gopls/internal/template: fix panic in completion when cursor is between opening braces

[YLChen-007]

Summary

Fix a Denial of Service (DoS) vulnerability in gopls where the language server crashes with panic: slice bounds out of range [8:7] when code completion is requested at a cursor position between the two { characters of a template action delimiter {{.

Root Cause

In completer.complete(), the expression c.p.buf[c.offset:start] panics when c.offset > start. This occurs when:

• A .tmpl file contains a template action like hello {{ }}
• The cursor is placed between the first { and second { (e.g., at byte offset 7)
• c.offset is computed as tk.Start + len("{{") = 8
• start (cursor position) = 7
• Resulting slice buf[8:7] triggers panic: runtime error: slice bounds out of range [8:7]

Since there is no recovery mechanism, this crashes the entire gopls process, degrading the user's IDE workspace.

Fix

Added a bounds check before the slice operation in completer.complete():

if c.offset > start {
    return ans, nil
}

This returns an empty completion list when the cursor is inside the delimiter characters, which is the correct behavior — there is no meaningful text to complete at that position.

Testing

• Added TestCompleteBoundsCheck regression test that directly constructs the problematic state (offset=8, cursor=7) and verifies complete() returns gracefully without panicking
• All existing tests continue to pass (21 TestParsed subtests + TestSymbols + TestWordAt + TestQuotes)

Reproduction

1. Open a workspace with gopls and configure templateExtensions: ["tmpl"]
2. Create hi.tmpl with content: hello {{ }}
3. Place cursor between the two { characters (byte offset 7)
4. Trigger code completion → gopls crashes

Impact

Denial of Service. An attacker can craft a malicious Go project containing a .tmpl file that crashes gopls when the victim opens it and the cursor enters the trigger position.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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