[Libguestfs] [PATCH nbdkit] curl: Add effective-url flag

Laszlo Ersek
Thu, 14 Oct 2021 15:32:01 -0000

On 10/13/21 12:06, Richard W.M. Jones wrote:
> This commit adds a new command line option (effective-url=true) which
> causes nbdkit-curl-plugin the first time it fetches a URL to update
> its internal 'url' variable with the CURLINFO_EFFECTIVE_URL.  That
> means, the URL after all redirects have been done.  Further
> connections will be done using this post-redirect URL, ensuring that
> those connections are stable.
>
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2013000
> ---
>  plugins/curl/nbdkit-curl-plugin.pod | 35 ++++++++++++++++++++++++
>  plugins/curl/curl.c                 | 42 ++++++++++++++++++++++++++---
>  2 files changed, 73 insertions(+), 4 deletions(-)
>
> diff --git a/plugins/curl/nbdkit-curl-plugin.pod
b/plugins/curl/nbdkit-curl-plugin.pod
> index c7acf6225..673f1d327 100644
> --- a/plugins/curl/nbdkit-curl-plugin.pod
> +++ b/plugins/curl/nbdkit-curl-plugin.pod
> @@ -126,6 +126,40 @@ Run C (a command or shell script
fragment) to generate the
>  HTTP/HTTPS cookies.  C<cookie-script> cannot be used with
C<cookie>.
> See L below.
>
> +=item B<effective-url=true>
> +
> +(nbdkit E<ge> 1.30)
> +
> +Replace the URL supplied on the command line with the effective URL
> +(from L<CURLINFO_EFFECTIVE_URL(3)>, the final URL fetched after
server
> +redirects).  This can be used with mirror services that redirect to a
> +geographical region ? for example file download sites ? to ensure the
> +URL will always be the same.
> +
> +Note use of this feature in long-lived nbdkit instances can cause
> +subtle problems:
> +
> +=over 4
> +
> +=item *
> +
> +The effective URL persists across connections for the lifetime of the
> +nbdkit instance.  If nbdkit is used for a long time then it is
> +possible for the redirected URL to become stale.
> +
> +=item *
> +
> +It will defeat some mirror load-balancing techniques.
> +
> +=item *
> +
> +If the mirror service sometimes redirects to a broken URL and it
> +happens that the URL you fetch first is broken then nbdkit will no
> +longer recover on subsequent connections (instead you will need to
> +restart nbdkit).
> +
> +=back
> +
>  =item B<followlocation=false>
>
>  (nbdkit E<ge> 1.26)
> @@ -481,6 +515,7 @@ C<nbdkit-curl-plugin> first appeared in nbdkit
1.2.
>
>  L<curl(1)>,
>  L<libcurl(3)>,
> +L<CURLINFO_EFFECTIVE_URL(3)>,
>  L<CURLOPT_CAINFO(3)>,
>  L<CURLOPT_CAPATH(3)>,
>  L<CURLOPT_COOKIE(3)>,
> diff --git a/plugins/curl/curl.c b/plugins/curl/curl.c
> index a1b0afba7..8c016509f 100644
> --- a/plugins/curl/curl.c
> +++ b/plugins/curl/curl.c
> @@ -44,6 +44,8 @@
>  #include <errno.h>
>  #include <assert.h>
>
> +#include <pthread.h>
> +
>  #include <curl/curl.h>
>
>  #include <nbdkit-plugin.h>
> @@ -73,6 +75,7 @@ const char *cookiefile = NULL;
>  const char *cookiejar = NULL;
>  const char *cookie_script = NULL;
>  unsigned cookie_script_renew = 0;
> +bool effectiveurl = false;
>  bool followlocation = true;
>  struct curl_slist *headers = NULL;
>  const char *header_script = NULL;
> @@ -93,6 +96,12 @@ const char *unix_socket_path = NULL;
>  const char *user = NULL;
>  const char *user_agent = NULL;
>
> +/* For handling the effective-url flag, we save the first effective
> + * URL we visit in this variable.
> + */
> +char *url_effective = NULL;
> +pthread_mutex_t url_effective_lock = PTHREAD_MUTEX_INITIALIZER;
> +
>  /* Use '-D curl.verbose=1' to set. */
>  NBDKIT_DLL_PUBLIC int curl_debug_verbose = 0;
>
> @@ -118,6 +127,7 @@ curl_unload (void)
>    free (proxy_password);
>    scripts_unload ();
>    curl_global_cleanup ();
> +  free (url_effective);
>  }
>
>  /* See <curl/curl.h> */
> @@ -248,6 +258,13 @@ curl_config (const char *key, const char *value)
>        return -1;
>    }
>
> +  else if (strcmp (key, "effective-url") == 0) {
> +    r = nbdkit_parse_bool (value);
> +    if (r == -1)
> +      return -1;
> +    effectiveurl = r;
> +  }
> +
>    else if (strcmp (key, "followlocation") == 0) {
>      r = nbdkit_parse_bool (value);
>      if (r == -1)
> @@ -404,6 +421,7 @@ curl_config_complete (void)
>    "cookiejar=<FILENAME>       Read and write cookies to
jar.\n" \
> "cookie-script= Script to set HTTP/HTTPS
cookies.\n" \
>    "cookie-script-renew=<SECS> Time to renew HTTP/HTTPS
cookies.\n" \
> +  "effective-url=true         Always use redirected URL.\n" \
>    "followlocation=false       Do not follow redirects.\n" \
> "header= Set HTTP/HTTPS header.\n" \
> "header-script= Script to set HTTP/HTTPS
headers.\n" \
> @@ -486,10 +504,14 @@ curl_open (int readonly)
>    }
>
>    /* Set the URL. */
> -  r = curl_easy_setopt (h->c, CURLOPT_URL, url);
> -  if (r != CURLE_OK) {
> -    display_curl_error (h, r, "curl_easy_setopt: CURLOPT_URL
[%s]", url);
> -    goto err;
> +  {
> +    ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&url_effective_lock);
> +
> +    r = curl_easy_setopt (h->c, CURLOPT_URL, url_effective ? : url);
> +    if (r != CURLE_OK) {
> +      display_curl_error (h, r, "curl_easy_setopt: CURLOPT_URL
[%s]", url);
> +      goto err;
> +    }
>    }
>
>    /* Various options we always set.
> @@ -594,6 +616,18 @@ curl_open (int readonly)
>      goto err;
>    }
>
> +  if (effectiveurl) {
> +    ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&url_effective_lock);
> +
> +    if (url_effective == NULL) {
> +      r = curl_easy_getinfo (h->c, CURLINFO_EFFECTIVE_URL,
&url_effective);
> +      if (r != CURLE_OK) {
> +        display_curl_error (h, r, "could not get effective
URL");
> +        goto err;
> +      }
> +    }
> +  }
> +
>  #ifdef HAVE_CURLINFO_CONTENT_LENGTH_DOWNLOAD_T
>    r = curl_easy_getinfo (h->c, CURLINFO_CONTENT_LENGTH_DOWNLOAD_T,
&o);
>    if (r != CURLE_OK) {
>

