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Eric Blake

2020-Aug-24 12:52 UTC

[Libguestfs] [RFC nbdkit PATCH] protocol: Alter .list_exports, add .default_export

I'm about to add an 'exportname' filter, and in the process, I
noticed a few shortcomings in our API.  Time to fix those before
the 1.22 release locks our API in stone.  First, .list_exports
needs to know if it is pre- or post-TLS, as that may affect which
names are exported.  Next, overloading .list_exports to do both
NBD_OPT_LIST and mapping "" to a canonical name is proving to be
awkward; the canonical mapping is only needed during an
NBD_INFO_NAME response to NBD_OPT_GO, and making .open try to
grab the entire .list_exports list just to use only the first
entry (even if the plugin optimized based on the bool to only
provide one entry) is awkward, compared to just having a
dedicated function.  Finally, as long as we are going to support
NBD_INFO_NAME, we can also support NBD_INFO_DESCRIPTION; but
while we map "" to a canonical name prior to calling .open,
getting the description makes more sense after the connection
is established, alongside .get_size.
---

I obviously need to finish the code to go with this, but here's where
I would like to see the API before we finalize the 1.22 release.

 docs/nbdkit-filter.pod | 41 +++++++++++++++++++----
 docs/nbdkit-plugin.pod | 74 ++++++++++++++++++++++++++++++++++++------
 2 files changed, 98 insertions(+), 17 deletions(-)

diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
index bf01db14..f15bdd96 100644
--- a/docs/nbdkit-filter.pod
+++ b/docs/nbdkit-filter.pod
@@ -339,12 +339,18 @@ an error message and return C<-1>.
 =head2 C<.list_exports>

  int (*list_exports) (nbdkit_next_list_exports *next, void *nxdata,
-                      int readonly, int default_only,
+                      int readonly, int is_tls,
                       struct nbdkit_exports *exports);

 This intercepts the plugin C<.list_exports> method and can be used to
 filter which exports are advertised.

+The C<readonly> parameter matches what is passed to <.preconnect>
and
+C<.open>, and may be changed by the filter when calling into the
+plugin.  The C<is_tls> parameter informs the filter whether TLS
+negotiation has been completed by the client, but is not passed on to
+C<next> because it cannot be altered.
+
 It is possible for filters to transform the exports list received back
 from the layer below.  Without error checking it would look like this:

@@ -355,8 +361,8 @@ from the layer below.  Without error checking it would look
like this:
    struct nbdkit_export e;
    char *name, *desc;

-   exports2 = nbdkit_exports_new (default_only);
-   next_list_exports (nxdata, readonly, default_only, exports);
+   exports2 = nbdkit_exports_new ();
+   next_list_exports (nxdata, readonly, exports);
    for (i = 0; i < nbdkit_exports_count (exports2); ++i) {
      e = nbdkit_get_export (exports2, i);
      name = adjust (e.name);
@@ -376,11 +382,9 @@ an error message and return C<-1>.
 Two functions are provided to filters only for allocating and freeing
 the list:

- struct nbdkit_exports *nbdkit_exports_new (int default_only);
+ struct nbdkit_exports *nbdkit_exports_new (void);

-Allocates and returns a new, empty exports list.  The C<default_only>
-parameter should match whether the list is intended to grab the
-canonical name of the default export, or all exports.
+Allocates and returns a new, empty exports list.

 On error this function can return C<NULL>.  In this case it calls
 C<nbdkit_error> as required.  C<errno> will be set to a suitable
@@ -408,6 +412,20 @@ Returns the number of exports in the list.

 Returns a copy of the C<i>'th export.

+=head2 C<.default_export>
+
+ const char *default_export (nbdkit_next_default_export *next, void *nxdata,
+                             int readonly, int is_tls)
+
+This intercepts the plugin C<.default_export> method and can be used to
+alter the canonical export name used in place of the default
C<"">.
+
+The C<readonly> parameter matches what is passed to <.preconnect>
and
+C<.open>, and may be changed by the filter when calling into the
+plugin.  The C<is_tls> parameter informs the filter whether TLS
+negotiation has been completed by the client, but is not passed on to
+C<next> because it cannot be altered.
+
 =head2 C<.open>

  void * (*open) (nbdkit_next_open *next, void *nxdata,
@@ -527,6 +545,15 @@ This function is only called once per connection and cached
by nbdkit.
 Similarly, repeated calls to C<next_ops-E<gt>get_size> will return
a
 cached value.

