Libguestfs - Aug 2020 - [nbdkit PATCH] server: Permit - and _ in plugin names

Neither - nor _ need shell quoting, and as long as they are not the
first character, permitting them in a plugin or filter name may make
the command line easier to read.  A restriction against a leading
digit is new, but hopefully does not break any existing plugins.

Signed-off-by: Eric Blake <eblake@redhat.com>
---

It turns out I can't name a filter 'tls-fallback' without a slight
tweak.  Our code for is_short_name() already accepts such names, but
our documentation and backend_load did not.

I'm undecided on whether rejecting a plugin beginning with a digit is
a good idea; as written, this patch would forbid a plugin named '9p',
but it is a one-line tweak to allow it (ascii_isalnum instead of
ascii_isalpha).

 docs/nbdkit-filter.pod |  3 ++-
 docs/nbdkit-plugin.pod |  3 ++-
 server/backend.c       | 10 ++++++++--
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
index 3c7527d4..361a70f1 100644
--- a/docs/nbdkit-filter.pod
+++ b/docs/nbdkit-filter.pod
@@ -201,7 +201,8 @@ methods.
  const char *name;

 This field (a string) is required, and B<must> contain only ASCII
-alphanumeric characters and be unique amongst all filters.
+alphanumeric characters as well as dash or underscore, must begin with
+a letter, and be unique amongst all filters.

 =head2 C<.longname>

diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
index 3d573b33..c961c116 100644
--- a/docs/nbdkit-plugin.pod
+++ b/docs/nbdkit-plugin.pod
@@ -471,7 +471,8 @@ an indication of failure.  It has the following prototype:
  const char *name;

 This field (a string) is required, and B<must> contain only ASCII
-alphanumeric characters and be unique amongst all plugins.
+alphanumeric characters as well as dash or underscore, must begin with
+a letter, and be unique amongst all filters.

 =head2 C<.version>

diff --git a/server/backend.c b/server/backend.c
index 8f4fed9d..046aacb4 100644
--- a/server/backend.c
+++ b/server/backend.c
@@ -98,10 +98,16 @@ backend_load (struct backend *b, const char *name, void
(*load) (void))
              program_name, b->filename, b->type);
     exit (EXIT_FAILURE);
   }
-  for (i = 0; i < len; ++i) {
+  if (! ascii_isalpha (*name)) {
+    fprintf (stderr,
+             "%s: %s: %s.name ('%s') field must begin with an
ASCII letter\n",
+             program_name, b->filename, b->type, name);
+    exit (EXIT_FAILURE);
+  }
+  for (i = 1; i < len; ++i) {
     unsigned char c = name[i];

-    if (! ascii_isalnum (c)) {
+    if (! ascii_isalnum (c) && c != '-' && c !=
'_') {
       fprintf (stderr,
                "%s: %s: %s.name ('%s') field "
                "must contain only ASCII alphanumeric characters\n",
-- 
2.28.0