Fix #502: Capture Maven stderr in error messages (#822)

Maven error messages were being discarded because stderr was not captured.
Now stderr is properly captured and displayed when Maven builds fail.

Changes:
- Modified build() and copy_dependency() to use stderr in error messages
- Updated tests to verify stderr is captured correctly

Author: sriram-mv

aws_lambda_builders/workflows/java_maven/maven.py

@@ -28,21 +28,21 @@ def __init__(self, maven_binary, os_utils=None):
 
     def build(self, scratch_dir):
         args = ["clean", "install"]
-        ret_code, stdout, _ = self._run(args, scratch_dir)
+        ret_code, stdout, stderr = self._run(args, scratch_dir)
 
         LOG.debug("Maven logs: %s", decode(stdout))
 
         if ret_code != 0:
-            raise MavenExecutionError(message=decode(stdout))
+            raise MavenExecutionError(message=decode(stderr))
 
     def copy_dependency(self, scratch_dir):
         include_scope = "runtime"
         LOG.debug("Running copy_dependency with scope: %s", include_scope)
         args = ["dependency:copy-dependencies", f"-DincludeScope={include_scope}", "-Dmdep.prependGroupId=true"]
-        ret_code, stdout, _ = self._run(args, scratch_dir)
+        ret_code, stdout, stderr = self._run(args, scratch_dir)
 
         if ret_code != 0:
-            raise MavenExecutionError(message=decode(stdout))
+            raise MavenExecutionError(message=decode(stderr))
 
     def _run(self, args, cwd=None):
         p = self.os_utils.popen(

tests/unit/workflows/java_maven/test_maven.py

@@ -50,12 +50,12 @@ def test_build_project(self):
         )
 
     def test_build_raises_exception_if_retcode_not_0(self):
-        self.popen = FakePopen(retcode=1, out=b"Some Error Message")
+        self.popen = FakePopen(retcode=1, err=b"Maven Error from stderr")
         self.os_utils.popen.side_effect = [self.popen]
         maven = SubprocessMaven(maven_binary=self.maven_binary, os_utils=self.os_utils)
         with self.assertRaises(MavenExecutionError) as err:
             maven.build(self.source_dir)
-        self.assertEqual(err.exception.args[0], "Maven Failed: Some Error Message")
+        self.assertEqual(err.exception.args[0], "Maven Failed: Maven Error from stderr")
 
     def test_copy_dependency(self):
         maven = SubprocessMaven(maven_binary=self.maven_binary, os_utils=self.os_utils)
@@ -68,9 +68,9 @@ def test_copy_dependency(self):
         )
 
     def test_copy_dependency_raises_exception_if_retcode_not_0(self):
-        self.popen = FakePopen(retcode=1, out=b"Some Error Message")
+        self.popen = FakePopen(retcode=1, err=b"Maven Error from stderr")
         self.os_utils.popen.side_effect = [self.popen]
         maven = SubprocessMaven(maven_binary=self.maven_binary, os_utils=self.os_utils)
         with self.assertRaises(MavenExecutionError) as err:
             maven.copy_dependency(self.source_dir)
-        self.assertEqual(err.exception.args[0], "Maven Failed: Some Error Message")
+        self.assertEqual(err.exception.args[0], "Maven Failed: Maven Error from stderr")