httpd: Support caching via ETag header

If server responds with ETag then next time client can resend it via If-None-Match header.
Then httpd will check if file wasn't modified and if not return 304 Not Modified status code.
The ETag value is constructed from file's last modification date in unix epoch and it's size:
"hex(last_mod)-hex(file_size)" e.g. "5e132e20-417" (with quotes).
That means that it's not completely reliable as hash functions but fair enough.
The same form of ETag is used by Nginx so load balancing of static content is safe.

function                                             old     new   delta
handle_incoming_and_exit                            2135    2201     +66
http_response                                         88      96      +8
send_headers                                         676     683      +7
parse_conf                                          1362    1365      +3
http_response_type                                    22      24      +2
send_file_and_exit                                   847     841      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/1 up/down: 86/-6)              Total: 80 bytes

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 54 insertions, 3 deletions

diff --git a/networking/httpd.c b/networking/httpd.c
index 94f7297ad..0297cf9ec 100644
--- a/networking/httpd.c
+++ b/networking/httpd.c
@@ -215,6 +215,19 @@
 //config:       Makes httpd send files using GZIP content encoding if the
 //config:       client supports it and a pre-compressed <file>.gz exists.
 //config:
+//config:config FEATURE_HTTPD_ETAG
+//config:       bool "Support caching via ETag header"
+//config:       default y
+//config:       depends on HTTPD
+//config:       help
+//config:       If server responds with ETag then next time client (browser)
+//config:       resend it via If-None-Match header.
+//config:       Then httpd will check if file wasn't modified and if not,
+//config:       return 304 Not Modified status code.
+//config:       The ETag value is constructed from last modification date
+//config:       in unix epoch, and size: "hex(last_mod)-hex(file_size)".
+//config:       It's not completely reliable as hash functions but fair enough.
+//config:
 //config:config FEATURE_HTTPD_LAST_MODIFIED
 //config:       bool "Add Last-Modified header to response"
 //config:       default y
@@ -328,6 +341,7 @@ enum {
         HTTP_OK = 200,
         HTTP_PARTIAL_CONTENT = 206,
         HTTP_MOVED_TEMPORARILY = 302,
+        HTTP_NOT_MODIFIED = 304,
         HTTP_BAD_REQUEST = 400,       /* malformed syntax */
         HTTP_UNAUTHORIZED = 401, /* authentication needed, respond with auth hdr */
         HTTP_NOT_FOUND = 404,
@@ -345,7 +359,6 @@ enum {
         HTTP_NO_CONTENT = 204,
         HTTP_MULTIPLE_CHOICES = 300,
         HTTP_MOVED_PERMANENTLY = 301,
-        HTTP_NOT_MODIFIED = 304,
         HTTP_PAYMENT_REQUIRED = 402,
         HTTP_BAD_GATEWAY = 502,
         HTTP_SERVICE_UNAVAILABLE = 503, /* overload, maintenance */
@@ -358,6 +371,9 @@ static const uint16_t http_response_type[] ALIGN2 = {
         HTTP_PARTIAL_CONTENT,
 #endif
         HTTP_MOVED_TEMPORARILY,
+#if ENABLE_FEATURE_HTTPD_ETAG
+        HTTP_NOT_MODIFIED,
+#endif
         HTTP_REQUEST_TIMEOUT,
         HTTP_NOT_IMPLEMENTED,
 #if ENABLE_FEATURE_HTTPD_BASIC_AUTH
@@ -374,7 +390,6 @@ static const uint16_t http_response_type[] ALIGN2 = {
         HTTP_NO_CONTENT,
         HTTP_MULTIPLE_CHOICES,
         HTTP_MOVED_PERMANENTLY,
-        HTTP_NOT_MODIFIED,
         HTTP_BAD_GATEWAY,
         HTTP_SERVICE_UNAVAILABLE,
 #endif
@@ -389,6 +404,9 @@ static const struct {
         { "Partial Content", NULL },
 #endif
         { "Found", NULL },
+#if ENABLE_FEATURE_HTTPD_ETAG
+        { "Not Modified" },
+#endif
         { "Request Timeout", "No request appeared within 60 seconds" },
         { "Not Implemented", "The requested method is not recognized" },
 #if ENABLE_FEATURE_HTTPD_BASIC_AUTH
@@ -405,7 +423,6 @@ static const struct {
         { "No Content" },
         { "Multiple Choices" },
         { "Moved Permanently" },
-        { "Not Modified" },
         { "Bad Gateway", "" },
         { "Service Unavailable", "" },
 #endif
@@ -419,6 +436,9 @@ struct globals {
         smallint content_gzip;
 #endif
         time_t last_mod;
+#if ENABLE_FEATURE_HTTPD_ETAG
+        char *if_none_match;
+#endif
         char *rmt_ip_str;       /* for $REMOTE_ADDR and $REMOTE_PORT */
         const char *bind_addr_or_port;
 
