Add separate article specifically on Apache 2.0 and GPL v2 compatibil…

…ity question

Signed-off-by: Andy Fingerhut <andy.fingerhut@gmail.com>

license-details/README.md

@@ -62,7 +62,11 @@ https://github.com/p4lang projects, unless this is not legally
 allowed.
 
 
-# Source files that cannot be released under the Apache 2.0 license
+# Source files that should not be released under the Apache 2.0 license
+
+See [this article](apache-and-gpl-v2-licenses.md) for background on
+questions of compatibility of the Apache 2.0 and GPL v2 licenses.
+
 
 ## Python source files that import the Scapy package
 

license-details/apache-and-gpl-v2-licenses.md

@@ -0,0 +1,110 @@
+# Compatibility of GPL v2.0 and Apache 2.0 licenses
+
+The claims of incompatibility in this section could use some good
+references to back them up.  Please suggest references if you know of
+them.
+
+| Python program A  |  ... imports Python     |          |
+| with license      |  package B with license | Allowed? |
+| ----------------- | ----------------------- | -------- |
+| Apache-2.0        | Apache-2.0              | yes, same licenses |
+| GPL-2.0-only      | GPL-2.0-only            | yes, same licenses |
+| Apache-2.0        | GPL-2.0-only            | very questionable |
+| GPL-2.0-only      | Apache-2.0              | very questionable |
+
+
+There is no publicly available advice on the last two rows of the
+table above that makes the answer obviously "compatible or obviously
+"incompatible", hence the "very questionable" statement on whether it
+is allowed.  See later sections for examples arguing both ways.
+
+Given that lack of clarity on whether it is allowed, it seems prudent
+that the P4 Consortium should not risk its limited funds and available
+volunteer time by using questionable combinations of software
+licenses.  We have a way to avoid that risk: use combinations of
+licenses that are well known to be compatible.
+
+If someone proposes using other approaches, it seems wise that they
+should be able to provide advice backed by an organization with at
+least one intellectual property lawyer, and willing to defend the
+approach in court.
+
+
+# Article published in a law journal
+
+See [5].
+
+Here is the text of the abstract of this 29-page article:
+
+    License “incompatibility” in free and open source software
+    licensing means that, when two differently licensed pieces of software
+    are combined, one cannot comply with both licenses at the same time.
+    It is commonly accepted that the GNU General Public License version
+    2 is incompatible with the Apache License, version 2 because certain
+    provisions of the Apache License would be considered “further
+    restrictions” not permitted by the GPLv2. However, this article will
+    explain why there is no legally cognizable claim for combining the two,
+    either under a copyright infringement theory or a breach of contract
+    theory.
+
+I have not read this article in full.  I have no idea whether this
+author's ideas would hold up in a court case.
+
+
+# Open Source Automation Development Lab (OSADL)
+
+The OSADL [1] publishes a license compatibility matrix [2].  The
+details in this section are for the version of this matrix downloaded
+on 2025-Jan-23.
+
+The matrix entry for the leading license being GPL-2.0-only and the
+subordinate license being Apache-2.0 says:
+```
+    { "name": "Apache-2.0", "compatibility": "No", "explanation": "Incompatibility of the Apache-2.0 license with the GPL-2.0-only license is explicitly stated in the GPL-2.0-only license checklist."},
+```
+
+The matrix entry for the leading license being Apache-2.0 and the
+subordinate license being GPL-2.0-only says:
+```
+    { "name": "GPL-2.0-only", "compatibility": "No", "explanation": "Software under a copyleft license such as the GPL-2.0-only license normally cannot be redistributed under a non-copyleft license such as the Apache-2.0 license, except if it were explicitly permitted in the licenses."},
+```
+
+
+# Free Software Foundation (FSF)
+
+"By the same token, lax licenses are usually compatible with any
+copyleft license. In the combined program, the parts that came in
+under lax licenses still carry them, and the combined program as a
+whole carries the copyleft license. One lax license, Apache 2.0, has
+patent clauses which are incompatible with GPL version 2; since I
+think those patent clauses are good, I made GPL version 3 compatible
+with them." [4]
+
+The above was written by Richard Stallman, who often speaks for the
+FSF, but I do not know whether the FSF specifically endorses the legal
+interpretations in the quoted paragraph above.  It is published on the
+gnu.org site quite prominently, though.
+
+
+# Randomly found StackExchange discussion
+
+See [3].  I don't claim anything in there as authoritative, but it
+might have useful links that lead to more authoritiatve information.
+
+
+# References
+
+[1] https://osadl.org
+
+[2] "OASDL license compatibility matrix with explanations",
+    https://www.osadl.org/html/CompatMatrix.html
+
+
+[3] https://opensource.stackexchange.com/questions/1357/can-i-link-a-apache-2-0-library-into-software-under-gplv2
+
+[4] https://www.gnu.org/licenses/license-compatibility.html
+
+[5] Pamela S. Chestek, "A promise without a remedy: The supposed
+    incompatibility of the GPL v2 and Apache v2 licenses", Santa Clara
+    High Technology Law Journal, Vol 40, Issue 3, Article 2,
+    https://digitalcommons.law.scu.edu/cgi/viewcontent.cgi?article=1701&context=chtlj#:~:text=It%20is%20commonly%20accepted%20that,not%20permitted%20by%20the%20GPLv2.

