Support interactive password prompts in HTTP proxy.

In HTTP proxying, we can (and do) send the username and password
immediately in the form of HTTP Basic, if we have them in the Conf.
But if they get rejected, or if we never sent them in the first place
and the server won't let us in without auth, then we get back an HTTP
407 response with a full set of headers and an error-document.

Assuming the HTTP connection doesn't close after that (which in
sensible HTTP/1.1 proxies it won't), this gives us the opportunity to
respond by sending a second CONNECT request, containing a fresh
username and password we just requested interactively from the user.

proxy/http.c

@@ -43,32 +43,125 @@ static bool read_line(bufchain *input, strbuf *output, bool is_header)
 
 typedef struct HttpProxyNegotiator {
     int crLine;
-    strbuf *line;
+    strbuf *response, *header, *token;
+    int http_status_pos;
+    size_t header_pos;
+    strbuf *username, *password;
+    int http_status;
+    bool connection_close;
+    prompts_t *prompts;
+    int username_prompt_index, password_prompt_index;
+    size_t content_length;
     ProxyNegotiator pn;
 } HttpProxyNegotiator;
 
 static ProxyNegotiator *proxy_http_new(const ProxyNegotiatorVT *vt)
 {
     HttpProxyNegotiator *s = snew(HttpProxyNegotiator);
+    memset(s, 0, sizeof(*s));
     s->pn.vt = vt;
-    s->crLine = 0;
-    s->line = strbuf_new();
+    s->response = strbuf_new();
+    s->header = strbuf_new();
+    s->token = strbuf_new();
+    s->username = strbuf_new();
+    s->password = strbuf_new_nm();
     return &s->pn;
 }
 
 static void proxy_http_free(ProxyNegotiator *pn)
 {
     HttpProxyNegotiator *s = container_of(pn, HttpProxyNegotiator, pn);
-    strbuf_free(s->line);
+    strbuf_free(s->response);
+    strbuf_free(s->header);
+    strbuf_free(s->token);
+    strbuf_free(s->username);
+    strbuf_free(s->password);
+    if (s->prompts)
+        free_prompts(s->prompts);
     sfree(s);
 }
 
+#define HTTP_HEADER_LIST(X) \
+    X(HDR_CONNECTION, "Connection") \
+    X(HDR_CONTENT_LENGTH, "Content-Length") \
+    X(HDR_PROXY_AUTHENTICATE, "Proxy-Authenticate") \
+    /* end of list */
+
+typedef enum HttpHeader {
+    #define ENUM_DEF(id, string) id,
+    HTTP_HEADER_LIST(ENUM_DEF)
+    #undef ENUM_DEF
+    HDR_UNKNOWN
+} HttpHeader;
+
+static inline bool is_whitespace(char c)
+{
+    return (c == ' ' || c == '\t' || c == '\n');
+}
+
+static inline bool is_separator(char c)
+{
+    return (c == '(' || c == ')' || c == '<' || c == '>' || c == '@' ||
+            c == ',' || c == ';' || c == ':' || c == '\\' || c == '"' ||
+            c == '/' || c == '[' || c == ']' || c == '?' || c == '=' ||
+            c == '{' || c == '}');
+}
+
+#define HTTP_SEPARATORS
+
+static bool get_token(HttpProxyNegotiator *s)
+{
+    size_t pos = s->header_pos;
+
+    while (pos < s->header->len && is_whitespace(s->header->s[pos]))
+        pos++;
+
+    if (pos == s->header->len)
+        return false;                  /* end of string */
+
+    if (is_separator(s->header->s[pos]))
+        return false;
+
+    strbuf_clear(s->token);
+    while (pos < s->header->len &&
+           !is_whitespace(s->header->s[pos]) &&
+           !is_separator(s->header->s[pos]))
+        put_byte(s->token, s->header->s[pos++]);
+
+    s->header_pos = pos;
+    return true;
+}
+
+static bool get_separator(HttpProxyNegotiator *s, char sep)
+{
+    size_t pos = s->header_pos;
+
+    while (pos < s->header->len && is_whitespace(s->header->s[pos]))
+        pos++;
+
+    if (pos == s->header->len)
+        return false;                  /* end of string */
+
+    if (s->header->s[pos] != sep)
+        return false;
+
+    s->header_pos = ++pos;
+    return true;
+}
+
 static void proxy_http_process_queue(ProxyNegotiator *pn)
 {
     HttpProxyNegotiator *s = container_of(pn, HttpProxyNegotiator, pn);
 
     crBegin(s->crLine);
 
