gdb/testsuite: update expected results in gdb.python/py-startup-opt.exp

The test gdb.python/py-startup-opt.exp checks the behaviour of GDB's:

  set python dont-write-bytecode on

This flag (when on) stops Python creating .pyc files.  The test first
checks that .pyc files will be created, then turns this option on and
checks .pyc files will not be created.

However, if the user has PYTHONDONTWRITEBYTECODE set in their
environment then this will prevent Python from creating .pyc files, as
such the first test, that .pyc files are being created, currently
fails.

We could unset PYTHONDONTWRITEBYTECODE, however, until Python 3.8
there is no way to control where Python writes the .pyc files.  As the
GDB developer clearly doesn't want .pyc files created in their
file-system it feels wrong to silently unset this environment
variable.

My proposal then, is that we just spot when this environment variable
is set and adjust the expected results.  My hope is that across all
GDB developers some will be running with PYTHONDONTWRITEBYTECODE
unset, so this feature will be fully tested at least some of the time.

gdb/testsuite/ChangeLog:

	PR testsuite/27788
	* gdb.python/py-startup-opt.exp (test_python_settings): Change the
	expected results when environment variable PYTHONDONTWRITEBYTECODE
	is set.

2 files changed, 36 insertions, 1 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index be39527..16c0df1 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2021-05-03  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	PR testsuite/27788
+	* gdb.python/py-startup-opt.exp (test_python_settings): Change the
+	expected results when environment variable PYTHONDONTWRITEBYTECODE
+	is set.
+
 2021-04-30  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.ada/enum_idx_packed/pck.ads (My_Enum, My_Array_Type)
diff --git a/gdb/testsuite/gdb.python/py-startup-opt.exp b/gdb/testsuite/gdb.python/py-startup-opt.exp
index 842add3..f0a735b 100644
--- a/gdb/testsuite/gdb.python/py-startup-opt.exp
+++ b/gdb/testsuite/gdb.python/py-startup-opt.exp
@@ -62,13 +62,41 @@ proc test_python_settings { exp_state } {
     gdb_test_no_output "python import sys"
 
     foreach_with_prefix attr {ignore_environment dont_write_bytecode} {
+
+	# If we are checking 'dont_write_bytecode', and we are
+	# expecting this attribute to be 'off', then, if the user has
+	# PYTHONDONTWRITEBYTECODE set in their environment, the result
+	# will be 'on' instead of 'off', so override the expected
+	# result here.
+	#
+	# The reason for this is, 'set python dont-write-bytecode' by
+	# default is set to 'auto', which means, so long as 'set
+	# python ignore-environment' is 'off', GDB will check for the
+	# above environment variable.
+	#
+	# We could unset the environment variable, but until Python
+	# 3.8 there was no way to control where .pyc files are placed,
+	# and it feels bad to cause .pyc files to be created within
+	# the users filesystem when they clearly don't want them.
+	#
+	# And so, we adjust the expected results.  Hopefully, between
+	# all GDB developers some will test GDB with this environment
+	# variable unset.
+	if { $attr == "dont_write_bytecode" \
+		 && $exp_state == "off"
+		 && [info exists ::env(PYTHONDONTWRITEBYTECODE)] } {
+	    set answer "on"
+	} else {
+	    set answer $exp_state
+	}
+
 	gdb_test_multiline "testname" \
 	    "python" "" \
 	    "if hasattr(sys, 'flags') and getattr(sys.flags, '${attr}', False):" "" \
 	    "  print (\"${attr} is on\")" "" \
 	    "else:" "" \
 	    "  print (\"${attr} is off\")" "" \
-	    "end" "${attr} is ${exp_state}"
+	    "end" "${attr} is ${answer}"
     }
 
     gdb_exit