Add support for ETag in 'plain' view

When downloading a blob identified by its path, the client might want
to know if the blob has been modified since a previous download of the
same path. To this end, an ETag containing the blob SHA1 seems to be
ideal.

Todo: add support for HEAD requests...

Suggested-by: Owen Taylor <otaylor@redhat.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.c

@@ -206,6 +206,7 @@ static void prepare_context(struct cgit_context *ctx)
 	ctx->page.size = 0;
 	ctx->page.modified = time(NULL);
 	ctx->page.expires = ctx->page.modified;
+	ctx->page.etag = NULL;
 }
 
 struct refmatch {

cgit.h

@@ -180,6 +180,7 @@ struct cgit_page {
 	char *mimetype;
 	char *charset;
 	char *filename;
+	char *etag;
 	char *title;
 };
 

ui-plain.c

@@ -34,6 +34,7 @@ static void print_object(const unsigned char *sha1, const char *path)
 	ctx.page.mimetype = "text/plain";
 	ctx.page.filename = fmt("%s", path);
 	ctx.page.size = size;
+	ctx.page.etag = sha1_to_hex(sha1);
 	cgit_print_http_headers(&ctx);
 	html_raw(buf, size);
 	match = 1;

ui-shared.c

@@ -468,6 +468,8 @@ void cgit_print_http_headers(struct cgit_context *ctx)
 		      ctx->page.filename);
 	htmlf("Last-Modified: %s\n", http_date(ctx->page.modified));
 	htmlf("Expires: %s\n", http_date(ctx->page.expires));
+	if (ctx->page.etag)
+		htmlf("ETag: \"%s\"\n", ctx->page.etag);
 	html("\n");
 }