+=head2 C<.export_description>
+
+ const char *export_description (struct nbdkit_next_ops *next_ops,
+                                 void *nxdata, void *handle);
+
+This intercepts the plugin C<.export_description> method and can be
+used to read or modify the export description that the NBD client
+will see.
+
 =head2 C<.can_write>

 =head2 C<.can_flush>
diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
index 5c641e83..ecb6bba8 100644
--- a/docs/nbdkit-plugin.pod
+++ b/docs/nbdkit-plugin.pod
@@ -245,6 +245,12 @@ Early in option negotiation the client may try to list the
exports
 served by the plugin, and plugins can optionally implement this
 callback to answer the client.  See L</EXPORT NAME> below.

+=item C<.default_export>
+
+During option negotiation, if the client requests the default export
+name (C<"">), this optional callback provides a canonical name
to
+use in its place prior to calling C<.open>.
+
 =item C<.open>

 A new client has connected and finished the NBD handshake.  TLS
@@ -664,15 +670,16 @@ error message and return C<-1>.

 =head2 C<.list_exports>

- int list_exports (int readonly, int default_only,
+ int list_exports (int readonly, int is_tls,
                    struct nbdkit_exports *exports);

 This optional callback is called if the client tries to list the
 exports served by the plugin (using C<NBD_OPT_LIST>).  If the plugin
-does not supply this callback then a single export called C<"">
is
-returned.  The NBD protocol defines C<""> as the default
export, so
-this is suitable for plugins which ignore the export name and always
-serve the same content.  See also L</EXPORT NAME> below.
+does not supply this callback then the reply is determined by the
+result of C<.default_export>, which in turn defaults to advertising
+the name C<"">.  The NBD protocol defines C<"">
as the default export,
+so this default is suitable for plugins which ignore the export name
+and always serve the same content.  See also L</EXPORT NAME> below.

 The C<readonly> flag informs the plugin that the server was started
 with the I<-r> flag on the command line, which is the same value
@@ -681,11 +688,11 @@ not yet have a way to let the client advertise an intent
to be
 read-only even when the server allows writes, so this parameter may
 not be as useful as it appears.

-If the C<default_only> flag is set then the client is querying for the
-name of the default export, and the plugin may optimize by adding only
-a single export to the returned list (the default export name, usually
-C<"">).  The plugin can ignore this flag and return all exports
if it
-wants.
+The C<is_tls> flag informs the plugin whether this listing was
+requested after the client has completed TLS negotiation.  When
+running the server in a mode that permits but not requires TLS,
+be careful that any exports listed when C<is_tls> is C<false> do
+not leak unintended information.

 The C<exports> parameter is an opaque object for collecting the list
 of exports.  Call C<nbdkit_add_export> to add a single export to the
@@ -709,6 +716,37 @@ Returning C<0> will send the list of exports back to
the client.  If
 there is an error, C<.list_exports> should call C<nbdkit_error>
with
 an error message and return C<-1>.

+=head2 C<.default_export>
+
+ const char *default_export (int readonly, int is_tls);
+
+This optional callback is called if the client tries to connect to
+the default export C<"">.  If the plugin does not supply this
callback,
+the connection continues with the empty name; if the plugin returns
+a string, nbdkit returns that name to clients that support it (see
+the C<NBD_INFO_NAME> response to C<NBD_OPT_GO>), and behaves as if
+the client had passed that string instead of an empty name.  Similarly,
+if the plugin does not supply a C<.list_exports> callback, the result
+of this callback determines what export name to advertise to a client
+requesting an export list.
+
+The C<readonly> flag informs the plugin that the server was started
+with the I<-r> flag on the command line, which is the same value
+passed to C<.preconnect> and C<.open>.  However, the NBD protocol
does
+not yet have a way to let the client advertise an intent to be
+read-only even when the server allows writes, so this parameter may
+not be as useful as it appears.
+
+The C<is_tls> flag informs the plugin whether the canonical name for
+the default export is being requested after the client has completed
+TLS negotiation.  When running the server in a mode that permits but
+not requires TLS, be careful that a default export name does not leak
+unintended information.
+
+If the plugin returns C<NULL>, the client is not permitted to connect
+to the default export.  However, this is not an error in the protocol,
+so it is not necessary to call C<nbdkit_error>.
+
 =head2 C<.open>

  void *open (int readonly);
@@ -769,6 +807,22 @@ to get the size (in bytes) of the block device being
exported.
 The returned size must be E<ge> 0.  If there is an error,
C<.get_size>
 should call C<nbdkit_error> with an error message and return C<-1>.

