curl: Case insensitive check for accept-ranges (RHBZ#1837337).

When accessing an HTTP/2 server we read lowercase headers so the
existing test for byte range support did not work.  You would see an
error like this:

  nbdkit: curl[1]: error: server does not support 'range' (byte range) requests

This commit copies the bug fix which was recently added to qemu’s
block/curl.c.

qemu commits:

  commit 69032253c33ae1774233c63cedf36d32242a85fc
  Author: David Edmondson <david.edmondson@oracle.com>
  Date:   Mon Feb 24 10:13:10 2020 +0000

    block/curl: HTTP header field names are case insensitive

  commit 7788a319399f17476ff1dd43164c869e320820a2
  Author: David Edmondson <david.edmondson@oracle.com>
  Date:   Mon Feb 24 10:13:09 2020 +0000

    block/curl: HTTP header fields allow whitespace around values

Thanks: David Edmondson, Pino Toscano, Zi Liu

plugins/curl/Makefile.am

@@ -44,6 +44,7 @@ nbdkit_curl_plugin_la_SOURCES = \
 
 nbdkit_curl_plugin_la_CPPFLAGS = \
 	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include \
 	-I$(top_srcdir)/common/utils \
 	$(NULL)
 nbdkit_curl_plugin_la_CFLAGS = \

plugins/curl/curl.c

@@ -57,6 +57,7 @@
 #include <nbdkit-plugin.h>
 
 #include "cleanup.h"
+#include "ascii-ctype.h"
 
 static const char *url = NULL;  /* required */
 
@@ -555,12 +556,30 @@ header_cb (void *ptr, size_t size, size_t nmemb, void *opaque)
 {
   struct curl_handle *h = opaque;
   size_t realsize = size * nmemb;
-  const char *accept_line = "Accept-Ranges: bytes";
-  const char *line = ptr;
-
-  if (realsize >= strlen (accept_line) &&
-      strncmp (line, accept_line, strlen (accept_line)) == 0)
-    h->accept_range = true;
+  const char *header = ptr;
+  const char *end = header + realsize;
+  const char *accept_ranges = "accept-ranges:";
+  const char *bytes = "bytes";
+
+  if (realsize >= strlen (accept_ranges) &&
+      strncasecmp (header, accept_ranges, strlen (accept_ranges)) == 0) {
+    const char *p = strchr (header, ':') + 1;
+
+    /* Skip whitespace between the header name and value. */
+    while (p < end && *p && ascii_isspace (*p))
+      p++;
+
+    if (end - p >= strlen (bytes)
+        && strncmp (p, bytes, strlen (bytes)) == 0) {
+      /* Check that there is nothing but whitespace after the value. */
+      p += strlen (bytes);
+      while (p < end && *p && ascii_isspace (*p))
+        p++;
+
+      if (p == end || !*p)
+        h->accept_range = true;
+    }
+  }
 
   return realsize;
 }

tests/web-server.c

@@ -235,7 +235,7 @@ handle_request (int s, bool headers_only)
   const char response1_ok[] = "HTTP/1.1 200 OK\r\n";
   const char response1_partial[] = "HTTP/1.1 206 Partial Content\r\n";
   const char response2[] =
-    "Accept-Ranges: bytes\r\n"
+    "Accept-rANGES:     bytes\r\n" /* See RHBZ#1837337 */
     "Connection: keep-alive\r\n"
     "Content-Type: application/octet-stream\r\n";
   char response3[64];