configure discards a user-supplied BASH_SHELL

configure.ac

@@ -57,7 +57,8 @@ LT_INIT
 
 # A real bash, for "make deb" and the test harness. Not searched into BASH: bash presets
 # that to its own path, and macOS /bin/sh is a bash, so the macro never searches (#895).
-AS_UNSET([BASH_SHELL])
+# BASH_SHELL isn't preset the way BASH is, so AC_ARG_VAR needs no guard.
+AC_ARG_VAR([BASH_SHELL], [path to a real (non-POSIX-mode) bash])
 AC_PATH_PROGS([BASH_SHELL], [bash], [/bin/bash])
 
 # Export LD_LIBRARY_PATH name or equivalent.

tests/146_bash-shell.test

@@ -43,4 +43,48 @@ test "$out" = procsub || {
     exit 1
 }
 
+# Whatever configure picks, an absolute BASH_SHELL from the user must win over the search.
+configure="${abs_top_srcdir:-}/configure"
+test -r "$configure" || {
+    echo "no configure script at $configure; tests/Makefile.am must export abs_top_srcdir" >&2
+    exit 1
+}
+
+help=$(bash "$configure" --help)
+grep -q '^  *BASH_SHELL ' <<<"$help" || {
+    echo "configure --help does not advertise BASH_SHELL (AC_ARG_VAR missing)" >&2
+    exit 1
+}
+
+tmp=$(mktemp -d)
+trap 'set +e; rm -rf "$tmp"' EXIT
+mkdir "$tmp/src" "$tmp/bld"
+ln -s "$sh" "$tmp/mybash"
+
+# Symlink farm, not the real srcdir: an in-tree config.status makes autoconf refuse it.
+for f in "$abs_top_srcdir"/*; do
+    case "${f##*/}" in
+    config.status | config.log | config.h | stamp-h1 | Makefile) continue ;;
+    esac
+    ln -s "$f" "$tmp/src/"
+done
+
+# Prefer the Makefile: it proves the value that reaches $(BASH_SHELL), not just the macro's
+# decision. configure may die on a library check before writing one, so fall back to the
+# trace it printed earlier.
+(cd "$tmp/bld" && BASH_SHELL="$tmp/mybash" bash "$tmp/src/configure" --disable-https >conf.log 2>&1) || true
+if test -f "$tmp/bld/Makefile"; then
+    got=$(sed -n 's/^BASH_SHELL = //p' "$tmp/bld/Makefile")
+    from="Makefile"
+else
+    got=$(sed -n 's/^checking for bash\.\.\. //p' "$tmp/bld/conf.log")
+    got=${got#"(cached) "}
+    from="trace"
+fi
+test "$got" = "$tmp/mybash" || {
+    echo "configure discarded BASH_SHELL=$tmp/mybash, $from has: ${got:-<nothing>}" >&2
+    tail -20 "$tmp/bld/conf.log" >&2
+    exit 1
+}
+
 echo "configured bash is $sh ($version)"

tests/Makefile.am

@@ -27,6 +27,7 @@ TESTS_ENVIRONMENT += V6_SUPPORT=$(V6_SUPPORT)
 # Asserted by 146_bash-shell.test.
 TESTS_ENVIRONMENT += BASH_SHELL=$(BASH_SHELL)
 TESTS_ENVIRONMENT += top_srcdir=$(top_srcdir)
+TESTS_ENVIRONMENT += abs_top_srcdir=$(abs_top_srcdir)
 TESTS_ENVIRONMENT += abs_top_builddir=$(abs_top_builddir)
 TESTS_ENVIRONMENT += CONFIGURED_DATADIR=$(datadir)
 TESTS_ENVIRONMENT += RENAMEFAIL_LA=$(abs_builddir)/librenamefail.la