Add Support for Variant ETags

[Aaron Peschel]

Hello,

Weak ETAGs proved to be insufficient to meet my requirements.

Apache has solved their issues with ETAGs by appending the type of the variant
to the existing ETAG. For example, the ETAG for a gzip variant would have -gzip
appended (EG "abc123" becomes "abc123-gzip"). This seems like a reasonable
solution to me, and it should work with Nginx as well.

Here is a link to the discussion:

https://issues.apache.org/bugzilla/show_bug.cgi?id=39727

I have created a patch to provide support for variant ETAGs in Nginx. Feedback
would be appreciated! I would like to see this merged into the main line, and
I'm willing to put in any additional work required.

Currently I have only switched the gzip module to using variant ETAGs, but
others could be switched as well. I have not tested this patch very thoroughly
yet either.

Thank you,

--Aaron Peschel

----

# HG changeset patch
# User Aaron Peschel <aaron.peschel at gmail.com>
# Date 1386735423 28800
#      Tue Dec 10 20:17:03 2013 -0800
# Node ID 1d645b958ca1d086f4ef1d4c0dd63dff6cb2f2af
# Parent  1bdf906bba99b4c03d306dd8f2756d4d8ffa4a13
Add Support for Variant ETags

Rather than stripping strict ETAGs, we can generate a variant of ETAG on the
fly that will act as a strict ETAG for the variant. The solution used by Apache
is to simply append the type of the variant to the existing ETAG. This seems
like a reasonable solution, and it is what this commit implments.

diff -r 1bdf906bba99 -r 1d645b958ca1
src/http/modules/ngx_http_gzip_filter_module.c
--- a/src/http/modules/ngx_http_gzip_filter_module.c Fri Dec 06
19:58:27 2013 +0400
+++ b/src/http/modules/ngx_http_gzip_filter_module.c Tue Dec 10
20:17:03 2013 -0800
@@ -306,7 +306,10 @@

     ngx_http_clear_content_length(r);
     ngx_http_clear_accept_ranges(r);
-    ngx_http_clear_etag(r);
+
+    if (ngx_http_set_etag_variant(r, "gzip") != NGX_OK) {
+        return NGX_ERROR;
+    }

     return ngx_http_next_header_filter(r);
 }
diff -r 1bdf906bba99 -r 1d645b958ca1 src/http/ngx_http_core_module.c
--- a/src/http/ngx_http_core_module.c Fri Dec 06 19:58:27 2013 +0400
+++ b/src/http/ngx_http_core_module.c Tue Dec 10 20:17:03 2013 -0800
@@ -1855,6 +1855,42 @@


 ngx_int_t
+ngx_http_set_etag_variant(ngx_http_request_t *r, const char *append_str)
+{
+    ngx_str_t  *etag;
+    u_char     *variant_str;
+    ngx_int_t   append_str_len;
+    ngx_int_t   variant_str_len;
+    ngx_int_t   closing_quote;
+
+    etag = &r->headers_out.etag->value;
+    append_str_len = ngx_strlen(append_str);
+    closing_quote = etag->len - 1;
+
+    if (etag->len <= 0) {
+        return NGX_OK;
+    }
+
+    // Add one byte for the dash between the etag and the append string.
+    variant_str = ngx_pnalloc(r->pool, etag->len + append_str_len + 1);
+    if (variant_str == NULL) {
+        return NGX_ERROR;
+    }
+
+    ngx_cpystrn(variant_str, etag->data, closing_quote);
+    variant_str_len = ngx_sprintf(variant_str + closing_quote,
+                                  "-%s\"",
+                                  append_str)
+                      - variant_str;
+
+    etag->len = variant_str_len;
+    etag->data = variant_str;
+
+    return NGX_OK;
+}
+
+
+ngx_int_t
 ngx_http_send_response(ngx_http_request_t *r, ngx_uint_t status,
     ngx_str_t *ct, ngx_http_complex_value_t *cv)
 {
diff -r 1bdf906bba99 -r 1d645b958ca1 src/http/ngx_http_core_module.h
--- a/src/http/ngx_http_core_module.h Fri Dec 06 19:58:27 2013 +0400
+++ b/src/http/ngx_http_core_module.h Tue Dec 10 20:17:03 2013 -0800
@@ -499,6 +499,7 @@
 ngx_int_t ngx_http_set_content_type(ngx_http_request_t *r);
 void ngx_http_set_exten(ngx_http_request_t *r);
 ngx_int_t ngx_http_set_etag(ngx_http_request_t *r);
+ngx_int_t ngx_http_set_etag_variant(ngx_http_request_t *r, const char
*append_str);
 ngx_int_t ngx_http_send_response(ngx_http_request_t *r, ngx_uint_t status,
     ngx_str_t *ct, ngx_http_complex_value_t *cv);
 u_char *ngx_http_map_uri_to_path(ngx_http_request_t *r, ngx_str_t *name,
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