This issue causes method in a Nil object to be read when a mirror download fails. In my case, this occurs due to my office's proxy blocking certain mirrors. This patch fixes the Nil access, exposing a more helpful error message.
## Before patch
```
$ brew install maven
Error: An exception occurred within a child process:
NoMethodError: undefined method `unlock' for nil:NilClass
```
## After patch
```
$ brew install maven
Error: An exception occurred within a child process:
DownloadError: Failed to download resource "maven"
Download failed: Couldn't determine mirror, try again later.
```
## Steps to reproduce
1. Use brew 1.8.0
2. Ensure maven is not installed:
```
brew remove maven
```
3. Ensure the download is not cached:
```
rm "$HOME/Library/Caches/Homebrew/downloads/*--apache-maven-*-bin.tar.gz"
```
4. Make the mirrors unreachable by pointing them at a nonfunctional proxy:
```
export http{,s}_proxy=http://localhost:8080
export no_proxy=*.github.com,*.brew.sh
```
5. Attempt package installation
```
brew install maven
```
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.
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