Improve detection of file extensions in CurlDownloadStrategy

[claui]

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The issue

Sometimes, brew cask fetch/install fails with an error message similar to this:

==> Downloading https://w3g3a5v6.ssl.hwcdn.net/upload2/game/214692/735
Error: Download failed on Cask 'steamed-hams' with message: Operation
not supported @ rb_sysopen -
/Users/claudia/Documents/dev/brew/var/homebrew/locks/steamed-hams--1.0
.com&Expires=1520937180&Signature=CGmDulxL8pmutKTlCleNTUY%2FyO9Xyl5u9y
VZUE0uWrjadjuz67Jp7zx3H7NEOhSyOhu8nzicEHRBjr3uSoOJzwkLC8LBLKnz%2B2X%2B
iq5m6IdwSVFcLp2Q1Hr2kR7ETn3rF1DIq5o0lHCyzMmyNe5giEKJNW8WF0KXriULhzLTWL
SA3ZTLCIofAdRiiGje1kNYY3C0SBqymQB8CG3ONn5kj7CIGbxrDOq5xI2ZSJdIyPysSX7S
LvEDBw2KdR24q9t1wfjS9LUzelf5TWk6ojj8p9%2FHjl%2Fi%2FVCXNN4o1mW%2FMayy2t
TY1qcC%2FTmqI1ulZS8SNuaSgr9Iys9oDF1%2BPK%2B4Sg==&hwexp=1520937440&hwsi
g=55bc66884b925ef22f8673c33bfcc33b.incomplete.lock

Steps to reproduce (in the lab)

To reproduce the issue, check out this branch and run the cask/download_strategy test suite:

brew tests --only=cask/download_strategy

Steps to reproduce (in the wild)

One real-life example would be a Cask whose URL has 1. no . character anywhere in the URL path itself (outside of the domain name), and 2. at least one query parameter with a . character in it; and 3. other query parameters following that with a combined length of more than 255 characters.

This combination may be uncommon but it exists, especially in URLs that change on every visit, for example steamed-hams from my claui/cask-games tap:

$ brew tap claui/cask-games

$ brew cask fetch steamed-hams

Analysis

In a nutshell, URL query strings sometimes look like a very long file extension to Homebrew, which it then proceeds to use as a file name.

In CurlDownloadStrategy, Homebrew seems to apply a heuristic to the cask URL in order to figure out a possibly meaningful file extension. Sometimes this heuristic produces immensely long file extensions, especially when there’s a query parameter with a . character in it but not in the URL path itself (outside of the domain name).

Homebrew then believes that everything after the . is a file extension. In one of the later steps, it tries to create a lock file containing that extension, which fails because HFS+ cannot handle files whose base file name has more than 255 characters.

Fix

This PR improves Homebrew’s extension detector a bit so that it won’t cross individual URL query param boundaries any longer. This is done by adding & as a stop character:

def ext
  Pathname.new(@url).extname[/[^?&]+/]
end

This appears to fix the issue for most (if not all) practical purposes.

[MikeMcQuaid]

Looks good to me! It would be really good if we could get Cask using Homebrew's download strategies, though!