implement BORG_SELFTEST env variable, fixes #5871

on my laptop, this saved ~100ms cpu time (sys+user) per borg command invocation.

docs/usage/general/environment.rst.inc

@@ -68,6 +68,14 @@ General:
         When set to no (default: yes), system information (like OS, Python version, ...) in
         exceptions is not shown.
         Please only use for good reasons as it makes issues harder to analyze.
+    BORG_SELFTEST
+        This can be used to influence borg's builtin self-tests. The default is to execute the tests
+        at the beginning of each borg command invocation.
+
+        BORG_SELFTEST=disabled can be used to switch off the tests and rather save some time.
+        Disabling is not recommended for normal borg users, but large scale borg storage providers can
+        use this to optimize production servers after at least doing a one-time test borg (with
+        selftests not disabled) when installing or upgrading machines / OS / borg.
     BORG_WORKAROUNDS
         A list of comma separated strings that trigger workarounds in borg,
         e.g. to work around bugs in other software.

src/borg/selftest.py

@@ -12,8 +12,7 @@ To assert that self test discovery works correctly the number of tests is kept i
 variable. SELFTEST_COUNT must be updated if new tests are added or removed to or from any of the tests
 used here.
 """
-
-
+import os
 import sys
 import time
 from unittest import TestResult, TestSuite, defaultTestLoader
@@ -56,6 +55,9 @@ class SelfTestResult(TestResult):
 
 
 def selftest(logger):
+    if os.environ.get('BORG_SELFTEST') == 'disabled':
+        logger.debug("borg selftest disabled via BORG_SELFTEST env variable")
+        return
     selftest_started = time.perf_counter()
     result = SelfTestResult()
     test_suite = TestSuite()

src/borg/testsuite/archiver.py

@@ -251,6 +251,7 @@ class ArchiverTestCaseBase(BaseTestCase):
         os.environ['BORG_CHECK_I_KNOW_WHAT_I_AM_DOING'] = 'YES'
         os.environ['BORG_DELETE_I_KNOW_WHAT_I_AM_DOING'] = 'YES'
         os.environ['BORG_PASSPHRASE'] = 'waytooeasyonlyfortests'
+        os.environ['BORG_SELFTEST'] = 'disabled'
         self.archiver = not self.FORK_DEFAULT and Archiver() or None
         self.tmpdir = tempfile.mkdtemp()
         self.repository_path = os.path.join(self.tmpdir, 'repository')