Truncate stdout and stderr in upstream test log

Truncate stdout and stderr in the way that it hopefully shows all
important information. At least 15 lines of stdout (Ansible tasks) and
remaining from stderr to fill up to maxlen size.

The test log mostly only showed lines from stderr as the log was a
concatenation of stdout and stderr and only the last 2500 chars of this
have been used. This was hiding the test result from stdout and failures
have not beeen visible.

tests/utils.py

@@ -162,6 +162,22 @@ def _run_playbook(playbook):
     return process
 
 
+def _truncate(lines, charcount, minlines=0):
+    output = ""
+    line_count = 1
+    for i in range(len(lines) - 1, -1, -1):
+        if len(output) + len(lines[i]) + 1 <= charcount or \
+           line_count < minlines:
+            output = lines[i] + "\n" + output
+            line_count += 1
+        else:
+            remaining = charcount - len(output) - 1 - 4
+            if remaining > 60:
+                output = "... " + lines[i][-(remaining):] + "\n" + output
+            break
+    return output
+
+
 def run_playbook(playbook, allow_failures=False):
     """
     Run an Ansible playbook and assert the return code.
@@ -183,13 +199,26 @@ def run_playbook(playbook, allow_failures=False):
     status_code_msg = "ansible-playbook return code: {0}".format(
         result.returncode
     )
+    _stdout = result.stdout.decode("utf8")
+    _stderr = result.stderr.decode("utf8")
+    # Truncate stdout and stderr in the way that it hopefully
+    # shows all important information. At least 15 lines of stdout
+    # (Ansible tasks) and remaining from stderr to fill up to
+    # maxlen size.
+    maxlen = 2000
+    factor = maxlen / (len(_stdout) + len(_stderr))
+    stdout = _truncate(_stdout.splitlines(),
+                       int(factor * len(_stdout)),
+                       minlines=15)
+    stderr = _truncate(_stderr.splitlines(), maxlen - len(stdout))
+
     assert_msg = "\n".join(
         [
             "",
             "-" * 30 + " Captured stdout " + "-" * 30,
-            result.stdout.decode("utf8"),
+            stdout,
             "-" * 30 + " Captured stderr " + "-" * 30,
-            result.stderr.decode("utf8"),
+            stderr,
             "-" * 30 + " Playbook Return Code " + "-" * 30,
             status_code_msg,
         ]