+    /*
+     * Initialise our username and password strbufs from the Conf.
+     */
+    put_dataz(s->username, conf_get_str(pn->ps->conf, CONF_proxy_username));
+    put_dataz(s->password, conf_get_str(pn->ps->conf, CONF_proxy_password));
+
+    while (true) {
         /*
          * Standard prefix for the HTTP CONNECT request.
          */
@@ -86,19 +179,21 @@ static void proxy_http_process_queue(ProxyNegotiator *pn)
          * password.
          */
         {
-        const char *username = conf_get_str(pn->ps->conf, CONF_proxy_username);
-        const char *password = conf_get_str(pn->ps->conf, CONF_proxy_password);
-        if (username[0] || password[0]) {
+            if (s->username->len || s->password->len) {
                 put_datalit(pn->output, "Proxy-Authorization: Basic ");
 
-            char *base64_input = dupcat(username, ":", password);
+                strbuf *base64_input = strbuf_new_nm();
+                put_datapl(base64_input, ptrlen_from_strbuf(s->username));
+                put_byte(base64_input, ':');
+                put_datapl(base64_input, ptrlen_from_strbuf(s->password));
+
                 char base64_output[4];
-            for (size_t i = 0, e = strlen(base64_input); i < e; i += 3) {
-                base64_encode_atom((const unsigned char *)base64_input + i,
+                for (size_t i = 0, e = base64_input->len; i < e; i += 3) {
+                    base64_encode_atom(base64_input->u + i,
                                        e-i > 3 ? 3 : e-i, base64_output);
                     put_data(pn->output, base64_output, 4);
                 }
-            burnstr(base64_input);
+                strbuf_free(base64_input);
                 smemclr(base64_output, sizeof(base64_output));
                 put_datalit(pn->output, "\r\n");
             }
@@ -110,38 +205,167 @@ static void proxy_http_process_queue(ProxyNegotiator *pn)
         put_datalit(pn->output, "\r\n");
         crReturnV;
 
+        s->content_length = 0;
+        s->connection_close = false;
+
         /*
          * Read and parse the HTTP status line, and check if it's a 2xx
          * for success.
          */
-    strbuf_clear(s->line);
-    crMaybeWaitUntilV(read_line(pn->input, s->line, false));
+        strbuf_clear(s->response);
+        crMaybeWaitUntilV(read_line(pn->input, s->response, false));
         {
-        int maj_ver, min_ver, status_pos = -1;
-        sscanf(s->line->s, "HTTP/%d.%d %n", &maj_ver, &min_ver, &status_pos);
+            int maj_ver, min_ver, n_scanned;
+            n_scanned = sscanf(
+                s->response->s, "HTTP/%d.%d %n%d",
+                &maj_ver, &min_ver, &s->http_status_pos, &s->http_status);
 
-        /* If status_pos is still -1 then the sscanf didn't get right
-         * to the end of the string */
-        if (status_pos == -1) {
+            if (n_scanned < 3) {
                 pn->error = dupstr("HTTP response was absent or malformed");
                 crStopV;
             }
 
-        if (s->line->s[status_pos] != '2') {
-            pn->error = dupprintf("HTTP response %s", s->line->s + status_pos);
-            crStopV;
+            if (maj_ver < 1 && (maj_ver == 1 && min_ver < 1)) {
+                /* Before HTTP/1.1, connections close by default */
+                s->connection_close = true;
             }
         }
 
         /*
-     * Read and skip the rest of the HTTP response headers, terminated
-     * by a blank line.
+         * Read the HTTP response header section.
          */
         do {
-        strbuf_clear(s->line);
-        crMaybeWaitUntilV(read_line(pn->input, s->line, true));
-    } while (s->line->len > 0);
+            strbuf_clear(s->header);
+            crMaybeWaitUntilV(read_line(pn->input, s->header, true));
+            s->header_pos = 0;
 