+=head2 C<.export_description>
+
+ const char *export_description (void *handle);
+
+This is called during the option negotiation phase only if the
+client specifically requested an export description (see the
+C<NBD_INFO_DESCRIPTION> response to C<NBD_OPT_GO>).  Any
+description provided must be human-readable UTF-8, no longer
+than 4096 bytes.  Ideally, this description should match any
+description set during C<.list_exports>, but that is not
+enforced.
+
+If the plugin returns C<NULL>, or if this callback is omitted, no
+description is offered to the client.  As this is not an error in the
+protocol, it is not necessary to call C<nbdkit_error>.
+
 =head2 C<.can_write>

  int can_write (void *handle);
-- 
2.28.0

Richard W.M. Jones

2020-Aug-24 16:16 UTC

head link

Re: [Libguestfs] [RFC nbdkit PATCH] protocol: Alter .list_exports, add .default_export

On Mon, Aug 24, 2020 at 07:52:33AM -0500, Eric Blake
wrote:> I'm about to add an 'exportname' filter, and in the process, I
> noticed a few shortcomings in our API.  Time to fix those before
> the 1.22 release locks our API in stone.  First, .list_exports
> needs to know if it is pre- or post-TLS, as that may affect which
> names are exported.  Next, overloading .list_exports to do both
> NBD_OPT_LIST and mapping "" to a canonical name is proving to be
> awkward; the canonical mapping is only needed during an
> NBD_INFO_NAME response to NBD_OPT_GO, and making .open try to
> grab the entire .list_exports list just to use only the first
> entry (even if the plugin optimized based on the bool to only
> provide one entry) is awkward, compared to just having a
> dedicated function.  Finally, as long as we are going to support
> NBD_INFO_NAME, we can also support NBD_INFO_DESCRIPTION; but
> while we map "" to a canonical name prior to calling .open,
> getting the description makes more sense after the connection
> is established, alongside .get_size.
> ---
> 
> I obviously need to finish the code to go with this, but here's where
> I would like to see the API before we finalize the 1.22 release.
Yes, looks like a good change with a reasonable motive behind it.