license-details/ptf-and-scapy.md

@@ -14,17 +14,7 @@ The ptf package is released under the Apache 2.0 license:
 
 # Compatibility of GPL v2.0 and Apache 2.0 licenses
 
-The claims of incompatibility in this section could use some good
-references to back them up.  Please suggest references if you know of
-them.
-
-| Python program A  |  ... imports Python     |          |
-| with license      |  package B with license | Allowed? |
-| ----------------- | ----------------------- | -------- |
-| Apache-2.0        | Apache-2.0              | yes, same licenses        |
-| GPL-2.0-only      | GPL-2.0-only            | yes, same licenses        |
-| Apache-2.0        | GPL-2.0-only            | no, incompatible licenses |
-| GPL-2.0-only      | Apache-2.0              | no, incompatible licenses |
+See [this article](apache-and-gpl-v2-licenses.md).
 
 
 # Consequences
@@ -33,17 +23,19 @@ Assuming everything above is correct, consider a Python program that
 imports both the scapy and ptf packages _directly_, i.e. it imports
 ptf directly, and it also imports scapy directly.
 
-Can anyone publish such a program, and if so, under what license can
-it be released?
+Should P4.org publish such a program, and if so, under what license
+should it be released?
 
-It seems that the answer is "no", because there is no license that it
-can be released under.
+It seems that the answer is "no", because the argument for whether
+this is allowed is too unclear.
 
-It cannot be released under Apache-2.0, because it imports scapy,
-which is GPL-2.0-only.
+Should P4.org publish such a program under Apache-2.0?  It seems
+that the answer is "no", because the program imports scapy, which is
+GPL-2.0-only.  It is too unclear whether this is allowed.
 
-It cannot be released under GPL-2.0-only, because it imports ptf,
-which is Apache-2.0.
+SHould P4.org publish such a program under GPL-2.0-only?  It seems
+that the answer is "no", because the program imports ptf, which is
+Apache-2.0.  Again, it is too unclear whether this is allowed.
 
 There are at least a few Python programs in p4lang repositories that
 currently import both the scapy and ptf packages, including these:
@@ -72,3 +64,7 @@ user.
 Then in the cases where they write a program A that imports both scapy
 and ptf in in A, they choose to use ptf under the GPL v2 license, and
 also release program A under the GPL v2 license.
+
+In cases where they write a program A that imports ptf, but not scapy,
+then they have a choice whether to publish program A under the GPL v2
+license, or the Apache-2.0 license.