19.3
environment: Visual Studio Code win32
initialization error: undefined
issue timestamp: Tue, 20 Dec 2022 12:17:00 GMT
restart history:
Tue, 20 Dec 2022 09:27:58 GMT: activation (enabled: true)

ATTENTION: PLEASE PROVIDE THE DETAILS REQUESTED BELOW.

Describe what you observed.

panic: runtime error: slice bounds out of range [39:37]

goroutine 163 [running]:
golang.org/x/tools/gopls/internal/lsp/template.(*completer).complete(0xc001ea7350)
	  completion.go:116  0xd05
golang.org/x/tools/gopls/internal/lsp/template.Completion({0xc0016cd740%3F, 0xc001a4b048%3F}, {0x164d118%3F, 0xc0009a0160%3F}, {0x1644920, 0xc0004b7ce0}, {0x0%3F, 0x0%3F}, {0x1, {0x0, ...}})
	  completion.go:55  0x30e
golang.org/x/tools/gopls/internal/lsp.(*Server).completion(0x1257ec0%3F, {0x1640c78, 0xc000192300}, 0xc0018d5d10)
	  completion.go:43  0x31d
golang.org/x/tools/gopls/internal/lsp.(*Server).Completion(0xc000196320%3F, {0x1640c78%3F, 0xc000192300%3F}, 0x1257ec0%3F)
	  server_gen.go:28  0x25
golang.org/x/tools/gopls/internal/lsp/protocol.serverDispatch({0x1640c78, 0xc000192300}, {0x1650ae0, 0xc00037a100}, 0xc0018d2270, {0x1641068, 0xc000192280})
	  tsserver.go:269  0x20c2
golang.org/x/tools/gopls/internal/lsp/protocol.ServerHandler.func1({0x1640c78, 0xc000192300}, 0xc0018d2270, {0x1641068, 0xc000192280})
	  protocol.go:157  0x90
golang.org/x/tools/gopls/internal/lsp/lsprpc.handshaker.func1({0x1640c78, 0xc000192300}, 0xc0018d2270, {0x1641068%3F, 0xc000192280%3F})
	  lsprpc.go:519  0xa39
golang.org/x/tools/internal/jsonrpc2.MustReplyHandler.func1({0x1640c78, 0xc000192300}, 0xc00028e378, {0x1641068%3F, 0xc000192280%3F})
	  handler.go:35  0xf6
golang.org/x/tools/internal/jsonrpc2.AsyncHandler.func1.2()
	  handler.go:103  0xa3
created by golang.org/x/tools/internal/jsonrpc2.AsyncHandler.func1
	  handler.go:100  0x20a
[Error - 13:16:48] 

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[findleyr]

CC @pjweinb

[findleyr]

CC @pjweinb

[adonovan]

I'm not at all familiar with the internal/lsp/template package, but from the crash information we can tell that the failure is below in completion.go, and that the cause is that c.offset is greater than start (39 > 37) but start itself is within bounds (since the error didn't mention "with capacity").

func (c *completer) complete() (*protocol.CompletionList, error) {
	ans := &protocol.CompletionList{IsIncomplete: true, Items: []protocol.CompletionItem{}}
	start := c.p.FromPosition(c.pos)
	sofar := c.p.buf[c.offset:start] // panic (39:37)

Reading a little more, we can tell that:

• c.offset is 39, computed from start (37) + len("{{") (2).
• inTemplate must have returned 37.
• c.p.FromPosition(c.pos) must also be 37, since it provides the upper bound of the slice.
• offset within inTemplate must be 37, since it comes from the same expression.
• the first loop within inTemplate does not return, because it can only return a value less than offset, but the function returns 37, which is equal to offset.
• therefore the second loop must have returned, since the final return is negative.
• thus 37 must have come from fc.elided, and the region being tested for {{ and }} is empty.
• so elideAt(37) must have been called. This means there was "{{" at 37. In short, the completion point was before a "{{".

I just tried creating a new template file in VSCode, inserting {{, backing up by two characters, and inserting a, and I got a panic similar to this one. Yay.

[findleyr]

This seems like it's not getting fixed for v0.12.0. That's ok. It was marked as a release blocker because it is a crasher, but I don't think the template feature should block the release. Moving to v0.13.0.

[gopherbot]

Change https://go.dev/cl/622875 mentions this issue: gopls/internal/template: fix completion token boundary conditions