Rich.
>  docs/nbdkit-filter.pod | 41 +++++++++++++++++++----
>  docs/nbdkit-plugin.pod | 74 ++++++++++++++++++++++++++++++++++++------
>  2 files changed, 98 insertions(+), 17 deletions(-)
> 
> diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
> index bf01db14..f15bdd96 100644
> --- a/docs/nbdkit-filter.pod
> +++ b/docs/nbdkit-filter.pod
> @@ -339,12 +339,18 @@ an error message and return C<-1>.
>  =head2 C<.list_exports>
> 
>   int (*list_exports) (nbdkit_next_list_exports *next, void *nxdata,
> -                      int readonly, int default_only,
> +                      int readonly, int is_tls,
>                        struct nbdkit_exports *exports);
> 
>  This intercepts the plugin C<.list_exports> method and can be used
to
>  filter which exports are advertised.
> 
> +The C<readonly> parameter matches what is passed to
<.preconnect> and
> +C<.open>, and may be changed by the filter when calling into the
> +plugin.  The C<is_tls> parameter informs the filter whether TLS
> +negotiation has been completed by the client, but is not passed on to
> +C<next> because it cannot be altered.
> +
>  It is possible for filters to transform the exports list received back
>  from the layer below.  Without error checking it would look like this:
> 
> @@ -355,8 +361,8 @@ from the layer below.  Without error checking it would
look like this:
>     struct nbdkit_export e;
>     char *name, *desc;
> 
> -   exports2 = nbdkit_exports_new (default_only);
> -   next_list_exports (nxdata, readonly, default_only, exports);
> +   exports2 = nbdkit_exports_new ();
> +   next_list_exports (nxdata, readonly, exports);
>     for (i = 0; i < nbdkit_exports_count (exports2); ++i) {
>       e = nbdkit_get_export (exports2, i);
>       name = adjust (e.name);
> @@ -376,11 +382,9 @@ an error message and return C<-1>.
>  Two functions are provided to filters only for allocating and freeing
>  the list:
> 
> - struct nbdkit_exports *nbdkit_exports_new (int default_only);
> + struct nbdkit_exports *nbdkit_exports_new (void);
> 
> -Allocates and returns a new, empty exports list.  The
C<default_only>
> -parameter should match whether the list is intended to grab the
> -canonical name of the default export, or all exports.
> +Allocates and returns a new, empty exports list.
> 
>  On error this function can return C<NULL>.  In this case it calls
>  C<nbdkit_error> as required.  C<errno> will be set to a
suitable
> @@ -408,6 +412,20 @@ Returns the number of exports in the list.
> 
>  Returns a copy of the C<i>'th export.
> 
> +=head2 C<.default_export>
> +
> + const char *default_export (nbdkit_next_default_export *next, void
*nxdata,
> +                             int readonly, int is_tls)
> +
> +This intercepts the plugin C<.default_export> method and can be used
to
> +alter the canonical export name used in place of the default
C<"">.
> +
> +The C<readonly> parameter matches what is passed to
<.preconnect> and
> +C<.open>, and may be changed by the filter when calling into the
> +plugin.  The C<is_tls> parameter informs the filter whether TLS
> +negotiation has been completed by the client, but is not passed on to
> +C<next> because it cannot be altered.
> +
>  =head2 C<.open>
> 
>   void * (*open) (nbdkit_next_open *next, void *nxdata,
> @@ -527,6 +545,15 @@ This function is only called once per connection and
cached by nbdkit.
>  Similarly, repeated calls to C<next_ops-E<gt>get_size> will
return a
>  cached value.
> 
> +=head2 C<.export_description>
> +
> + const char *export_description (struct nbdkit_next_ops *next_ops,
> +                                 void *nxdata, void *handle);
> +
> +This intercepts the plugin C<.export_description> method and can be
> +used to read or modify the export description that the NBD client
> +will see.
> +
>  =head2 C<.can_write>
> 
>  =head2 C<.can_flush>
> diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
> index 5c641e83..ecb6bba8 100644
> --- a/docs/nbdkit-plugin.pod
> +++ b/docs/nbdkit-plugin.pod
> @@ -245,6 +245,12 @@ Early in option negotiation the client may try to list
the exports
>  served by the plugin, and plugins can optionally implement this
>  callback to answer the client.  See L</EXPORT NAME> below.
> 
> +=item C<.default_export>
> +
> +During option negotiation, if the client requests the default export
> +name (C<"">), this optional callback provides a canonical
name to
> +use in its place prior to calling C<.open>.
> +
>  =item C<.open>
> 
>  A new client has connected and finished the NBD handshake.  TLS
> @@ -664,15 +670,16 @@ error message and return C<-1>.
> 
>  =head2 C<.list_exports>
> 
> - int list_exports (int readonly, int default_only,
> + int list_exports (int readonly, int is_tls,
>                     struct nbdkit_exports *exports);
> 
>  This optional callback is called if the client tries to list the
>  exports served by the plugin (using C<NBD_OPT_LIST>).  If the plugin
> -does not supply this callback then a single export called
C<""> is
> -returned.  The NBD protocol defines C<""> as the default
export, so
> -this is suitable for plugins which ignore the export name and always
> -serve the same content.  See also L</EXPORT NAME> below.
> +does not supply this callback then the reply is determined by the
> +result of C<.default_export>, which in turn defaults to advertising
> +the name C<"">.  The NBD protocol defines
C<""> as the default export,
> +so this default is suitable for plugins which ignore the export name
> +and always serve the same content.  See also L</EXPORT NAME> below.
> 
>  The C<readonly> flag informs the plugin that the server was started
>  with the I<-r> flag on the command line, which is the same value
> @@ -681,11 +688,11 @@ not yet have a way to let the client advertise an
intent to be
>  read-only even when the server allows writes, so this parameter may
>  not be as useful as it appears.
> 
> -If the C<default_only> flag is set then the client is querying for
the
> -name of the default export, and the plugin may optimize by adding only
> -a single export to the returned list (the default export name, usually
> -C<"">).  The plugin can ignore this flag and return all
exports if it
> -wants.
> +The C<is_tls> flag informs the plugin whether this listing was
> +requested after the client has completed TLS negotiation.  When
> +running the server in a mode that permits but not requires TLS,
> +be careful that any exports listed when C<is_tls> is C<false>
do
> +not leak unintended information.
> 
>  The C<exports> parameter is an opaque object for collecting the list
>  of exports.  Call C<nbdkit_add_export> to add a single export to the
> @@ -709,6 +716,37 @@ Returning C<0> will send the list of exports
back to the client.  If
>  there is an error, C<.list_exports> should call
C<nbdkit_error> with
>  an error message and return C<-1>.
> 
> +=head2 C<.default_export>
> +
> + const char *default_export (int readonly, int is_tls);
> +
> +This optional callback is called if the client tries to connect to
> +the default export C<"">.  If the plugin does not supply
this callback,
> +the connection continues with the empty name; if the plugin returns
> +a string, nbdkit returns that name to clients that support it (see
> +the C<NBD_INFO_NAME> response to C<NBD_OPT_GO>), and behaves
as if
> +the client had passed that string instead of an empty name.  Similarly,
> +if the plugin does not supply a C<.list_exports> callback, the
result
> +of this callback determines what export name to advertise to a client
> +requesting an export list.
> +
> +The C<readonly> flag informs the plugin that the server was started
> +with the I<-r> flag on the command line, which is the same value
> +passed to C<.preconnect> and C<.open>.  However, the NBD
protocol does
> +not yet have a way to let the client advertise an intent to be
> +read-only even when the server allows writes, so this parameter may
> +not be as useful as it appears.
> +
> +The C<is_tls> flag informs the plugin whether the canonical name for
> +the default export is being requested after the client has completed
> +TLS negotiation.  When running the server in a mode that permits but
> +not requires TLS, be careful that a default export name does not leak
> +unintended information.
> +
> +If the plugin returns C<NULL>, the client is not permitted to
connect
> +to the default export.  However, this is not an error in the protocol,
> +so it is not necessary to call C<nbdkit_error>.
> +
>  =head2 C<.open>
> 
>   void *open (int readonly);
> @@ -769,6 +807,22 @@ to get the size (in bytes) of the block device being
exported.
>  The returned size must be E<ge> 0.  If there is an error,
C<.get_size>
>  should call C<nbdkit_error> with an error message and return
C<-1>.
> 
> +=head2 C<.export_description>
> +
> + const char *export_description (void *handle);
> +
> +This is called during the option negotiation phase only if the
> +client specifically requested an export description (see the
> +C<NBD_INFO_DESCRIPTION> response to C<NBD_OPT_GO>).  Any
> +description provided must be human-readable UTF-8, no longer
> +than 4096 bytes.  Ideally, this description should match any
> +description set during C<.list_exports>, but that is not
> +enforced.
> +
> +If the plugin returns C<NULL>, or if this callback is omitted, no
> +description is offered to the client.  As this is not an error in the
> +protocol, it is not necessary to call C<nbdkit_error>.
> +
>  =head2 C<.can_write>
> 
>   int can_write (void *handle);
> -- 
> 2.28.0
> 
> _______________________________________________
> Libguestfs mailing list
> Libguestfs@redhat.com
> https://www.redhat.com/mailman/listinfo/libguestfs
-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW

