Suppose I have a GPL v2-licensed library that is of interest to a closed-source project (e.g. Wolfram Alpha). If some of my code were used in Wolfram Alpha, or a product that interfaces with Wolfram Alpha, would they have to release all of their code for Wolfram Alpha under the GPL v2 license?
GPL- How much source must be released?
open source;gpl;source code
Standard disclaimer: I am not a lawyer. You should get one, if you are concerned about possible legal issues.If some of my code were used in Wolfram Alpha ... would they have to release all of their code for Wolfram Alpha under the GPL?No. While GNU GPL indeed is hereditary license, that forces one to apply GPL to his program, that use GPL-licensed library, distributing it (speaking in the terms of GPLv3, conveying it), GPL does not force anyone to distribute software he wrote. Wolfam Alpha is a web-application or, if you will, software as a service (SaaS). That kind of usage does not qualify as distributing / conveying under terms of GNU GPL, so you cannot require Wolfram Alpha LCC to publish sources of their product.If you do not like this (reasonably), you have to consider using GNU AGPL (Affero General Public License) for the next versions of your libraries  it was purposefully created to prevent that problem. (That decision, of course, would not be retroactive.)Suppose you develop and release a free program under the ordinary GNU GPL. If developer D modifies the program and releases it, the GPL requires him to distribute his version under the GPL too. Thus, if you get a copy of his version, you are free to incorporate some or all of his changes into your own version.But suppose the program is mainly useful on servers. When D modifies the program, he might very likely run it on his own server and never release copies. Then you would never get a copy of the source code of his version, so you would never have the chance to include his changes in your version. You may not like that outcome.Using the GNU Affero GPL avoids that outcome. If D runs his version on a server that everyone can use, you too can use it. Assuming he has followed the license requirement to let the server's users download the source code of his version, you can do so, and then you can incorporate his changes into your version. GNU.org. Why Affero GPLP. S. @whatsisname pointed out that Wolfram Alpha is actually offered for deploying at customers hardware (what the price might be, I wonder). That should overturn the situation. But I believe, OP had not knew about it, choosing the hypothetical example, as well as I had not realized it, answering.