credential: add WWW-Authenticate header to cred requests

Add the value of the WWW-Authenticate response header to credential
requests. Credential helpers that understand and support HTTP
authentication and authorization can use this standard header (RFC 2616
Section 14.47 [1]) to generate valid credentials.

WWW-Authenticate headers can contain information pertaining to the
authority, authentication mechanism, or extra parameters/scopes that are
required.

The current I/O format for credential helpers only allows for unique
names for properties/attributes, so in order to transmit multiple header
values (with a specific order) we introduce a new convention whereby a
C-style array syntax is used in the property name to denote multiple
ordered values for the same property.

In this case we send multiple `wwwauth[]` properties where the order
that the repeated attributes appear in the conversation reflects the
order that the WWW-Authenticate headers appeared in the HTTP response.

Add a set of tests to exercise the HTTP authentication header parsing
and the interop with credential helpers. Credential helpers will receive
WWW-Authenticate information in credential requests.

[1] https://datatracker.ietf.org/doc/html/rfc2616#section-14.47

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

1 files changed, 18 insertions, 1 deletions

diff --git a/Documentation/git-credential.txt b/Documentation/git-credential.txt
index ac2818b9f6..50759153ef 100644
--- a/Documentation/git-credential.txt
+++ b/Documentation/git-credential.txt
@@ -113,7 +113,13 @@ separated by an `=` (equals) sign, followed by a newline.
 The key may contain any bytes except `=`, newline, or NUL. The value may
 contain any bytes except newline or NUL.
 
-In both cases, all bytes are treated as-is (i.e., there is no quoting,
+Attributes with keys that end with C-style array brackets `[]` can have
+multiple values. Each instance of a multi-valued attribute forms an
+ordered list of values - the order of the repeated attributes defines
+the order of the values. An empty multi-valued attribute (`key[]=\n`)
+acts to clear any previous entries and reset the list.
+
+In all cases, all bytes are treated as-is (i.e., there is no quoting,
 and one cannot transmit a value with newline or NUL in it). The list of
 attributes is terminated by a blank line or end-of-file.
 
@@ -160,6 +166,17 @@ empty string.
 Components which are missing from the URL (e.g., there is no
 username in the example above) will be left unset.
 
+`wwwauth[]`::
+
+	When an HTTP response is received by Git that includes one or more
+	'WWW-Authenticate' authentication headers, these will be passed by Git
+	to credential helpers.
++
+Each 'WWW-Authenticate' header value is passed as a multi-valued
+attribute 'wwwauth[]', where the order of the attributes is the same as
+they appear in the HTTP response. This attribute is 'one-way' from Git
+to pass additional information to credential helpers.
+
 Unrecognised attributes are silently discarded.
 
 GIT