sharness: add function to check for COLUMNS usability

Problem: With some versions of bash (likely >= 5.0), the COLUMNS shell
variable, when overridden in the environment for testing purposes, is
not preserved across invocation of the flux(1) command driver during
sharness tests. This appears to occur because COLUMNS is reset by
the libtool wrapper script (which has the shebang line #!/bin/bash),
and only seems to occur if the sharness test is run with `-d -v`
for unknown reason.

Add a function to sharness.d/flux-sharness.sh that checks if COLUMNS
can be preserved in the testsuite environment across an invocation of
`flux(1)`. This can be used by tests that require this functionality
to set a prereq, allowing them to skip the tests if necessary instead
of erroneously failing.

Author: grondo

t/sharness.d/flux-sharness.sh

@@ -292,6 +292,23 @@ test_on_rank() {
     flux exec --rank=${ranks} "$@"
 }
 
+#  Note: Some versions of bash may cause the `flux` libtool wrapper script
+#  to reset the COLUMNS shell variable even if it is explicitly set for
+#  for testing purposes. This causes tests that check for output truncation
+#  based on COLUMNS to erroneously fail. (Note: this only seems to be the
+#  case when tests are run with --debug --verbose for unknown reasons.
+#
+#  Add a script for tests that use COLUMNS to check if the variable will
+#  be preserved across an invovation of flux(1) so they may set a prereq
+#  and skip tests that might erroneous fail if COLUMNS is not preserved.
+#
+test_columns_variable_preserved() {
+	local cols=$(COLUMNS=12 \
+	             flux python -c \
+	             "import shutil; print(shutil.get_terminal_size().columns)")
+	test "$cols" = "12"
+}
+
 #  Export a shorter name for this test
 TEST_NAME=$SHARNESS_TEST_NAME
 export TEST_NAME