[Libguestfs] [PATCH nbdkit 1/3] sh: Implement inline scripts.

[Richard W.M. Jones]

On Sat, Dec 15, 2018 at 07:22:41AM +0000, Richard W.M. Jones wrote:
> On Fri, Dec 14, 2018 at 04:31:32PM -0600, Eric Blake wrote:
> > On 12/14/18 4:16 PM, Richard W.M. Jones wrote:
> > >This implements something like a readonly 1MB disk reading as zeroes:
> > >
> > >nbdkit sh script=- <<'EOF'
> > >   case "$1" in
> > >     get_size) echo 1M ;;
> > >     pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
> > >     *) exit 2 ;;
> > >   esac
> > >EOF
> > >
> > >Use of "-" is analogous to reading passwords from stdin.
> > >---
> > 
> > >@@ -26,6 +30,19 @@ like this:
> > >  You may have to add further C<key=value> arguments to the command
> > >  line.
> > >+=head2 Inline shell scripts
> > >+
> > >+It is also possible to write a shell script plugin "inline" using C<->
> > >+as the name of the script, like this:
> > >+
> > >+ nbdkit sh script=- <<'EOF'
> > >+   case "$1" in
> > >+     get_size) echo 1M ;;
> > >+     pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
> > >+     *) exit 2 ;;
> > >+   esac
> > >+ EOF
> > >+
> > 
> > Worth a specific mention that 'script=' is required, since otherwise
> > the '-' looks like an option to nbdkit proper rather than a script
> > parameter?
> 
> Actually I tried using just "-" (for the first time, I didn't try it
> before) and it works.  So the above can be written:
> 
>  nbdkit sh - <<'EOF'
>    case "$1" in
>      get_size) echo 1M ;;
>      pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
>      *) exit 2 ;;
>    esac
>  EOF
> 
> At least on Linux.  I wonder if that also works on FreeBSD etc and if
> it depends on an implementation detail of the parser.  I'll have to do
> some more investigations.

This works on FreeBSD, and looking at the way that coreutils cat(1) is
implemented (as an example of presumably portable code) it also simply
assumes that "-" is parsed as a non-option.

I'm going to rework this, along with the other review comments you
made.

Thanks,

Rich.

> Rich.
> 
> 
> > Probably not worth a mention that a local file named '-' can still
> > be invoked via [script=]./- (there, script is optional because the
> > leading ./ avoids the leading -).
> > 
> > >+static char *
> > >+inline_script (void)
> > >+{
> > >+  const char scriptname[] = "inline-script.sh";
> > >+  char *filename = NULL;
> > >+  char *cmd = NULL;
> > >+
> > >+  if (asprintf (&filename, "%s/%s", tmpdir, scriptname) == -1) {
> > >+    nbdkit_error ("asprintf: %m");
> > >+    goto err;
> > >+  }
> > >+
> > >+  /* Safe because both the tmpdir and script name are controlled by us
> > >+   * and don't contain characters that need quoting.
> > >+   */
> > >+  if (asprintf (&cmd, "cat > %s", filename) == -1) {
> > 
> > Useful comment :) And I verified it is true.
> > 
> > Looks good.
> > 
> > -- 
> > Eric Blake, Principal Software Engineer
> > Red Hat, Inc.           +1-919-301-3266
> > Virtualization:  qemu.org | libvirt.org
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
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-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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