+            if (!get_token(s)) {
+                /* Possibly we ought to panic if we see an HTTP header
+                 * we can't make any sense of at all? But whatever,
+                 * ignore it and hope the next one makes more sense */
+                continue;
+            }
+
+            /* Parse the header name */
+            HttpHeader hdr = HDR_UNKNOWN;
+            {
+                #define CHECK_HEADER(id, string) \
+                    if (!stricmp(s->token->s, string)) hdr = id;
+                HTTP_HEADER_LIST(CHECK_HEADER);
+                #undef CHECK_HEADER
+            }
+
+            if (!get_separator(s, ':'))
+                continue;
+
+            if (hdr == HDR_CONTENT_LENGTH) {
+                if (!get_token(s))
+                    continue;
+                s->content_length = strtoumax(s->token->s, NULL, 10);
+            } else if (hdr == HDR_CONNECTION) {
+                if (!get_token(s))
+                    continue;
+                if (!stricmp(s->token->s, "close"))
+                    s->connection_close = true;
+                else if (!stricmp(s->token->s, "keep-alive"))
+                    s->connection_close = false;
+            } else if (hdr == HDR_PROXY_AUTHENTICATE) {
+                if (!get_token(s))
+                    continue;
+
+                if (!stricmp(s->token->s, "Basic")) {
+                    /* fine, we know how to do Basic auth */
+                } else {
+                    pn->error = dupprintf("HTTP proxy asked for unsupported "
+                                          "authentication type '%s'",
+                                          s->token->s);
+                    crStopV;
+                }
+            }
+        } while (s->header->len > 0);
+
+        /* Read and ignore the entire response document */
+        crMaybeWaitUntilV(bufchain_try_consume(
+                              pn->input, s->content_length));
+
+        if (200 <= s->http_status && s->http_status < 300) {
+            /* Any 2xx HTTP response means we're done */
+            goto authenticated;
+        } else if (s->http_status == 407) {
+            /* 407 is Proxy Authentication Required, which we may be
+             * able to do something about. */
+            if (s->connection_close) {
+                pn->error = dupprintf("HTTP proxy closed connection after "
+                                      "asking for authentication");
+                crStopV;
+            }
+
+            /* Either we never had a password in the first place, or
+             * the one we already presented was rejected. We can only
+             * proceed from here if we have a way to ask the user
+             * questions. */
+            if (!pn->itr) {
+                pn->error = dupprintf("HTTP proxy requested authentication "
+                                      "which we do not have");
+                crStopV;
+            }
+
+            /*
+             * Send some prompts to the user. We'll assume the
+             * password is always required (since it's just been
+             * rejected, even if we did send one before), and we'll
+             * prompt for the username only if we don't have one from
+             * the Conf.
+             */
+            s->prompts = proxy_new_prompts(pn->ps);
+            s->prompts->to_server = true;
+            s->prompts->from_server = false;
+            s->prompts->name = dupstr("HTTP proxy authentication");
+            if (!s->username->len) {
+                s->username_prompt_index = s->prompts->n_prompts;
+                add_prompt(s->prompts, dupstr("Proxy username: "), true);
+            } else {
+                s->username_prompt_index = -1;
+            }
+
+            s->password_prompt_index = s->prompts->n_prompts;
+            add_prompt(s->prompts, dupstr("Proxy password: "), false);
+
+            while (true) {
+                int prompt_result = seat_get_userpass_input(
+                    interactor_announce(pn->itr), s->prompts);
+                if (prompt_result > 0) {
+                    break;
+                } else if (prompt_result == 0) {
+                    pn->aborted = true;
+                    crStopV;
+                }
+                crReturnV;
+            }
+
+            if (s->username_prompt_index != -1) {
+                strbuf_clear(s->username);
+                put_dataz(s->username,
+                          prompt_get_result_ref(
+                              s->prompts->prompts[s->username_prompt_index]));
+            }
+
+            strbuf_clear(s->password);
+            put_dataz(s->password,
+                      prompt_get_result_ref(
+                          s->prompts->prompts[s->password_prompt_index]));
+
+            free_prompts(s->prompts);
+            s->prompts = NULL;
+        } else {
+            /* Any other HTTP response is treated as permanent failure */
+            pn->error = dupprintf("HTTP response %s",
+                                  s->response->s + s->http_status_pos);
+            crStopV;
+        }
+    }
+
+  authenticated:
     /*
      * Success! Hand over to the main connection.
      */