Eric Blake

2020-Aug-24 22:02 UTC

head link

Re: [Libguestfs] [RFC nbdkit PATCH] protocol: Alter .list_exports, add .default_export

On 8/24/20 7:52 AM, Eric Blake wrote:> I'm about to add an 'exportname' filter, and in the process, I
> noticed a few shortcomings in our API.  Time to fix those before
> the 1.22 release locks our API in stone.  First, .list_exports
> needs to know if it is pre- or post-TLS, as that may affect which
> names are exported.  Next, overloading .list_exports to do both
> NBD_OPT_LIST and mapping "" to a canonical name is proving to be
> awkward; the canonical mapping is only needed during an
> NBD_INFO_NAME response to NBD_OPT_GO, and making .open try to
> grab the entire .list_exports list just to use only the first
> entry (even if the plugin optimized based on the bool to only
> provide one entry) is awkward, compared to just having a
> dedicated function.  Finally, as long as we are going to support
> NBD_INFO_NAME, we can also support NBD_INFO_DESCRIPTION; but
> while we map "" to a canonical name prior to calling .open,
> getting the description makes more sense after the connection
> is established, alongside .get_size.
> ---
> 
> I obviously need to finish the code to go with this, but here's where
> I would like to see the API before we finalize the 1.22 release.
> 
> +++ b/docs/nbdkit-plugin.pod
> +=head2 C<.default_export>
> +
> + const char *default_export (int readonly, int is_tls);
Oh fun.  For some plugins (like ondemand), this is trivial: return a 
compile-time constant string.  But for others (like sh and eval), 
there's a lifetime issue: this callback is used _before_ .open, ergo 
there is no handle struct that it can be associated with.  What's more, 
this is called _after_ .preconnect, which means it is logical to expect 
that the default export name might change over time (consider a plugin 
that advertises the largest file in a directory as its default, but 
where the directory can change _which_ file is largest between when the 
first client connects and when the second client connects).  And the 
string returned by the sh script is in malloc'd memory (by it's very 
nature of coming from the user script, rather than being a compile-time 
constant).  Without a handle to store this string in, we would have a 
memory leak: there is no way to associate this inside the handle's 
struct so that .close can reclaim it, but storing it globally is not 
thread-safe to parallel client connections.  So I'm thinking I need to 
add a helper function:

