Bazel aspects: allow sharing source jars and allow empty sourcejars (#757)

* Use labelname when declaring output for unzipped sourcejar
* Handle empty source jars

Author: antonsviridov-src

examples/bazel-example/src/main/java/srcjar_example/BUILD

@@ -4,10 +4,25 @@ genrule(
     cmd = "echo 'package com.testing; public class Bar {};' > Bar.java && jar cf $(@) Bar.java",
 )
 
+genrule(
+    name = "empty-srcjar",
+    outs = ["empty.srcjar"],
+    cmd = "touch test.txt && zip $(@) test.txt && zip -d $(@) test.txt",
+)
+
 java_library(
     name = "testing",
     srcs = [
         "Foo.java",
         ":generated-srcjar",
+        ":empty-srcjar"
+    ],
+)
+
+java_library(
+    name = "other_library",
+    srcs = [
+        "Baz.java", # create a new file in source at test/Baz.java, alongside test/Foo.java
+        ":generated-srcjar",
     ],
 )

examples/bazel-example/src/main/java/srcjar_example/Baz.java

@@ -0,0 +1,7 @@
+package com.testing;
+
+public class Baz {
+    public Bar baz(Bar value) {
+        return value;
+    }
+}

scip-java/src/main/resources/scip-java/scip_java.bzl

@@ -70,15 +70,15 @@ def _scip_java(target, ctx):
     output_dir = []
 
     for source_jar in source_jars:
-        dir = ctx.actions.declare_directory("extracted_srcjar/" + source_jar.short_path)
+        dir = ctx.actions.declare_directory(ctx.label.name + "/extracted_srcjar/" + source_jar.short_path)
         output_dir.append(dir)
 
         ctx.actions.run_shell(
             inputs = javac_action.inputs,
             outputs = [dir],
             mnemonic = "ExtractSourceJars",
             command = """
-                unzip {input_file} -d {output_dir}
+                [ "$(unzip -q -l {input_file} | wc -l)" -eq 0 ] || unzip {input_file} -d {output_dir}
             """.format(
                 output_dir = dir.path,
                 input_file = source_jar.path,