launcher: support Python 3.5 for now

The codebase still supports Python 3.5, so allow use of that instead
of requiring Python 3.6+.  Supporting this mode well is a bit tricky
as we want to first scan for newer versions before falling back to
older ones.  And we have to avoid infinite loops in the process.

Bug: https://crbug.com/gerrit/13795
Change-Id: I47949a173899bfa9ab20d3fefa1a97bf002659f6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/292442
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

repo

@@ -32,6 +32,13 @@ import subprocess
 import sys
 
 
+# These should never be newer than the main.py version since this needs to be a
+# bit more flexible with older systems.  See that file for more details on the
+# versions we select.
+MIN_PYTHON_VERSION_SOFT = (3, 6)
+MIN_PYTHON_VERSION_HARD = (3, 5)
+
+
 # Keep basic logic in sync with repo_trace.py.
 class Trace(object):
   """Trace helper logic."""
@@ -70,8 +77,6 @@ def check_python_version():
   def reexec(prog):
     exec_command([prog] + sys.argv)
 
-  MIN_PYTHON_VERSION = (3, 6)
-
   ver = sys.version_info
   major = ver.major
   minor = ver.minor
@@ -80,17 +85,24 @@ def check_python_version():
   if (major, minor) < (2, 7):
     print('repo: error: Your Python version is too old. '
           'Please use Python {}.{} or newer instead.'.format(
-              *MIN_PYTHON_VERSION), file=sys.stderr)
+              *MIN_PYTHON_VERSION_SOFT), file=sys.stderr)
     sys.exit(1)
 
   # Try to re-exec the version specific Python 3 if needed.
-  if (major, minor) < MIN_PYTHON_VERSION:
+  if (major, minor) < MIN_PYTHON_VERSION_SOFT:
     # Python makes releases ~once a year, so try our min version +10 to help
     # bridge the gap.  This is the fallback anyways so perf isn't critical.
-    min_major, min_minor = MIN_PYTHON_VERSION
+    min_major, min_minor = MIN_PYTHON_VERSION_SOFT
     for inc in range(0, 10):
       reexec('python{}.{}'.format(min_major, min_minor + inc))
 
+    # Fallback to older versions if possible.
+    for inc in range(MIN_PYTHON_VERSION_SOFT[1] - MIN_PYTHON_VERSION_HARD[1], 0, -1):
+      # Don't downgrade, and don't reexec ourselves (which would infinite loop).
+      if (min_major, min_minor - inc) <= (major, minor):
+        break
+      reexec('python{}.{}'.format(min_major, min_minor - inc))
+
     # Try the generic Python 3 wrapper, but only if it's new enough.  We don't
     # want to go from (still supported) Python 2.7 to (unsupported) Python 3.5.
     try:
@@ -103,18 +115,19 @@ def check_python_version():
     except (OSError, subprocess.CalledProcessError):
       python3_ver = None
 
-    # The python3 version looks like it's new enough, so give it a try.
-    if python3_ver and python3_ver >= MIN_PYTHON_VERSION:
+    # If the python3 version looks like it's new enough, give it a try.
+    if (python3_ver and python3_ver >= MIN_PYTHON_VERSION_HARD
+        and python3_ver != (major, minor)):
       reexec('python3')
 
     # We're still here, so diagnose things for the user.
     if major < 3:
       print('repo: warning: Python 2 is no longer supported; '
-            'Please upgrade to Python {}.{}+.'.format(*MIN_PYTHON_VERSION),
+            'Please upgrade to Python {}.{}+.'.format(*MIN_PYTHON_VERSION_HARD),
             file=sys.stderr)
-    else:
+    elif (major, minor) < MIN_PYTHON_VERSION_HARD:
       print('repo: error: Python 3 version is too old; '
-            'Please use Python {}.{} or newer.'.format(*MIN_PYTHON_VERSION),
+            'Please use Python {}.{} or newer.'.format(*MIN_PYTHON_VERSION_HARD),
             file=sys.stderr)
       sys.exit(1)