HADOOP-18487. Protobuf 2.5 removal part 2: stop exporting protobuf-2.5 (#6185)

Followup to the previous HADOOP-18487 patch: changes the scope of
protobuf-2.5 in hadoop-common and elsewhere from "compile" to "provided".

This means that protobuf-2.5 is
* No longer included in hadoop distributions
* No longer exported by hadoop common POM files
* No longer exported transitively by other hadoop modules.
* No longer listed in LICENSE-binary.

Contributed by Steve Loughran

BUILDING.txt

@@ -301,15 +301,11 @@ Controlling the redistribution of the protobuf-2.5 dependency
     the Hadoop codebase; alongside the move to Protobuf 3.x a private successor
     class, org.apache.hadoop.ipc.internal.ShadedProtobufHelper is now used.
 
-    The hadoop-common JAR still declares a dependency on protobuf-2.5, but this
-    is likely to change in the future. The maven scope of the dependency can be
-    set with the common.protobuf2.scope option.
-    It can be set to "provided" in a build:
-       -Dcommon.protobuf2.scope=provided
-    If this is done then protobuf-2.5.0.jar will no longer be exported as a dependency,
-    and will then be omitted from the share/hadoop/common/lib/ directory of
-    any Hadoop distribution built. Any application declaring a dependency on hadoop-commmon
-    will no longer get the dependency; if they need it then they must explicitly declare it:
+    The hadoop-common module no longer exports its compile-time dependency on
+    protobuf-2.5. Hadoop distributions no longer include it.
+    Any application declaring a dependency on hadoop-commmon will no longer get
+    the artifact added to their classpath.
+    If is still required, then they must explicitly declare it:
 
       <dependency>
         <groupId>com.google.protobuf</groupId>
@@ -317,6 +313,16 @@ Controlling the redistribution of the protobuf-2.5 dependency
         <version>2.5.0</version>
       </dependency>
 
+    In Hadoop builds the scope of the dependency can be set with the
+    option "common.protobuf2.scope".
+    This can be upgraded from "provided" to "compile" on the maven command line:
+
+           -Dcommon.protobuf2.scope=compile
+
+    If this is done then protobuf-2.5.0.jar will again be exported as a
+    hadoop-common dependency, and included in the share/hadoop/common/lib/
+    directory of any Hadoop distribution built.
+
 ----------------------------------------------------------------------------------
 Building components separately
 

LICENSE-binary

@@ -397,7 +397,6 @@ hadoop-tools/hadoop-sls/src/main/html/js/thirdparty/d3.v3.js
 hadoop-hdfs-project/hadoop-hdfs/src/main/webapps/static/d3-v4.1.1.min.js
 leveldb v1.13
 
-com.google.protobuf:protobuf-java:2.5.0
 com.google.protobuf:protobuf-java:3.6.1
 com.google.protobuf:protobuf-java:3.7.1
 com.google.re2j:re2j:1.1

hadoop-project/pom.xml

@@ -90,8 +90,8 @@
     <!-- This is used in hadoop-common for compilation only -->
     <protobuf.version>2.5.0</protobuf.version>
     <!-- Protobuf scope in hadoop common -->
-    <!-- set to "provided" and protobuf2 will no longer be exported as a dependency  -->
-    <common.protobuf2.scope>compile</common.protobuf2.scope>
+    <!-- set to "provided" so protobuf2 is no longer exported as a dependency  -->
+    <common.protobuf2.scope>provided</common.protobuf2.scope>
     <!-- Protobuf scope in other modules which explicitly import the libarary -->
     <transient.protobuf2.scope>${common.protobuf2.scope}</transient.protobuf2.scope>
     <!-- ProtocolBuffer version, actually used in Hadoop -->