[Patch] Handle all four etags types in HTTP client

[Patch] Handle all four etags types in HTTP client

[Derek Upham]

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The HTTP client code is aware of and handles strong and weak etags 
as quoted strings (matching the spec), and strong etags as 
unquoted strings (which don’t match the spec, but which some 
servers send anyway).  Some servers also violate the spec by 
returning *weak* tags as unquoted strings.  This patch updates the 
parsing code to recognize and handle that case.


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From dfd68f4f2dc6c2b3b86191e550a50a19533522d3 Mon Sep 17 00:00:00 2001
From: Derek Upham <derek_upham@mailfence.com>
Date: Sun, 19 Jan 2020 16:55:10 -0800
Subject: [PATCH 2/2] Properly handle unquoted and quoted etags.

---
 module/web/http.scm            | 20 +++++++++++++-------
 test-suite/tests/web-http.test |  1 +
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/module/web/http.scm b/module/web/http.scm
index c5e559bea..0d495ff69 100644
--- a/module/web/http.scm
+++ b/module/web/http.scm
@@ -875,8 +875,10 @@ as an ordered alist."
 
 ;; Following https://tools.ietf.org/html/rfc7232#section-2.3, an entity
 ;; tag should really be a qstring.  However there are a number of
-;; servers that emit etags as unquoted strings.  Assume that if the
-;; value doesn't start with a quote, it's an unquoted strong etag.
+;; servers that emit strong and weak etags as unquoted strings.  We have
+;; to handle all four combinations of quoted/unquoted and strong/weak.
+;; (Use the heuristic that an unquoted entity tag with an initial 'W/'
+;; is a weak tag, with the tag following the slash.)
 (define* (parse-entity-tag val #:optional (start 0) (end (string-length val))
                            #:key sloppy-delimiters)
   (define (parse-proper-etag-at start strong?)
@@ -888,16 +890,20 @@ as an ordered alist."
           (values (cons tag strong?) next))))
      (else
       (values (cons (parse-qstring val start end) strong?) end))))
+  (define (parse-unquoted-etag-at start strong?)
+    (let ((delim (or (and sloppy-delimiters
+                          (string-index val sloppy-delimiters start end))
+                     end)))
+      (values (cons (substring val start delim) strong?) delim)))
   (cond
-   ((string-prefix? "W/" val 0 2 start end)
+   ((string-prefix? "W/\"" val 0 3 start end)
     (parse-proper-etag-at (+ start 2) #f))
+   ((string-prefix? "W/" val 0 2 start end)
+    (parse-unquoted-etag-at (+ start 2) #f))
    ((string-prefix? "\"" val 0 1 start end)
     (parse-proper-etag-at start #t))
    (else
-    (let ((delim (or (and sloppy-delimiters
-                          (string-index val sloppy-delimiters start end))
-                     end)))
-      (values (cons (substring val start delim) #t) delim)))))
+    (parse-unquoted-etag-at start #t))))
 
 (define (entity-tag? val)
   (match val
diff --git a/test-suite/tests/web-http.test b/test-suite/tests/web-http.test
index 843936bb5..0cf3b6129 100644
--- a/test-suite/tests/web-http.test
+++ b/test-suite/tests/web-http.test
@@ -410,6 +410,7 @@
   (pass-if-parse etag "\"foo\"" '("foo" . #t))
   (pass-if-parse etag "W/\"foo\"" '("foo" . #f))
   (pass-if-parse etag "foo" '("foo" . #t))
+  (pass-if-parse etag "W/foo" '("foo" . #t))
   (pass-if-parse location "http://other-place"
                  (build-uri 'http #:host "other-place"))
   (pass-if-only-parse location "#foo"
-- 
2.25.1


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Derek

-- 
Derek Upham
derek_upham@mailfence.com