http: read HTTP WWW-Authenticate response headers

Read and store the HTTP WWW-Authenticate response headers made for
a particular request.

This will allow us to pass important authentication challenge
information to credential helpers or others that would otherwise have
been lost.

libcurl only provides us with the ability to read all headers recieved
for a particular request, including any intermediate redirect requests
or proxies. The lines returned by libcurl include HTTP status lines
delinating any intermediate requests such as "HTTP/1.1 200". We use
these lines to reset the strvec of WWW-Authenticate header values as
we encounter them in order to only capture the final response headers.

The collection of all header values matching the WWW-Authenticate
header is complicated by the fact that it is legal for header fields to
be continued over multiple lines, but libcurl only gives us each
physical line a time, not each logical header. This line folding feature
is deprecated in RFC 7230 [1] but older servers may still emit them, so
we need to handle them.

In the future [2] we may be able to leverage functions to read headers
from libcurl itself, but as of today we must do this ourselves.

[1] https://www.rfc-editor.org/rfc/rfc7230#section-3.2
[2] https://daniel.haxx.se/blog/2022/03/22/a-headers-api-for-libcurl/

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/git-compat-util.h b/git-compat-util.h
index 4f0028ce60..f671a0ec3f 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1287,6 +1287,25 @@ static inline int skip_iprefix(const char *str, const char *prefix,
 	return 0;
 }
 
+/*
+ * Like skip_prefix_mem, but compare case-insensitively. Note that the
+ * comparison is done via tolower(), so it is strictly ASCII (no multi-byte
+ * characters or locale-specific conversions).
+ */
+static inline int skip_iprefix_mem(const char *buf, size_t len,
+				   const char *prefix,
+				   const char **out, size_t *outlen)
+{
+	do {
+		if (!*prefix) {
+			*out = buf;
+			*outlen = len;
+			return 1;
+		}
+	} while (len-- > 0 && tolower(*buf++) == tolower(*prefix++));
+	return 0;
+}
+
 static inline int strtoul_ui(char const *s, int base, unsigned int *result)
 {
 	unsigned long ul;