@@ -453,6 +473,9 @@ struct globals {
 #define sizeof_hdr_buf COMMON_BUFSIZE
         char *hdr_ptr;
         int hdr_cnt;
+#if ENABLE_FEATURE_HTTPD_ETAG
+        char etag[sizeof("'%llx-%llx'") + 2 * sizeof(long long)*3];
+#endif
 #if ENABLE_FEATURE_HTTPD_ERROR_PAGES
         const char *http_error_page[ARRAY_SIZE(http_response_type)];
 #endif
@@ -1208,6 +1231,10 @@ static void send_headers(unsigned responseNum)
 #if ENABLE_FEATURE_HTTPD_LAST_MODIFIED
                         "Last-Modified: %s\r\n"
 #endif
+#if ENABLE_FEATURE_HTTPD_ETAG
+                        "ETag: %s\r\n"
+#endif
+
         /* Because of 4.4 (5), we can forgo sending of "Content-Length"
          * since we close connection afterwards, but it helps clients
          * to e.g. estimate download times, show progress bars etc.
@@ -1218,6 +1245,9 @@ static void send_headers(unsigned responseNum)
 #if ENABLE_FEATURE_HTTPD_LAST_MODIFIED
                                 date_str,
 #endif
+#if ENABLE_FEATURE_HTTPD_ETAG
+                                G.etag,
+#endif
                                 file_size
                 );
         }
@@ -1730,6 +1760,7 @@ static NOINLINE void send_file_and_exit(const char *url, int what)
                 }
         } else {
                 fd = open(url, O_RDONLY);
+                /* file_size and last_mod are already populated */
         }
         if (fd < 0) {
                 if (DEBUG)
@@ -1741,6 +1772,19 @@ static NOINLINE void send_file_and_exit(const char *url, int what)
                         send_headers_and_exit(HTTP_NOT_FOUND);
                 log_and_exit();
         }
+#if ENABLE_FEATURE_HTTPD_ETAG
+        /* ETag is "hex(last_mod)-hex(file_size)" e.g. "5e132e20-417" */
+        sprintf(G.etag, "\"%llx-%llx\"", (unsigned long long)last_mod, (unsigned long long)file_size);
+
+        if (G.if_none_match) {
+                if (DEBUG)
+                        bb_perror_msg("If-None-Match and file's ETag are: '%s' '%s'\n", G.if_none_match, G.etag);
+                /* Weak ETag comparision.
+                 * If-None-Match may have many ETags but they are quoted so we can use simple substring search */
+                if (strstr(G.if_none_match, G.etag))
+                        send_headers_and_exit(HTTP_NOT_MODIFIED);
+        }
+#endif
         /* If you want to know about EPIPE below
          * (happens if you abort downloads from local httpd): */
         signal(SIGPIPE, SIG_IGN);
@@ -2480,6 +2524,13 @@ static void handle_incoming_and_exit(const len_and_sockaddr *fromAddr)
                         continue;
                 }
 #endif
+#if ENABLE_FEATURE_HTTPD_ETAG
+                if (STRNCASECMP(iobuf, "If-None-Match:") == 0) {
+                        free(G.if_none_match);
+                        G.if_none_match = xstrdup(skip_whitespace(iobuf + sizeof("If-None-Match:") - 1));
+                        continue;
+                }
+#endif
 #if ENABLE_FEATURE_HTTPD_CGI
                 if (cgi_type != CGI_NONE) {
                         bool ct = (STRNCASECMP(iobuf, "Content-Type:") == 0);