What are the "special cases" that it will fail on. I just tried "Install from URL" task with the following URL:
https://github.com/jtester/TestExtension/archive/1.0.zip
and it worked perfectly

it worked perfectly

Umm, I didn't expect this...

The "special case" I am talking about is actually what GitHub is doing here.
It took me a while to figure out why it doesn't work (for me), but since you asked for the details, here they are

The URL
https://github.com/jtester/TestExtension/archive/1.0.zip
does not point to a "real file" but triggers the GitHub build thingy generating a redirect to:
https://codeload.github.com/jtester/TestExtension/zip/1.0
passing the filename in the header:
attachment; filename=TestExtension-1.0.zip
Note that there are no quotes so this code:

https://github.com/joomla/joomla-cms/blob/master/libraries/cms/installer/helper.php#L63

$contentfilename = explode("\"", $response->headers['Content-Disposition']);
$target = $contentfilename[1];

will fail miserably, producing also a PHP notice when accessing the item 1 of an array with 1 entry... all this happens silently leaving the URL path as a fall back:
/path/to/joomla/tmp/1.0
with the resulting file name
1.0
Since JArchive relies on the file extension, it can not unpack a file with the extension 0, resulting in a Exception with the message: Unknown Archive Type which is also silently ignored...

Final result: "Unable to find install package"

Now, why this happens to me and not to you, I have honestly no idea.

If you have debugging capabilities, please set a breakpoint in the installer helper #64, otherwise could you just hack a

echo $response->headers['Content-Disposition'];die;

in there ?

Another test (which is why this is bugging me...) Could you switch to the "Install from web" tab - the JED thingy - and search for the "EasyCreator" extension? Does this also install OK?

Thanks for testing :)

yes easy creator installs fine

Not sure I added the echo correctly but it gave
Notice: Undefined offset: 1 in /Applications/MAMP/htdocs/newjab/libraries/cms/installer/helper.php on line 64
attachment; filename=TestExtension-1.0.zip

Now I am even more confused since you even get the notice I was talking about...

Could I ask you again to hack a

echo $target; die;

and then

echo $url; die;

at the end of the method (before the return statement)?
In my case this gives me the URL /path/to/joomla/tmp/1.0 leading to the errors described.

Thanks again ;)

Sorry do you mean making the code look like

// Return the name of the downloaded package
    echo $target; die;
    echo $url; die;
    return basename($target);

like this:

// Bump the max execution time because not using built in php zip libs are slow
@set_time_limit(ini_get('max_execution_time'));

print_r(array($target, $url)); die;

// Return the name of the downloaded package
return basename($target);

Array ( [0] => /Applications/MAMP/htdocs/newjab/tmp/1.0.zip [1] => https://github.com/jtester/TestExtension/archive/1.0.zip )

That's not what I expected either... but you still get the notice ?

Your output contains the original URL while mine contains the URL from the redirect e.g.
Array ( [0] => /home/elkuku/repos/joomla-cms/tmp/1.0 [1] => https://codeload.github.com/jtester/TestExtension/zip/1.0 )

I'm lost :(

Yes i still get the notice
Sorry I cant help any further

Maybe related to PHP version?
I can reproduce the issue on my test server and patch fixes the issue.

I was testing locally on PHP Version 5.4.10
Just tested on my live server PHP Version 5.3.28 and got the error

Tested both on PHP 5.3.23 and 5.4.20 and got the error. So it's likely not related to that

I'm on PHP 5.5.9 ;)

@test
before patch I get the same error as Brian i.e.
PHP Notice: Undefined offset: 1 in /Applications/MAMP/htdocs/trunkgitnew/libraries/cms/installer/helper.php on line 71
But the component installs fine.

No more Notice after patch

I sorry. I missed this one and sent a PR for the same yesterday.

@elkuku can you review it? I think it's a simpler solution because instead of check double quotes and then basename= it direcly uses =.

Related PR: #3182

Closing in favor of #3182