[PATCH v4 1/2] http: honor empty http.proxy option to bypass proxy

[Sergey Ryazanov <ryazanov.s.a@gmail.com>]

Curl distinguishes between an empty proxy address and a NULL proxy
address. In the first case it completely disables proxy usage, but if
the proxy address option is NULL then curl attempts to determine the
proxy address from the http_proxy environment variable.

According to the documentation, if the http.proxy option is set to an
empty string, git should bypass proxy and connect to the server
directly:

    export http_proxy=http://network-proxy/
    cd ~/foobar-project
    git config remote.origin.proxy ""
    git fetch

Previously, proxy host was configured by one line:

    curl_easy_setopt(result, CURLOPT_PROXY, curl_http_proxy);

Commit 372370f167 ("http: use credential API to handle proxy
authentication", 2016-01-26) parses the proxy option, then extracts the
proxy host address and updates the curl configuration, making the
previous call a noop:

    credential_from_url(&proxy_auth, curl_http_proxy);
    curl_easy_setopt(result, CURLOPT_PROXY, proxy_auth.host);

But if the proxy option is empty then the proxy host field becomes NULL.
This forces curl to fall back to detecting the proxy configuration from
the environment, causing the http.proxy option to not work anymore.

Fix this issue by explicitly handling http.proxy being set the empty
string. This also makes the code a bit more clear and should help us
avoid such regressions in the future.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
---

Changes since v1:
 - explicitly handle this case instead of mangling the common code

Changes since v2:
 - fix grammar (thanks to Ævar)

Changes since v3:
 - no changes

 http.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/http.c b/http.c
index 96d84bb..8be75b2 100644
--- a/http.c
+++ b/http.c
@@ -836,8 +836,14 @@ static CURL *get_curl_handle(void)
 		}
 	}
 
-	if (curl_http_proxy) {
-		curl_easy_setopt(result, CURLOPT_PROXY, curl_http_proxy);
+	if (curl_http_proxy && curl_http_proxy[0] == '\0') {
+		/*
+		 * Handle case with the empty http.proxy value here to keep
+		 * common code clean.
+		 * NB: empty option disables proxying at all.
+		 */
+		curl_easy_setopt(result, CURLOPT_PROXY, "");
+	} else if (curl_http_proxy) {
 #if LIBCURL_VERSION_NUM >= 0x071800
 		if (starts_with(curl_http_proxy, "socks5h"))
 			curl_easy_setopt(result,
-- 
2.10.2