Introduce simple HTTP client demo application.

demo/http_client/README.adoc

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+= http_client
+
+This is a very simple HTTP client.  It only performs HTTP GET
+operations, and does not follow HTTP redirects.  Think of it as
+a trivialized version of cURL.  It is super simple, taking the
+URL on the command line, and emitting the results to stdout.
+
+For clarity, we are eliding TLS support.
+
+It may not work on all systems, but it should work anywhere that
+ both the standard C library and nng itself are available.
+
+We check for errors, but no effort is made to clean up resources,
+since this program just exits.  In longer running programs or libraries,
+callers should take care to clean up things that they allocate.
+
+Unfortunately many famous sites use redirects (usually to HTTPS
+sites), so it's not a very useful replacement for cURL.
+
+== Compiling
+
+The following is an example typical of UNIX and similar systems like
+Linux and macOS:
+
+[source, bash]
+----
+% export CPPFLAGS="-I /usr/local/include"
+% export LDFLAGS="-L /usr/local/lib -lnng"
+% export CC="cc"
+% ${CC} ${CPPFLAGS} http_client.c -o http_client ${LDFLAGS}
+----
+
+== Running
+
+Make sure you specify the full URL (if the root page include
+the simple "/".  The URL parser does not add it for you automatically.)
+
+[source, bash]
+----
+% ./http_client http://httpbin.org/ip
+----
+

demo/http_client/http_client.c

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+//
+// Copyright 2018 Staysail Systems, Inc. <[email protected]>
+// Copyright 2018 Capitar IT Group BV <[email protected]>
+//
+// This software is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+// This is a very simple HTTP client.  It only performs HTTP GET
+// operations, and does not follow HTTP redirects.  Think of it as
+// a trivialized version of CURL.  It is super simple, taking the
+// URL on the command line, and emitting the results to stdout.
+// For clarity, we are eliding TLS support.
+
+// It may not work on all systems, but it should work anywhere that
+// both the standard C library and nng itself are available.
+
+// We check for errors, but no effort is made to clean up resources,
+// since this program just exits.  In longer running programs or libraries,
+// callers should take care to clean up things that they allocate.
+
+// Unfortunately many famous sites use redirects, so you won't see that
+// emitted.
+
+// Example usage:
+//
+// % export CPPFLAGS="-I /usr/local/include"
+// % export LDFLAGS="-L /usr/local/lib -lnng"
+// % export CC="cc"
+// % ${CC} ${CPPFLAGS} http_client.c -o http_client ${LDFLAGS}
+// % ./http_client http://httpbin.org/ip
+// 
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <nng/nng.h>
+
+void
+fatal(int rv)
+{
+	fprintf(stderr, "%s\n", nng_strerror(rv));
+	exit(1);
+}
+
+int
+main(int argc, char **argv)
+{
+	nng_http_client *client;
+	nng_http_conn *conn;
+	nng_url *url;
+	nng_aio *aio;	
+	nng_http_req *req;
+	nng_http_res *res;
+	const char *hdr;
+	int rv;
+	int len;
+	void *data;
+	nng_iov iov;
+
+	if (argc < 2) {
+		fprintf(stderr, "No URL supplied!\n");
+		exit(1);
+	}
+
+	if (((rv = nng_url_parse(&url, argv[1])) != 0) ||
+	    ((rv = nng_http_client_alloc(&client, url)) != 0) ||
+	    ((rv = nng_http_req_alloc(&req, url)) != 0) ||
+	    ((rv = nng_http_res_alloc(&res)) != 0) ||
+	    ((rv = nng_aio_alloc(&aio, NULL, NULL)) != 0)) {
+		fatal(rv);
+	}
+
+	// Start connection process...
+	nng_http_client_connect(client, aio);
+
+	// Wait for it to finish.
+	nng_aio_wait(aio);
+	if ((rv = nng_aio_result(aio)) != 0) {
+		fatal(rv);
+	}
+
+	// Get the connection, at the 0th output.
+	conn = nng_aio_get_output(aio, 0);
+
+	// Request is already set up with URL, and for GET via HTTP/1.1.
+	// The Host: header is already set up too.
+
+	// Send the request, and wait for that to finish.
+	nng_http_conn_write_req(conn, req, aio);
+	nng_aio_wait(aio);
+
+	if ((rv = nng_aio_result(aio)) != 0) {
+		fatal(rv);
+	}
+
+	// Read a response.
+	nng_http_conn_read_res(conn, res, aio);
+	nng_aio_wait(aio);
+
+	if ((rv = nng_aio_result(aio)) != 0) {
+		fatal(rv);
+	}
+
+	if (nng_http_res_get_status(res) != NNG_HTTP_STATUS_OK) {
+		fprintf(stderr, "HTTP Server Responded: %d %s\n",
+		    nng_http_res_get_status(res),
+		    nng_http_res_get_reason(res));
+	}
+
+	// This only supports regular transfer encoding (no Chunked-Encoding,
+	// and a Content-Length header is required.)
+	if ((hdr = nng_http_res_get_header(res, "Content-Length")) == NULL) {
+		fprintf(stderr, "Missing Content-Length header.\n");
+		exit(1);
+	}
+
+	if (len == 0) {
+		return (0);
+	}
+
+	// Allocate a buffer to receive the body data.
+	len = atoi(hdr);
+	data = malloc(len);
+
+	// Set up a single iov to point to the buffer.
+	iov.iov_len = len;
+	iov.iov_buf = data;
+
+	// Following never fails with fewer than 5 elements.
+	nng_aio_set_iov(aio, 1, &iov);
+
+	// Now attempt to receive the data.
+	nng_http_conn_read_all(conn, aio);
+
+	// Wait for it to complete.
+	nng_aio_wait(aio);
+
+	if ((rv = nng_aio_result(aio)) != 0) {
+		fatal(rv);
+	}
+
+	fwrite(data, 1, len, stdout);
+	return (0);
+}