Asking just for my own education:

My understanding (or, well, assumption) from this code is that multiple
threads may run curl_open() concurrently. The first code snippet sets
the URL -- namely, in the thread that runs it first, it sets the
"main"
URL, and then the second code snippet grabs the redirected URL. The rest
of the threads are then supposed to set the previously redirected URL as
their primary URL (and skip the second code snippet).

While "url_effective" is indeed protected by the mutex, don't we
have
(at least) a TOCTOU issue here? If two threads start at the top of
curl_open() in close succession, they'll both pass the original URL as
CURLOPT_URL to their own curl contexts -- "url_effective_lock" will
serialize them, but, by the time the second thread sets CURLOPT_URL, the
first thread to exit the upper code snippet may not have reached the
second code snippet (with CURLINFO_EFFECTIVE_URL). Meaning that both
CURL contexts could end up with different effective URLs. My concern is
not that the second code snippet will corrupt anything in that case (it
will not), but that one of the resolved (effective) URLs is going to be
thrown away. It's just not intuitive.

In other words, the (CURLOPT_URL, CURLINFO_EFFECTIVE_URL) *pair* needs
to be atomic, and until one thread completes *both* steps, no other
threads must fetch any ranges (because they don't know what URL to use).

In my opinion this technical detail just strengthens my point that the
redirect should be resolved outside of nbdkit.

Thanks
Laszlo