const char *nbdkit_string_intern (const char *str);

Return a pointer to a copy of str, where nbdkit owns the lifetime of the 
copy (allowing the caller to not have to worry about persisting str 
indefinitely).  If called when there is no client connection (such as 
during .load), the copy will remain valid through .unload; if called in 
the context of a client connection (any callback from .preconnect 
through .close), the copy will remain valid through .close.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Richard W.M. Jones

2020-Aug-25 10:00 UTC

head link

Re: [Libguestfs] [RFC nbdkit PATCH] protocol: Alter .list_exports, add .default_export

On Mon, Aug 24, 2020 at 05:02:56PM -0500, Eric Blake
wrote:> On 8/24/20 7:52 AM, Eric Blake wrote:
> >I'm about to add an 'exportname' filter, and in the
process, I
> >noticed a few shortcomings in our API.  Time to fix those before
> >the 1.22 release locks our API in stone.  First, .list_exports
> >needs to know if it is pre- or post-TLS, as that may affect which
> >names are exported.  Next, overloading .list_exports to do both
> >NBD_OPT_LIST and mapping "" to a canonical name is proving to
be
> >awkward; the canonical mapping is only needed during an
> >NBD_INFO_NAME response to NBD_OPT_GO, and making .open try to
> >grab the entire .list_exports list just to use only the first
> >entry (even if the plugin optimized based on the bool to only
> >provide one entry) is awkward, compared to just having a
> >dedicated function.  Finally, as long as we are going to support
> >NBD_INFO_NAME, we can also support NBD_INFO_DESCRIPTION; but
> >while we map "" to a canonical name prior to calling .open,
> >getting the description makes more sense after the connection
> >is established, alongside .get_size.
> >---
> >
> >I obviously need to finish the code to go with this, but here's
where
> >I would like to see the API before we finalize the 1.22 release.
> >
> 
> >+++ b/docs/nbdkit-plugin.pod
> 
> >+=head2 C<.default_export>
> >+
> >+ const char *default_export (int readonly, int is_tls);
> 
> Oh fun.  For some plugins (like ondemand), this is trivial: return a
> compile-time constant string.  But for others (like sh and eval),
> there's a lifetime issue: this callback is used _before_ .open, ergo
> there is no handle struct that it can be associated with.  What's
> more, this is called _after_ .preconnect, which means it is logical
> to expect that the default export name might change over time
> (consider a plugin that advertises the largest file in a directory
> as its default, but where the directory can change _which_ file is
> largest between when the first client connects and when the second
> client connects).  And the string returned by the sh script is in
> malloc'd memory (by it's very nature of coming from the user
script,
> rather than being a compile-time constant).  Without a handle to
> store this string in, we would have a memory leak: there is no way
> to associate this inside the handle's struct so that .close can
> reclaim it, but storing it globally is not thread-safe to parallel
> client connections.
I'm not sure there's an actual problem here, because there is still a
connection object inside the server any time we have a TCP connection,
so you can just store it there, unless I'm misunderstanding something.
But anyway ...
> So I'm thinking I need to add a helper
> function:
> 
> const char *nbdkit_string_intern (const char *str);
> 
> Return a pointer to a copy of str, where nbdkit owns the lifetime of
> the copy (allowing the caller to not have to worry about persisting
> str indefinitely).  If called when there is no client connection
> (such as during .load), the copy will remain valid through .unload;
> if called in the context of a client connection (any callback from
> .preconnect through .close), the copy will remain valid through
> .close.
I'm a bit unclear why the plugin would have to call this (or this
function be in the public API at all).  Why can't string interning be
done inside the server.  Have a global struct where strings returned
from the plugin are interned, and free it on server exit.

Rich.

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