The ./configure script should produce --help output even if Python is
not installed.

Listing trace backends is simple: show the names of all Python modules
in scripts/tracetool/backend/ whose source code contains 'PUBLIC =
True'.

Perform the backend enumeration in shell instead of Python so that we
can move the Python check until after ./configure --help.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 configure | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/configure b/configure
index d1ce33b..52b6ab3 100755
--- a/configure
+++ b/configure
@@ -1186,21 +1186,6 @@ for opt do
   esac
 done
 
-if ! has $python; then
-  error_exit "Python not found. Use --python=/path/to/python"
-fi
-
-# Note that if the Python conditional here evaluates True we will exit
-# with status 1 which is a shell 'false' value.
-if ! $python -c 'import sys; sys.exit(sys.version_info < (2,6) or sys.version_info >= (3,))'; then
-  error_exit "Cannot use '$python', Python 2.6 or later is required." \
-      "Note that Python 3 or later is not yet supported." \
-      "Use --python=/path/to/python to specify a supported Python."
-fi
-
-# Suppress writing compiled files
-python="$python -B"
-
 case "$cpu" in
     ppc)
            CPU_CFLAGS="-m32"
@@ -1328,7 +1313,7 @@ Advanced options (experts only):
                            set block driver read-only whitelist
                            (affects only QEMU, not qemu-img)
   --enable-trace-backends=B Set trace backend
-                           Available backends: $($python $source_path/scripts/tracetool.py --list-backends)
+                           Available backends: $(grep -le '^PUBLIC\s*=\s*True$' scripts/tracetool/backend/*.py | sed -e 's/^.*\/\(.*\)\.py$/\1/' | xargs echo)
   --with-trace-file=NAME   Full PATH,NAME of file to store traces
                            Default:trace-<pid>
   --disable-slirp          disable SLIRP userspace network connectivity
@@ -1428,6 +1413,21 @@ EOF
 exit 0
 fi
 
+if ! has $python; then
+  error_exit "Python not found. Use --python=/path/to/python"
+fi
+
+# Note that if the Python conditional here evaluates True we will exit
+# with status 1 which is a shell 'false' value.
+if ! $python -c 'import sys; sys.exit(sys.version_info < (2,6) or sys.version_info >= (3,))'; then
+  error_exit "Cannot use '$python', Python 2.6 or later is required." \
+      "Note that Python 3 or later is not yet supported." \
+      "Use --python=/path/to/python to specify a supported Python."
+fi
+
+# Suppress writing compiled files
+python="$python -B"
+
 # Now we have handled --enable-tcg-interpreter and know we're not just
 # printing the help message, bail out if the host CPU isn't supported.
 if test "$ARCH" = "unknown"; then
-- 
2.9.3

On 28 March 2017 at 14:08, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> The ./configure script should produce --help output even if Python is
> not installed.
>
> Listing trace backends is simple: show the names of all Python modules
> in scripts/tracetool/backend/ whose source code contains 'PUBLIC =
> True'.
>
> Perform the backend enumeration in shell instead of Python so that we
> can move the Python check until after ./configure --help.
>
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Thanks. I think we should postpone this to 2.10 since we've had
this behaviour since forever, but it's a useful start to getting
our --help text into a less confusing place in configure.

> @@ -1328,7 +1313,7 @@ Advanced options (experts only):
>                             set block driver read-only whitelist
>                             (affects only QEMU, not qemu-img)
>    --enable-trace-backends=B Set trace backend
> -                           Available backends: $($python $source_path/scripts/tracetool.py --list-backends)
> +                           Available backends: $(grep -le '^PUBLIC\s*=\s*True$' scripts/tracetool/backend/*.py | sed -e 's/^.*\/\(.*\)\.py$/\1/' | xargs echo)


'\s' doesn't work with NetBSD grep (it isn't POSIX). I think you need
   '^PUBLIC[[:space:]]*=[[:space:]]True'

(or just use ' ' since we don't have to contend with tabs here, as
you suggested on IRC).

I think we should also move the determining of the list of backends
out of the --help text itself, so set an avail_trace_backends
variable first and then just substitute the variable here.

thanks
-- PMM