bazel: generate license list for documentation.

Example:

    $ bazel build //Documentation:pgm-licenses.txt
    ..
    $ cat bazel-genfiles/Documentation/pgm-licenses.txt
    //lib:jsch //lib:LICENSE-jsch
    //lib:servlet-api-3_1 //lib:LICENSE-Apache2.0

Still missing:

* check that there are dependencies on forbidden licenses

* check that there are no external dependencies except through the
  //lib directory.

Change-Id: I8f0ed94de20f68e0dbc1fe87dfd56ce11e02fab7

Documentation/BUILD

@@ -0,0 +1,7 @@
+
+load("//tools/bzl:license.bzl", "license_map")
+
+license_map(
+  name = "pgm-licenses",
+  target = "//gerrit-pgm:pgm",
+)

lib/BUILD

@@ -1,6 +1,7 @@
 java_library(
   name = 'servlet-api-3_1',
   neverlink = 1,
+  data = [ ":LICENSE-Apache2.0" ],
   exports = ['@servlet_api_3_1//jar'],
   visibility = ['//visibility:public'],
 )
@@ -75,6 +76,7 @@ java_library(
   name = 'jsch',
   exports = ['@jsch//jar'],
   visibility = ['//visibility:public'],
+  data = [ ":LICENSE-jsch" ],
 )
 
 java_library(

tools/bzl/BUILD

@@ -0,0 +1,2 @@
+
+exports_files(["license-map.py"])

tools/bzl/license-map.py

@@ -0,0 +1,25 @@
+#!/usr/bin/env python
+
+# reads a bazel query XML file, to join target names with their licenses.
+
+import sys
+import xml.etree.ElementTree as ET
+
+tree = ET.parse(sys.argv[1])
+root = tree.getroot()
+
+entries = {}
+
+for child in root:
+  rule_name = child.attrib["name"]
+  for c in child.getchildren():
+    if c.tag != "rule-input":
+      continue
+
+    license_name = c.attrib["name"]
+    if "//lib:LICENSE" in license_name:
+      assert rule_name not in entries, (license_name, entries[rule_name])
+      entries[rule_name] = license_name
+
+for k, v in sorted(entries.items()):
+  print k, v

tools/bzl/license.bzl

@@ -0,0 +1,22 @@
+
+def license_map(name, target):
+    """Generate XML for all targets that depend directly on a LICENSE file"""
+    native.genquery(
+        name = name + ".xml",
+        scope = [ target, ],
+
+        # Find everything that depends on a license file, but remove
+        # the license files themselves from this list.
+        expression = 'rdeps(%s, filter("//lib:LICENSE.*", deps(%s)),1) - filter("//lib:LICENSE.*", deps(%s))' % (target, target, target),
+
+        # We are interested in the edges of the graph ({java_library,
+        # license-file} tuples).  'query' provides this in the XML output.
+        opts = [ "--output=xml"],
+    )
+
+    # post process the XML into our favorite format.
+    native.genrule(
+        name = "gen_license_txt_" + name,
+        cmd = "python $(location //tools/bzl:license-map.py) $(location :%s.xml) > $@" % name,
+        outs = [ name + ".txt",],
+        tools = [ "//tools/bzl:license-map.py", name + ".xml"])